Louwrentius - solar

I made my blog solar-powered - a huge update

2026-07-12T12:00:00+02:00

Background

In 2020 - inspired by Low Tech Magazine - I decided to make this blog solar-powered. At that time, I was already self-hosting this blog on a Raspberry Pi at that time so it felt like a fun little project.

It all started with a small 60W solar panel and a few small 'motor cycle' lead acid batteries¹. In part due to the sub-optimal orientation of my balcony, the 60W panel did not generate enough energy. So I added a bigger 150W panel in parallel with the 60W panel and added an old lead acid car battery, as documented in my first blog post about the project.

As this still didn't generate enough solar power to keep the Raspberry Pi running 24/7, I added two 370 Watt solar panels and a large 280Ah 12V lead-acid battery, as documented here².

During spring and summer, the two large solar panels provided enough power to charge the battery and even run my Mac mini and dual monitor setup for many hours. So I added an inverter (converts DC to AC) to make this possible.

As this setup worked great, I decided to replace the lead-acid battery with an LFP battery, which was a huge improvement.

Four 230Ah LFP prismatic cells (~2.7 kWh)

As a ship of Theseus, not a single component of that first solar setup remains³. And I'm no longer only powering the 3W Raspberry Pi hosting this blog, or just my computer and monitors.

My solar-setup now powers my apartment.

I want to acknowledge that DIY solar-battery systems is starting to become more mainstream and in that sense my setup is not special. People building DIY 1-phase or 3-phase systems with 15kWh - 64kWh+ of battery storage is well-documented on Youtube.

Yet, I think my small setup might still be interesting and I just wanted to share my build.

About my apartment

I live in an apartment in The Netherlands. I've placed two solar panels on my balcony, which is facing west. Direct sunlight only appears at around 16:00 in the afternoon, which is far, far from ideal.

This is an important limitation of my setup and the daily solar yield of the two panels would be much higher if my balcony would have faced south. I'm convinced I could run my home most of the time on solar from April until early October with just two 370W panels⁴.

An overview of my current setup

It's a 12V system where all components are connected to a shared busbar. It consists of two 370W solar panels, each connected to its own solar (MPPT) controller. There are two batteries based on LFP with a combined capacity of around 6kW. The inverter is grid-connected and can charge from the grid or discharge towards the grid. A 20x4 LCD display connected to the Pis GPIO pins shows some basic info.

Some 12V 'cigarette lighter' style outlets are used to charge power banks, tablets and other battery-powered gizmos. I've also used a boost-converter to power 30V DC led lights to illuminate my living room.

The shunt is a device that measures how much energy is flowing through the system and it is the most accurate device to gauge state-of-charge of the batteries. Although statistics about the batteries and individual cells can be read through bluetooth, the state-of-charge estimate of the individual BMSses (Battery Management System) isn't reliable enough.

I want to acknowledge that the wiring of my setup isn't neat or pretty, but I've done my best to keep everything safe to the best of my knowledge.

Since the last blog post about my solar setup, the following changes have been made:

Added a second MPPT solar charge controller, one for each 370W panel
Added a grid-connected inverter to keep my house energy meter at zero
I'm reading P1 energy data in real-time from my energy meter via P1
Switched from a fixed to a dynamic energy tariff for my grid connection
Added a second 280Ah LFP battery (~3.3kWh for a total of ~6kWh)
Added two 12V 25A battery chargers to speed up charging (1 visible)
Use zigbee to enable / disable the auxiliary chargers
Bought proper battery cases for the cells and BMS
Added 1W temperature sensors

These changes will be discussed in this blogpost.

Extra MPPT Charge Controller

Initially, I had the two solar panels connected to a single solar controller (MPPT 100/30) in parallel, but the output of this MPPT is limited to 30A x 12V = 360W. By adding a second solar controller - one for each panel - 576W peak power was reached in June. Adding the second MPPT clearly removed a bottleneck.

The new grid-connected inverter

By far the most important change is the new grid-connected inverter. My original Victron Phoenix inverter is intended for off-grid usage only and can't be connected to the grid, so I had to replace it.

I bought the Victron Multiplus II 12/1200/50 model. This is a 12 Volt model with a max output of 1kW and can charge with 0.6kW.

The Multiplus inverter can use the grid to charge the batteries, but can also inject power back onto the grid. In other words: if your home is using 200W and the inverter injects 200W onto the grid, the utility energy meter reads zero. We often talk about 'keeping the meter at zero'.

Victron⁵ does sell a complete system to manage and control your solar-battery setup, but I wanted to build the energy management system myself, as a hobby project.

The main goal for this new inverter is to charge the batteries when energy is cheap, and use the energy from the batteries and inject the energy back into the grid when energy is more expensive. People may be confused by the idea of 'cheap' or 'more expensive' kWh prices, as many will pay a fixed flat kWh rate.

However, since a couple of years it's possible in The Netherlands (and elsewhere) to get a 'dynamic' electricity contract, with fluctuating kWh prices.

About dynamic energy tariffs (or day-ahead tariffs)

When you have an electricity contract with an energy provider that supports dynamic pricing, you don't pay a flat fee per kWh. Instead, the day is chopped up in 15 minute segments, and each 15-minute segment has its own kWh price.

Around early afternoon (14:00), you'll receive the quarter-hour prices for the next day, so you can anticipate when to run large consumers like a washing machine, dishwasher or anything else that uses a lot of energy (a car perhaps?).

Grafana dashboard showing dynamic energy prices per quarter-hour

By itself fluctuating kWh prices may sound like a nightmare: why would you ever want to switch to such a provider, where prices fluctuate every 15 minutes?

Fortunately, electricity prices aren't that erratic. The early mornings and early evenings are more expensive, the 10:00-17:00 time window is often very cheap due to (an excess of) solar energy. And sometimes the entire night can be cheap due to tons of wind power being available⁶. In addition, you can almost always count on low kWh prices in weekends, as many businesses and organizations are closed.

As the dynamic price data is available through an API, you can setup automation to take advantage of both cheap and expensive hours. You may decide to charge the batteries when a cheap rate is active, and inject the energy from the batteries back into the grid when energy prices are high, to keep the meter at zero.

My dishwasher and washing machine I can't control through software. But I can look at my day-ahead pricing dashboard and plan when to run these devices, which isn't an inconvenience and works well. It takes almost no effort to change my habits, save some money and be more gentle on the environment.

Some people have solar setups that produce much more energy than they consume on a daily basis. They decide to trade: to discharge their batteries as fast as possible when energy is expensive and make sure the batteries are as empty as possible for the next day, to capture as much solar yield as possible from their panels. I myself are only aiming to reduce my carbon footprint and I'm not trading.

Round-trip efficiency

Charging power from the grid into the batteries is around 90% efficient. Turning battery power back into grid power is also around 90% efficient. With some additional losses, it's best to use a 75% round-trip efficiency as a rule of thumb. The price difference between low and high prices must be substantial enough to offset the round-trip efficiency losses.

About inverter sizing

It's true that my 1000W inverter can't power large appliances like washing machines or dishwashers⁷. Because I run those appliances on grid power when energy is cheap, the inverter never has to power them (or compensate their energy usage).

The energy usage of my home hovers between maybe 70W (idle) to 400W tops when the fridge runs and my (lab) servers are running. Most of the day, the load never surpasses 300W and that's the kind of load I mostly target with this inverter.

I sized my inverter large enough that with most common loads (<300W) the inverter operates silently without it's fan spinning (The fan is really loud).

The batteries

I started out with 4 x 230Ah prismatic LFP cells priced around 500 Euro including taxes, thus around 125 euro a piece. Later I bought 4 x 280 Ah cells for a total of 223 Euro including taxes, thus 55 euro a piece. More capacity for less then half the price: amazing.

These battery cases fit both 230Ah and 280Ah cells

Only recently I've noticed that these very good deals have dried up, probably due to recent global events, but prices are still much lower than when I initially bought my 230Ah cells.

I bought these gray battery cases from a vendor on Ali-express (no affiliate) and I can recommend them. I paid around 63 euros a piece including shipping (for a pair).

A 280Ah prismatic LFP cell

Battery Management Systems (BMS)

The BMS⁸ is a critical safety device that prevents individual cells from going out of spec in terms of voltage and current. This prevents damage to the cells or can even prevent a fire (or explosion).

The 230 Ah battery is using an older Daly BMS. The 280 Ah battery is using a JK BMS. My software communicates with both batteries through Bluetooth to keep track of individual cell voltages.

The Daly BMS is terrible at balancing the cells, so I've added a separate balancer device that start balancing cells when they reach 3.45V. The JK BMS has an on-board 2A balancer and that's enough to keep the 280Ah cells balanced.

Technical setup of my solar management system

This blog is running on a Raspberry Pi 4. This Pi is also running my software (Python) that controls my solar setup. It runs Zigbee2MQTT with a Zigbee dongle to remote control a switch to enable/disable (extra) 12V chargers.

The Victron gear is mostly connected through ve.direct, which is a serial protocol (using USB-to-serial cables). Device data is read and submitted to an InfluxDB server, and Grafana is used to turn the data into dashboards.

The actual 'dashboard' located in my living room is a portable Raspberry Pi 15 inch monitor, hooked up to a Raspberry Pi 4 that runs a Linux desktop in kiosk mode, just to start a browser and show a playlist of various Grafana dashboards. The Pi monitor isn't power hungry but I'm using an Ikea Zigbee motion sensor to turn off the display when there's nobody in the room.

I've added several DS18B20 temperature sensors connected to GPIO pins of the Raspberry Pi. These temperature sensors are digital and use the 1-wire protocol. The temperature data is used as part of a safety mechanism in the software. If one of the sensors crosses the 65C mark, the system stops charging or discharging.

Controlling the solar-battery system

I've written software⁹ to extract relevant data from devices and API endpoints. Based on the input data, the system decides to charge, discharge or 'do nothing'.

data goes in and a decision to charge, discharge or stay idle comes out

The actual decision making can become complex very quickly because of all the variables involved. For example, tomorrow's prices may be low, but I have to keep the battery charged enough to get the apartment through the night (when prices are high). Although it would be more efficient to charge tomorrow, I do have to calculate how many hours to charge today as not to prematurely deplete the batteries.

There are so many different inputs that finding the optimal solution will be very difficult. The decision making can be captured with regular if-then-else statements - and that's what I'm currently doing - but it will probably never be optimal or efficient.

Some examples that can be used as input to create a charge / discharge plan:

Average base load of the home (expected power usage)
Expected solar yield
Expected electricity prices based on wind forecast
Expected temperature impacting heat pump usage
Expected temperature impacting air conditioning
Next day energy prices
Current battery state-of-charge.
State of Charge of EV

The optimization problem at hand seems akin to the knapsack problem. This kind of problem is solved with constrains programming. Google has a tool called OR-Tools that does exactly this.

I'm currently only aware of a Home Assistant plugin called "Day Ahead Optimizer" that uses the python-mip package (for Mixed-Integer Linear programs(MIPs)) to calculate the optimal charging and discharging strategy. This software actually integrates into a solar-battery management system, managing charging and discharging behavior. Maybe there are other solutions out there as well.

Keeping the energy meter at zero

To keep the energy meter at zero, the software must know how much energy is being consumed in the first place. Fortunately, Dutch smart grid energy meters all have a P1 port (Dutch), a standard serial interface that spits out energy usage data (and more) every second.

Reading and processing P1 data

I'm using a Raspberry Pi and read this data from the energy meter and send it to an MQTT server. My solar control software (as discussed previously) is reading the relevant MQTT topic and uses this P1 data to control the inverter, injecting more or less power, depending on current electricity usage.

When charging is required, the chargers are enabled by sending a MQTT message that gets picked up by Zigbee2MQTT and send to a Zigbee switch.

Communication with the Victron Multiplus requires a Victron MKIII-USB device as the Multiplus inverters talk a non-standard (but open, documented) protocol called VE.Bus.

Energy management software controlling chargers and inverter

From a safety perspective, I found it interesting that the Multiplus inverter requires a message every few seconds or it will stop whatever it is doing (charging or discharging) and become idle. This is a nice example of a fail-safe mechanism.

Extra 12V chargers

In addition to the 50A charge capacity of the Multiplus, I've added two extra 12V, 25A chargers to the system to speed up charging. The total charging current is 100A on paper, or about 1kW of power. With 6kWh of capacity, it takes around 6+ hours to fully charge the batteries.

I'm using the old Atlas charger/inverter as shown in the picture. This older model can only charge from the grid and not discharge into the grid.

I'm also using a Victron Blue Smart Charger that can also charge with 25A.

Both chargers are connected to a Zigbee 230V switch and they are enabled/disabled by software.

I do think that this approach with multiple chargers to speed up charging is not very efficient as each separate charger has their own charging losses. It would be better to have one single, beefy charger but those are expensive.

A 12 Volt system is not a great choice

As a rule of thumb, current is 'bad' because current times resistance results in heat losses. To keep resistance low, copper cables with less resistance are required, thus cables with a larger diameter are necessary. Such cables can become expensive quickly.

A 12V system needs to output 10A for a 120W load. A 24V system only needs 5A and a 48V system only 2.5A. 24V and 48V can therefore be build with much thinner cables and 24V and 48V equipment is much cheaper for a certain output than 12V equipment.

If possible, I would always try and use 48V or at least 24V for a solar-battery setup.

My solar setup as a UPS (Uninterruptible power supply)

As a nice bonus, the Multiplus inverter has a secondary output (AC OUT) that is battery protected. By default the AC OUT port is connected to the grid. In case of a power failure, relays within the inverter switch within 20ms to the battery, thus all (computer) equipment will keep running.

It's quite easy to grab a power extention cord and power my fridge from the batteries during a power outage.

Although not very cheap, it's quite easy to build your own LFP-powered UPS using an inverter/charger and an LFP battery. It's probably a better solution than most lead-acid battery based UPS devices where you often discover the batteries are shot during an actual power outage.

Some people are motivated by grid outages to put their entire home behind Multiplus inverters, so they can run 'off-grid' during a (prolonged) power outage.

Closing words

My particular hobby solar setup won't recuperate it's cost, but that's ok. I have an intrinsic afinity with renewables and I want to be mindful about my own carbon footprint. Working with solar, batteries and control software has been a ton of fun and I have a working system that makes me happy.

And maybe, just maybe, somebody else reading my blogposts about my solar setup will be inspired to start a solar-battery project of their own.

Take a look at Will Prowse's YT channel and The Offgrid Garage YT channel.

Also often used in UPSses (picture) ↩
I started with just a single 370W panel, but added another panel soon after as it still wasn't enough to power the Pi on cloudy days, especially during the winter. This is not because solar doesn't work, it's purely because of the sub-optimal orientation of the balcony. ↩
The 20x4 LCD screen was already an upgrade from a 20x2 LCD display 😅 ↩
The 'idle' power consumption of my home is around 120W on average. This idle power usage does include a fridge, a few Raspberry Pis, WiFi access points and four network switches. My NAS or other servers are turned off by default and only used when required. During the daytime, my computer setup, a Mac mini (M2) with two 1440p monitors requires around ~50-60W. With my fridge running, and my computer setup turned on, I observe a usage of around 235W. I only have my regular ventilation system running (70W) after a shower or cooking. I have no air conditioning and heating is (still) based on gas. ↩
I have no relationship with Victron and I'm not sponsored. All equipment is paid for with my own money. ↩
In April 2026, we had a few days with negative prices. Prices were so negative that even the energy tax and VAT was compensated with room to spare. This meant that I got paid to use electricity. It's obvious why the market tries to avoid this scenario, but it did happen. ↩
The heating elements use around 2.2kW, otherwise, the inverter is perfectly capable of powering the motors / pumps. ↩
In practice I believe the BMS should never have to intervene (except for balancing) as the parameters for the energy management system should keep the system in check. If the BMS has to intervene (stop (dis)charging), something might be misconfigured. ↩
This software is not publicly available because it's not generic, it's written for my specific setup. Also, I think the quality is not good enough for public release. ↩

My solar-powered blog is now on Lithium Iron Phosphate

2023-05-19T12:00:00+02:00

In my last blog post I discussed how a small solar project - to power this blog on a Raspberry Pi - escalated into a full-blown off-grid solar setup, large enough to power the computer I use at the moment to write this update¹. In this update, I want to discuss my battery upgrade.

For me, the huge lead acid battery (as pictured below) was always a relatively cheap temporary solution.

A 12 Volt 230 Ah lead-acid battery

Lead-acid batteries are not ideal for solar setups for multiple reasons, but the most problematic issue is the slow charging speed. I only have a few hours of direct sunlight per day due to my particular situation and the battery just could not absorb sunlight fast enough.

For the last 5-7 years, the go-to battery chemistry for solar is LiFePO4 or lithium iron phosphate as a replacement for lead-acid batteries. This battery chemistry is not as energy-dense as Lithium-ion, but the upside is price and safety. In particular, LiFePO4 cells aren't as volatile as Lithium-ion cells. They may start outgassing, but they don't start a fire.

More importantly for my situation: LiFePO4 batteries can charge and discharge at much higer rates than lead-acid batteries². It's possible to charge LiFePO4 cells with a C-rate of 1! This means that if a battery is rated for 100Ah (Ampere-hours) you can charge with a current of 100 Ampere! My solar setup will never come even close to that number, but at least it's good to have some headroom.

A single 3.2 volt 230Ah Lithium Iron Phosphate prismatic cell

I did contemplate buying an off-the-shelf battery but I decided against it. You have no control over the brand and quality of the LiFePO4 cells they use and more importantly, what's the fun in that anyway?

So I decided to order my own cells and build my own 12 Volt LiFePO4 battery consisting of four cells in series (4S) as my existing system is also based on 12 Volt. Other common configurations are 8S (24 Volt) and 16S (48 Volt³).

It turned out that I could just buy my cells locally in The Netherlands (instead of China) because of a company that specializes in batteries (no affiliate). As the price was right, I bought effectively 3 KWh for just shy of 500 Euros.

I decided to buy B-grade cells as those are cheaper than A(utomotive)-grade cells. I might have gone for A-grade cells as not to risk anything if I would build a more serious battery bank for my whole home. Yet a lot of people report no significant differences between A-grade and B-grade LiFePO4 cells for solar battery banks so in the end, it's all about your particular apetite for risk.

Just buying cells and putting them in series (in my case 4S) is not enough, a BMS or battery management system is needed, which you put in series with the battery on the negative terminal. I ordered a 100A Daly BMS from China which works fine. I'm even able to use Python to talk with the Daly BMS over bluetooth to extract data (voltages, current, State of Charge and so on).

The BMS is critical because it protects the cells against deep discharge and overcharging. In addition, the BMS tries to keep the voltage of the cells as equal as possible, which is called 'balancing'. Charging stops entirely when just one of the cells reach their maximum voltage. If other cells have a much lower voltage, it means that they can still be charged but the one cell with the high voltage is blocking them from doing so. That's why cell balancing is critical if you want to use as much of the capacity as possible.

The Daly BMS is quite bad at cell balancing so I've ordered a separate cell balancer for $18 to improve cell balancing (yet to be installed).

Ikea sells a Kuggis 32cmx32cmx32cm storage box that seems to be perfect for my small battery. As it has two holes on the sides, I just routed the positive and negative cables through them.

Now that I've put this battery in place I've seen a huge improvement regarding solar charge performance.

I've actually potentially created a new problem: my solar charge controller can only handle about 400 Watts of solar power at 12V and my setup is quite close to reaching this output. I may have undersized my solar charge controller and it has come back to bite me. For now, I'm going to just observe: if that peak of 400 Watts is only reached for a brief time - as it is right now - I don't think I'm going to upgrade my solar charge controller as that would not be worth it.

As we are still in May, my best yield is 1.2 KWh per day. Although that's paltry as compared to regular residential solar setups, that 1.2 KWh is more than a third of my battery capacity and can run my computer setup for 10 hours, so for me it's good enough.

It's funny to me that all of this started out with just a 60 Watt solar panel, a 20 Euro solar charge controller (non MPPT) and a few 12V 7Ah lead acid gel batteries in parallel.

I think it's beyond amazing that you can now build a 15KWh battery bank complete with BMS for less than €3000. For that amount of money, you can't come even close to this kind of capacity.

For context, it's also good to know that the longevity of LiFePO4 cells is amazing. A-grade cells are rated for 6000 cycles ( 16+ years at one cycle per day ) and my vendor rated B-grade cells at 4000 cycles (~11 years).

Maybe my battery build may inspire you to explore building your own battery. LiFePO4 cells come in a whole range of capacities, I've seen small 22Ah cells or huge 304Ah cells so you can select something that fits your need and budget.

If you're looking for more information: there are quite a few Youtubers that specialise in building large battery banks (48 Volt, 300Ah, ~15KWh) to power their homes and garages.

Although Will Prowse reviewed LiFePO4 cells in the past, he currently focusses mostly on off-the-shelf products, like "rack-mount" batteries and inverter/chargers.

I also like the off-grid-garage channel a lot, the channel as tested and explored quite a few products.

Harrold Halewijn (Dutch) also has quite a few videos about solar setups in general and solar battery setups. He's really into automation, in combination with flexible (next-day) energy prices.

Also in Dutch, a cool article about some people building their own large-scale home storage batteries (15KWh+)

Another Dutch person build a solar power factory with a battery capacity of 128 KWh for professional energy production. Truely amazing.

The Hacker News thread about this article.

https://news.ycombinator.com/item?id=35596959#35597492 ↩
To fully charge a lead-acid battery, the charging process spends a lot of time in the contstant-voltage phase, the voltage is kept constant so as the battery charges further, the charging current goes down, so the charge process slows down. More info can be found here ↩
It seems to me that most batteries build for home energy storage systems are standardising on 48 volt. ↩

I made my blog solar-powered, then things escalated

2023-04-17T12:00:00+02:00

In 2020 I wondered if I could run my blog on solar power, being inspired by Low-tech Magazine, doing the same thing (but better)¹. The answer was 'yes', but only through spring and summer.

I live in an apartment complex in The Netherlands and my balcony is facing west. This means it only receives direct sunlight from 16:00 onward during spring and summer. Most of the time, the panels only get indirect sunlight and therefore generate just a tiny fraction of their rated performance. The key issue is not solar, but the west-facing balcony (it should ideally be facing south).

original solar panel

It's fair to say that my experiment isn't rational because of the sub-optimal solar conditions. Yet, I'm unreasonably obsessed by solar power and I wanted to make it work, even if it didn't make sense from an economic or environmental perspective².

When I wrote my blog about my solar-powered setup, I was already on my second iteration: I started out with just a 60 Watt panel and a cheap $20 solar controller³. That didn't even come close to being sufficient, so I upgraded the solar controller and bought a second panel rated for 150 Watt, which is pictured above. With the 60 Watt and 150 Watt panels in parallel, it was still not enough to keep the batteries charged in the fall and winter, due to the west-facing balcony.

A Raspberry Pi 4B+ consumes around ~3.5 Watt of power continuously. Although that sounds like a very light load, if you run it for 24 hours, it's equivalent to using 84 Watts continuously for one hour. That's like running two 40 Watt fans for one hour, it's not insignificant and it doesn't even account for battery charging losses.

So 210 Watt of solar (receiving mostly indirect sunlight) still could not power my Raspberry Pi through the winter under my circumstances. Yet, in the summer, I had plenty of power available and had no problems charging my iPad and other devices.

As my solar setup could not keep the batteries charged from October onward, I decided to do something radical. I bought a 370 Watt⁴ solar panel (1690 x 1029 mm) and build a frame made of aluminium tubing⁵. Solar panels have become so cheap that the aluminium frame is more expensive than the panel.

Even this 370 Watt panel was not enough during the gloomy, cloudy winter days. So I bought a second panel and build a second tube frame. Only with a 740 Watt rated solar panel setup was I able to power my Raspberry Pi through the winter⁶.

I didn't create this over-powered setup just to power the Raspberry Pi during the winter. I knew that solar performed much better during spring and summer and I wanted to capture as much of that energy as possible. The real goal was to go beyond powering the Pi and power my computer desk, which includes an Intel Mac Mini, two 1440p 27" displays and some other components (using around 100 Watt on average)⁷.

I would not be able to power my desk 24/7 but I would be happy if I can work on solar power for a few hours every other day during spring and summer. I also wanted to light my house in the evening using this setup.

The original solar setup was enough to power the Raspberry Pi and charge an iPad in the spring/summer. The solar charge controller could not handle the increased solar capacity and needed replacement. So I decided to build a new setup inspired by Will Prowse solar demo setups, which is pictured below:

the latest iteration of my solar setup

First a brief disclaimer: I'm a hobbyist, not an expert (if you didn't notice already). I have no background in electrical systems. I've tried to make my setup safe, but I may have done things that are not recommended.

My setup is a 12-volt system⁸. The drawback of a 12-volt system is the relatively large currents required to charge the battery and power the inverter. This requires thicker, more expensive cabling to prevent energy losses in the cabling⁹.

Most components are self-explanatory, except for the shunt. This device precisely measures battery voltage and how much current is going in and out of the battery. The solar charge controller and the shunt are linked together in a bluetooth network, so the solar controller uses the precise voltage and current information from the shunt to regulate the battery charging process.

The solar controller, inverter and shunt have VE.direct interfaces (Victron-specific) which I use to collect data. I'm using a Python VE.direct module to gather this data, which just works without any issues. My Python script dumps the data into InfluxDB and I use Grafana for graphs (see below). The script also updates the 'solar status' bar to the right (or bottom for mobile users).

this image is updated periodically

The LCD display is just for fun, and mostly to keep an eye on the battery charge state.

the 20x4 LCD screen

The LCD screen is managed by the same python script that dumps the VE.direct data into Grafana. I focus on two metrics in particular. First of all the daily solar yield as a percentage: 100% means the load has been compensated by solar and anything higher means an energy 'profit'. In the bottom right we see the charger status (Bulk): if it's on 'Float' the battery is full. I tend to wait for the battery to recharge to a 'float' status before I use the inverter again.

Let's talk about the battery. I've chosen to use a large used lead-acid battery even though Lithium (LiFePO4) batteries beat lead-acid in every metric.

Update May 2023: I have since upgraded to Lithium Iron Phosphate, see this blogpost for more information.

A 12 Volt 230 Ah lead-acid battery¹⁰

I bought the battery¹¹ second-hand for €100 so that's not a significant investment for a battery. Although it's a bit worn-down and the capacity is reduced, it is still good enough for me to run my computer setup for 10 hours after a full charge¹². In practice, I won't use the battery for more than four to five hours at a time because recharging can take multiple days and lead-acid batteries should ideally be fully recharged within 24 hours or their aging is accelerated.

The lead-acid battery also serves another purpose: is a relatively cheap option for me to validate my setup. If it works as intended, I might opt to upgrade to lithium (LiFePO4) at some point.

Until recently, switching between the grid and solar for my computer setup was quite cumbersome. I had to power down all equipment, connect to the inverter and power everything up again. That got old very quickly. Fortunately, I stumbled on an advertisement for a Victron Filax 2 and it turns out that it does exactly what I need.

The Filax 2 switches between two 230 Volt input sources without any interruption, like a UPS (Uninterruptible power supply). Now that I've installed this device, I can switch between solar and grid power without any interruption. Brand new, The Filax 2 costs €350 which was beyond what I wanted to spend, but the second-hand price was acceptable.

My solar setup is not something that I can just turn on and forget about. I have to keep an eye on the battery, especially because lead-acid should ideally be recharged within 24 hours.

Happy case: the battery is full and my computer desk is 100% solar-powered

It's now April 2023 and my setup seems promising. Peak output of the two 370 Watt solar panels facing west was 230 Watts. Only for a very short period, but it makes me confident for spring and summer. I could automate enabling and disabling the inverter, with a relay and some logic in Python, but for now I'm good with manually operating the inverter.

You may have noticed that I've used a lot of Victron equipment¹³. Mostly because it seems high-quality and the data interfaces are documented and easy-to-use. The inverter was also chosen because of the low parasitic load (self-consumption) of around 6 watt. Victron equipment is not cheap. Buying Victron gear second-hand can save a lot of money.

Speaking of cost, if I include all the cost I made, including previous solar projects and mistakes, I think I spend around €2000.

That's all I have to say about my hobby solar project for now.

Link to Hacker News thread about this article.

Their attempt was quite serious and precise. They accounted for the energy used to produce the equipment. They went as far as dithering images to reduce bandwidth and thus energy usage. ↩
The cost can never be reclaimed by the electricity savings. Also, the energy produced to make all the components would never be recovered due to my west-facing setup. ↩
never buy those cheap non-MPPT solar charge controllers unless you really know what you are doing. You are better off with a MPPT controller which is much better at getting the most energy out of a solar panel. ↩
Jinko half cut 120 cell 370 WP JKM370N-6TL3-B ↩
I have absolutely no experience with designing and building aluminium tube frames. After you're done laughing at this contraption, if you have a better, more efficient design, I'm still interested. ↩
I may have cheated once by recharging the battery from the grid just to protect it against accelerated aging due to being in a prolonged (partially) discharged state. ↩
Suddenly you realise that making the background on all monitors black saves ~20 Watt. My blog should be dark-themed to reduce energy usage 😅 ↩
If you ever intend to build some kind of solar setup yourself, consider a 24 Volt or ideally an 48 Volt system to reduce currents and thus save on cabling cost. ↩
I've actually oversized the battery cabling for safety reasons. If a length of cable isn't rated for the current flowing through it, it becomes a resistor, generating heat, which can cause a fire so I want to be carefull. ↩
The + and - pole are temporary uncovered for this picture, but normally they are covered to prevent a short-circuit if anything would fall on the poles. ↩
A sealed lead-acid battery like the one I'm using is safe and won't release any (explosive) gasses unless overcharged or abused. ↩
The inverter uses a dynamic load algorithm to prevent deep discharge of the battery. Ideally a lead-acid battery should never be discharged beyond 50% of capacity and it seems to work perfectly. Dumb inverters, just discharge until 10.5 volt under load, which means the battery is almost depleted, causing rapid aging and significantly reduced life-span. ↩
No, I'm not sponsored by Victron, I wish 😅💸 ↩

A practical understanding of lead acid batteries

2021-08-29T12:00:00+02:00

Introduction

The goal of this article is to give you a practical understanding of Lead Acid batteries. We won't address the underlying chemistry, we'll treat them as a black-box and we will discover their characteristics and how to keep them healthy.

source

Disclaimer

I'm an amateur. I have absolutely zero relevant background in battery technology or electronics. I just scraped some information together in a hopefully useful manner.

A high-level overview of the lead acid battery

It can provide a ton of current / power
It hates to be deep-discharged and will die quickly if done repeatedly
It hates being in a discharged state
Only use 50% of total capacity if longevity matters (ideally only 30%)
It's usable capacity depends on the load
They are slow to charge (8-12 hours)
They don't perform as well in cold weather

Lead acid batteries can provide a lot of current

Lead acid batteries can put out so much current that you can use them to weld¹. They are widely used in ICE cars to power the starter motor, which needs hundreds of amps at 12 volt to turn over the engine.

They are also used to power mobility scooters, golf carts, trolly motors, small toy cars for children to ride in, or provide electricity on boats, caravans and in RVs. You can also find them in more stationary applications such in UPS systems² or - of course - solar battery banks.

Danger

Lead acid batteries typically don't have any kind of short-circuit protection build-in. This means that if you (accidentally) short-circuit a lead acid battery, the battery can explode or it can cause a fire. Whatever object caused the short-circuit, will probably be destroyed.

Because lead acid batteries can supply such high currents, it's important to assure that you use the right wire thickness / diameter. If the wire is too thin, it causes too much resistance and thus may overheat, causing the insulation to catch fire.

Lead acid batteries can be very dangerous, so you have to be very carefull with them. Personally, I always make sure that anything connected to a lead acid battery is properly fused.

Lead acid batteries hate being deep discharged

The common rule of thumb is that a lead acid battery should not be discharged below 50% of capacity, or ideally not beyond 70% of capacity. This is because lead acid batteries age / wear out faster if you deep discharge them.

The most important lesson here is this:

Although a lead acid battery may have a stated capacity of 100Ah, it's practical usable capacity is only 50Ah or even just 30Ah

If you buy a lead acid battery for a particular application, you probably expect a certain lifetime from it, probably in years. If the battery won't last this long, it may not be an economically viable solution.

image source - Please note that this chart is based on a heavy-duty lead acid battery and doesn't reflect the lifecycle of a regular consumer lead acid battery. It is advised to look up the relevant chart for the particular battery model you may be interested in buying.

If you cycle a battery (with the characteristics depicted in the chart) every day as part of some kind of off-grid solar setup and you use 80% of it's capacity, you'll probably have to replace it after about two years.

If you add a few extra batteries in parallel, individual batteries may only be used 20% to 30% of capacity, and those same batteries may last 6 - 9 years. So by spending 2 or 3 times the money on batteries, you get 3 to 4 times the lifetime out of your setup.

So, for example, if you really need 100Ah of battery capacity, you may need two 100Ah batteries in parallel to assure longevity. You even may decide to buy three 100Ah batteries just to assure that they will last for the desired number of cycles.

However, if the battery setup is only meant for emergency power and thus only expected to operate a few times a year, discharging a lead acid battery to 80% of capacity is not a big deal. There is no need to add extra battery capacity because the number of charge/discharge cycles is so low that there isn't that much wear on the battery.

Lead acid batteries eventually die from old age

A lead acid battery deteriorates just by ageing. So even if it's kept full charged most of the time, it will wear out and needs to be replaced after a few years. It doesn't matter how well you treat them, even with the best care, they need to be replaced eventually.

Lead acid batteries hate being in a discharged state

Lead acid batteries should never stay discharged for a long time, ideally not longer than a day. It's best to immediately charge a lead acid battery after a (partial) discharge to keep them from quickly deteriorating.

A battery that is in a discharged state for a long time (many months) will probably never recover or ever be usable again even if it was new and/or hasn't been used much.

Usable capacity depends on the load

A typical 12-volt battery has a rating stated in ampere hour that tells you the capacity. For example, a battery can be rated as 70Ah.

So this could mean that the battery can sustain a load of 7A for 10 hours or 70A for one hour, right?

Unfortunately, no

It turns out that the usable capacity of a lead acid battery depends on the applied load. Therefore, the stated capacity is actually the capacity at a certain load that would deplete the battery in 20 hours.

This is concept of the C-rate. 1C is the theoretical one hour discharge rate based on the capacity. Batteries are mostly sold with a capacity based on a 0.05C discharge rate for 20 hours.

The C-rate is important because the C-rate is related to the usable capacity of a battery. That 70Ah capacity rating is based on a 0.05 C-rate or 20-hour discharge rate. That would be 70Ah / 20 = 3.5A.

This is important to understand: if you would put a higher load on this battery, the usable capacity will be less than 70Ah. For example, with a 7A load, the usable capacity may only be 64Ah (fake number for illustration purposes).

It also works in your favor: if the load is less than the 0.05 C-rate, the actual usable capacity will be higher!

So why is this?

When you put a load on a battery, the voltage drops a bit. Higher loads cause larger voltage drops, or to put it differently: the battery 'struggles' to maintain voltage.

Image source

So if a load exceeds the standard 0.05C rate (C/20), you may have to select a higher capacity battery or accept a shorter run-time than you might expect based on the rated capacity on the label.

You even may consider putting multiple batteries in parallel to reach the desired usable capacity / runtime.

WARNING

The chart about the state-of-charge under load shows that you should keep an eye on the actual load and voltage. With a C/20 load, the battery is at 50% at 12.30 volt³.

A C/5 load on a 70Ah battery would be 14A. At that load, the battery is at 50% capacity at ~11.55 Volt under load. Only the load in combination with the voltage may give an indication of actual state-of-charge.

Predicting state-of-charge under load is doable with a static, constant load, but becomes more difficult when the load fluctuates, so take this into account.

ANOTHER WARNING

Different manufacturers produce different batteries that may have different discharge characteristics. This means that you should look up the battery specifications and hopefully find a discharge rate chart that will help you gauge actual capacity under load for this particular model.

How do you know the state of charge of a lead acid battery?

The state of charge is measured at rest: when the battery is not connected to any load or charger for 24 hours. The voltage will reflect the state of charge (SoC).

WARNING

There are many different, conflicting tables to be found on the internet that correlate voltage with a particular state of charge. Be sure you check that you pick the right one, consult the footnote⁴ for more information.

State of Charge (SoC)	Voltage at rest (24h)
100%	12.70+
75%	12.40
50%	12.20
25%	12.00
0%	11.80

Please note that this table is only valid at an ambient temperature of 25C / 77F. If the temperature is lower, usable capacity diminishes and the voltages at wich a certain SoC is reached, will be higher.

Furthermore, these numbers can deviate a little bit depending on the kind of lead acid battery.

If you measure the voltage under load - for example, when you power some lights - the voltage does not reflect the actual state of charge.

It is quite difficult to determine the state of charge under load. Sometimes, battery manufactures provide a discharge chart that allows you to determine the state-of-charge based on the current load.

But often it is something you have to measure or figure out yourself. A constant load makes estimating battery capacity under load more predictable, but if the load varies, it is more difficult to accurately gauge the state of charge.

The positive impact on capacity of connecting batteries in parallel

By using multiple batteries in parallel, the load is also shared across all batteries. Each individual battery only has to supply a fraction of the total load. This means that in addition to the extra usable capacity of the added batteries, there is also added usable capacity because of the reduced load on each individual battery.

For example, if a 100Ah battery has a 0.05C discharge rate of 5A. If it has to provide 10A, the usable capacity is lower than the advertised 100Ah as explained earlier. If we add a second 100A battery in parallel, each battery now needs to supply only half of the load and thus will be able to provide the stated capacity as it is precisely the 0.05C discharge rate.

Lead acid batteries need deep discharge protection

It is highly recommended to use lead acid batteries in combination with a low-voltage cut-off solution that protects the battery against deep discharge⁵.

this article is not sponsored by victron

Ideally you can configure the cut-off coltage, such as with the depicted unit.

So many lead acid batteries are 'murdered' because they are left connected (accidentally) to a power 'drain'.

Charging a lead acid battery

No matter the size, lead acid batteries are relatively slow to charge. It may take around 8 - 12 hours to fully charge a battery from fully depleted. It's not possible to just dump a lot of current into them and charge them quickly. That would just overload and destroy the battery⁶.

Lead acid batteries need a specific 3-stage charge process⁷ in order to preserve their condition.

In practice, if you don't discharge a battery beyond 50%, it takes less time to recharge the battery⁸.

It can be a good idea to hookup unused batteries permanently to a 'tricklecharger'. This is a charger that charges the battery with a maximum current of 0.8A.

As it can take a very long time to charge a larger capacity battery with a tricklecharger, you need a regular charger, that can supply a decent current, to charge a battery 'within a reasonable timeframe'.

Lead acid battery types

Flooded / FLA

This is the well-known older type of battery. It may be necessary to add distilled water from time to time, so they require maintenance.

The key problem with batteries that require maintenance is that most people (consumers) don't know and if they know, they forget. These batteries basically don't match well with 'human nature'.

It seems to me that these batteries are on their way out in the consumer space, but are still prevalent in commercial/industrial application. It's probably easy for a business to just have a trained employee or service company periodically maintain the batteries.

EFB or Enhanced Flooded Battery

These batteries are improved versions of the regular flooded battery. They are more expensive, but will last more charge/discharge cycles, especially with deeper discharges.

Although not as performant as AGM batteries (which will be discussed shortly), they provide a cheaper alternative to AGM batteries.

Sealed Lead Acid

This type of battery is fully sealed. SLA batteries essentially the same as VRLA batteries but this name is used for the smaller capacity batteries, as found in motorcycles, uninterruptible power supplies and such.

These are maintenance-free batteries. They never require any maintenance during their lifetime. You don't need to add distilled water or anything during their lifetime.

Valve-Regulated Lead Acid

This name is used for batteries like the SLA battery, but with higher capacities. See also wikipedia. They have liquid inside like the flooded battery, but they are sealed and don't need any maintenance. To be precise: they can't be maintained, only be replaced.

The 'valve(s)' are only there in case of emergency, to release pressure due to gas buildup within the battery case if charged incorrectly.

AGM (Absorbent Glass Mat)

This is also a fully sealed SLA/VRLA battery, but it is even more advanced. They are better able to withstand deep discharges and can be recharged faster. This comes at a relatively steep price.

The faster recharge cycle can be important if used within a solar power bank, because there are only a limited number of hours when the sun provides enough energy for charging.

Deep-Cycle

These batteries are build differently⁹ and are less suited for starting cars, but better suited to provide power to power boats, RC vans or form a solar power bank.

They are often not a kind of battery in and of itself: there are just regular flooded deep-cycle batteries, or AGM deep-cycle batteries. They are often specifically designed for solar power banks or similar applications.

Evaluation

Although regular flooded batteries will have the longest lifespan of all lead acid battery technology, they require regular maintenance and that may not be practical. Therefore, AGM or other maintenance-free batteries are better suited for residential battery applications, the relatively lower life expectancy is just the price for practicality/convenience.

Low self-discharge rate and storing batteries

Lead acid batteries needs to be stored fully charged. They should be recharged at least every six months due to self-discharge, although the self-discharge rate is rather low.

Buyer beware - ask for fresh batteries

I've ordered quite a few smaller SLA batteries from various brands to test their capacities. I noticed that the actual brand didn't matter much. The age of the battery seemed to matter.

some of the tested SLA batteries

While they are in storage at the vendor, they are probably never recharged, which deteriorates the battery. The batteries with a lower SoC correlated with a serial number that indicated that they were older than the other batteries.

So it might be beneficial to specifically ask for a 'fresh' battery when you order a lead acid battery.

Q & A

Can my lead acid battery be revived?

No.

If the voltage of a 12 volt battery at rest is close to zero, it is dead.

There are tips like 'using epsomsalts' or keeping them on a charger for weeks, but at best, you get only a small portion of usable capacity back, if any. A battery 'revived' like this should never power something you rely on. Personally I don't think it's worth the cost of epsom salt or your time, but you have to decide for yourself if that's true or not.

If a battery is totally dead, I would recommend to accept the loss and get a new one.

The impact of cold weather on performance

If a lead acid battery is exposed to colder or even freezing temperatures, it will work fine, but it can output less current. This is relevant for older, more worn-down batteries. Such batteries can still work fine in the summer, but may no longer be able to start a car or provide another utility with sufficient power when temperatures drop significantly.

Does it make sense to use Lead acid batteries for an off-grid solar setup?

You can do a lead acid solar setup if you can get those batteries cheap but otherwise it may be better to go for a LiFePo4 based setup. Although the initial investment is much higher, Lithium-based batteries will be cheaper long-term because they last so much longer than lead acid batteries (life-time).

I think lead acid batteries are suited for climates with a lot of sunlight available all year round, to power a livingspace through the night.

Since lead acid batteries don't 'like' to be in a discharged state for a long time (more than a day at most), I don't think they are suitable for a more temperate climate, with lots of overcast days.

So the first issue with lead acid batteries is that they don't take well being in a discharged state for more than a day or so. It will make them deteriorate faster.

I think the second issue with lead acid batteries as a solar power bank is their slow charging speed. Lead acid batteries often can't use all available solar power to charge because they just can't charge any faster, no matter their capacity.

This means that even though there would have been enough energy available to fully charge the batteries, it was not available long enough to fully charge the batteries. Maybe AGM batteries may help as they can be charged with higher currents, even though they may not last as long.

Lithium-based batteries can be charged with very large currents and can - in some sense - capture every bit of sunlight that's available. This is much better suited to climates with more intermittent sunny days or even sunny hours, I think.

Another thing that comes to mind is that if you really want to go with lead acid batteries for a solar bank, flooded may be the longest lasting, but the regular maintenance they require may quickly become a chore / unmanageable. I have zero experience with this, but please verify this beforehand. All the more reason to consider at least maintenance-free lead acid batteries, even if they may not last as long.

This is just my thought, I'm no expert on this.

Just remember that regular car batteries are just not suitable for this application. You need - more expensive - batteries that are build specifically for being used in a power bank¹⁰.

Why are lead acid batteries so widely used in cars?

Cars need a power source that can provide a lot of power to run the starter motor. Starter motors can use anywhere from 1.5 to 3 Kilowatt when cranking the engine. That's about 125A to 250A of current at 12 volts.

You may notice that batteries are often rated for much higher CCA or 'Cold Cranking Amps' values, but since they deteriorate over time, that extra margin will come in handy. Especially in colder weather.

Lead acid batteries as used in cars can last many years because they are used under near ideal conditions. They are always kept fully charged and are ony briefly and slightly discharged. They are immediately recharged after the car is started.

How can I check if a battery is healthy ?

You need a battery tester for this. They can be had for around 50 Euro's, which is not far off from just buying a new battery, which you might have to do anyway.

A demonstration video of such a cheap charger.

Just because you can, doesn't mean you should. Don't do it. ↩
A UPS can be quite small, to power just a single computer, running off a 'small' 12 volt 7Ah lead acid battery (depicted further down below in the acticle). A step up in size would be a 19-inch rackmounted UPS, which can often be expanded with multiple external battery packs. A datacenter scale UPS is build using many large batteries in both series for higher voltages and in parallel for higher capacity. Lead acid batteries are well-suited for these type of applications because they are always kept fully charged and rarely (fully) discharged. In datacenter applications, they often only need to last until the diesel generators kick in. ↩
Notice the voltages in the C/20 discharge rate - which should reflect the numbers in the table shown earlier - are actually a bit higher. If you want to be safe, using higher voltages is always safer for battery longevity, but at the cost of usable capacity. ↩
This article goes into more detail about this. Be sure you look at a table that correlates resting voltage against SoC and not the voltage under load. If you see a table with 10.8 volts at 0%, you are looking at a table for under load voltages. A battery at 10.5 - 10.8 volts at rest is probably damaged. A lead acid battery should never be below 11.80 volt at rest. ↩
'bad' battery protection solutions will just start to oscillate as the battery voltage recovers (above the cut-off threshold) when the load is removed. I bought a cheap 20 Euro unit and it was effectively useless because of this problem. ↩
It is critical that a proper battery charger is used. You should never just apply a static current as overcharging the battery may lead to the buildup of flammable gasses like hydrogen. There are many documented cases of car batteries exploding in this way. Not only can you get hurt by debris, the internal liquid is acidic which can cause significant burns and is especially dangerous for the eyes. ↩
https://en.wikipedia.org/wiki/IUoU_battery_charging ↩
If Lithium-based batteries have one big upside over lead acid batteries in energy storage applications, it might be this aspect: they can be charged much faster. It may make sense to oversize the solar power array just to charge the batteries as quickly as possible within the limited number of available 'sun-hours'. ↩
They have ticker plates that are better able to withstand deep discharges at the cost of lower peak current. ↩
I myself do use regular car batteries as part of my solar-powered blog because I got them for free and even if they are shot, they may last for quite a bit. I can also imagine that people would actually build a battery bank made of old car batteries and just ad a whole lot of them, if you have the space. I'm not sure if that kind of setup would be quite reliable. ↩
The car batteries are free, and I had no other use for the gel batteries so I hooked those up too (in parallel). The batteries have wildly different capacities and this is absolutely not recommended. If you hook up batteries in parallel, always use the same capacity. ↩

This blog is now running on solar power

2020-07-06T12:00:00+02:00

Introduction

This blog is now running on solar power.

I've put a solar panel on my balcony, which is connected to a solar charge controller. This device charges an old worn-out car battery and provides power to a Raspberry Pi ~~3b+~~ 4B, which in turn powers this (static) website.

For updates: scroll to the bottom of this article.

Some statistics about the current status of the solar setup is shown in the sidebar to the right. The historical graph below is updated every few minutes (European time).

Low-tech Magazine as inspiration

If you think you've seen a concept like this before, you are right.

The website Low-tech Magazine is the inspiration for my effort. I would really recommend visiting this site because it goes to incredible length to make the site energy-efficient. For example, images are dithered to save on bandwidth!

Low-tech Magazine goes off-line when there isn't enough sunlight and the battery runs out, which can happen after a few days of bad weather.

In January 2020, the site shared some numbers about the sustainability of the solar-powered website.

The build

My build is almost identical to that of Low-tech Magazine in concept, but not nearly as efficient. I've just performed a lift-and-shift of my blog from the cloud to a Raspberry Pi.

I've build my setup based on some parts I already owned, such as the old car battery and the Pi. The solar panel and solar charge controller were purchased new. The LCD display and current/voltage sensor have been recycled from an earlier hobby project.

I've used these parts:

Solar Panel	Monocrystalline 150 Watt 12V
Battery	12 Volt Lead Acid Battery (Exide 63Ah)
Solar Charge Controller	Victron BlueSolar MPPT 75\|10
Voltage/Current sensor	INA260
LCD Display	HD44780 20x4
Computer	Raspberry Pi 4B
Communications cable	VE.Direct to USB interface

The Solar Panel

The panel is extremely over-dimensioned because my balcony is directed towards the west, so it has only a few hours a day of direct sunlight. Furthermore, the angle of the solar panel is sub-optimal.

My main concern will be the winter. It is not unlikely that during the winter, the panel will not be able to generate enough energy to power the Pi and charge the battery for the night.

I have also noticed that under great sunlight conditions, the panel can easily produce 60+ Watt¹ but the battery cannot ingest power that fast.

I'm not sure about the actual brand of the panel, it was the cheapest panel I could find on Amazon for the rated wattage.

The Solar Charger

It's a standard solar charger made by Victron, for small solar setups (to power a shed or mobile home). I've bought the special data cable² so I can get information such as voltage, current and power usage.

The controller uses a documented protocol called ve.direct. I'm using a Python module to obtain the data.

According to the manual, this solar charger will assure that the battery is sufficiently charged and protects against deep discharge or other conditions that could damage the battery.

I feel that this is a very high-quality product. It seems sturdy and the communications port (which even supports a bluetooth dongle) giving you access to the data is really nice.

The controller is ever so slightly under-dimensioned for the solar panel, but since I will never get the theoretical full power of the panel due to the sub-optimal configuration, this should not be an issue.

The battery

In the day and age of Lithium-ion batteries it may be strange to use a Lead Acid battery. The fact is that this battery³ was free and - although too worn down for a car - can still power light loads for a very long time (days). And I could just hook up a few extra batteries to expand capacity (and increase solar energy absorption rates).

To protect against short-circuits, the battery is protected by a fuse. This is critical because car batteries can produce so much current that they can be used for welding. They are dangerous.

If you ever work with lead acid batteries, know this: don't discharge them beyond 50% of capacity, and ideally not beyond 70% of capacity. The deeper the discharge, the lower the life expectancy. A 100% discharge of a lead acid battery will kill it very quickly.

You may understand why Lead Acid batteries aren't that great for solar usage, because you need to buy enough of them to assure you never have to deep discharge them.

Voltage, Current and Power Sensor

I noticed that the load current sensor of the solar charge controller was not very precise, so I added an INA260 based sensor. This sensor uses I2C for communication, just like the LCD display. It measures voltage, current and power in a reasonable presice resolution.

Using the sensor is quite simple (pip3 install adafruit-circuitpython-ina260):

#!/usr/bin/env python3
import board
import adafruit_ina260
i2c = board.I2C()
ina260_L = adafruit_ina260.INA260(i2c,address=64)
print(ina260_L.current)
print(ina260_L.voltage)
print(ina260_L.power)

Please note that this sensor is purely optional, the precision it provides is not really required. I've used this sensor to observe that the voltage and current sensing sensors of the solar charge controller are fairly accurate, except for that of the load, which only measures the current in increments of 100 mAh.

The LCD Display

The display has four lines of twenty characters and uses a HD44780 controller. It's dirt-cheap and uses the I2C bus for communications. By default, the screen is very bright, but I've used a resistor on a header for the backlight to lower the brightness.

I'm using the Python RPLCD library (pip3 install RPLCD) for interfacing with the LCD display.

Using an LCD display in any kind of project is very simple.

#!/usr/bin/env python3
from RPLCD.i2c import CharLCD
lcd = CharLCD('PCF8574', 0x27, cols=20, rows=4)
lcd.clear()
lcd.cursor_pos = (0,0) # (line,column)
lcd.write_string("Hello")

12 volt to 5 Volt conversion

I'm just using a simple car cigarette lighter USB adapter to power the Raspberry Pi. I'm looking at a more power-efficient converter, although I'm not sure how much efficiency I'll be able to gain, if any.

Update: I've replaced the cigarette lighter usb adapter device with a buck converter, which resulted in a very slight reduction in power consumption.

Script to collect data

I've written a small Python script to collect all the data. The data is send to two places:

It is send to Graphite/Grafana for nice charts (serves no real purpose)
It is used to generate the infographic in the sidebar to the right

Because I don't want to wear out the SD card of the Raspberry Pi, the stats as shown in the sidebar to the right is written to a folder that is mounted on tmpfs.

The cloud as backup

When you connect to this site, you connect to a VPS running HAProxy. HAproxy determines if my blog is up and if so, will proxy between you and the Raspberry Pi. If the battery would run out, HAProxy will redirect you an instance of my blog on the same VPS (where it was running for years).

As you may understand, I still have to pay for the cloud VPS and that VPS also uses power. From an economical standpoint and from a ecological standpoint, this project may make little sense.

Possible improvements

VPS on-demand

The obvious flaw in my whole setup is the need for a cloud VPS that is hosting HAProxy and a backup instance of my blog.

A better solution would be to only spawn a cloud VPS on demand, when power is getting low. To move visitors to the VPS, the DNS records should be changed to point to the right IP-address, which could be done with a few API calls.

I could also follow the example of Low-tech Magazine and just accept that my blog would be offline for some time, but I don't like that.

Switching to Lithium-ion

As long as the car battery is still fine, I have no reason to switch to Lithium-ion. I've also purchased a few smaller Lead Acid batteries just to test their real-life capacity, to support projects like these. Once the car battery dies, I can use those to power this project.

The rest of the network is not solar-powered

The switches, router and modem that supply internet access are not solar-powered. Together, these devices use significantly more power, which I cannot support with my solar setup.

I would have to move to a different house to be able to install sufficient solar capacity.

Other applications

During good weather conditions, the solar panel provides way more power than is required to keep the battery charged and run the Raspberry Pi.

I've used the excess energy to charge my mobile devices. Although I think that's fun, if I just forget turning off my lights or amplifier for a few hours, I would already waste most of my solar gains.

I guess it's the tought that counts.

Conclusion

In the end, it it was a fun hobby project for me to realise. I want to thank Low-tech Magazine for the idea, I had a lot of fun creating my (significantly worse) copy of it.

If you have any ideas on how to improve this project, feel free to comment below or email me.

This blog post featured on hacker news and the Pi 3b+ had no problems handling the load.

Updates

Car battery died

After about two weeks the old and worn-down car battery finally died. Even after a whole day of charging, the voltage of the battery dropped to 11.5 Volts in about a minute. It would no longer hold a charge.

I have quite a lot of spare 12 volt 7Ah batteries that I can use as a replacement. I'm now using four of those batteries (older ones) in parallel.

Added wall charger as backup power (October 2020)

As we approached fall, the sun started to set earlier and earlier. The problem with my balcony is that I only have direct sunlight at 16:00 until sunset. My solar panel was therefore unable to keep the batteries charged.

I even added a smaller 60 watt solar panel I used for earlier tests in parallel to gain a few extra watts, but that didn't help much.

It is now at a point where I think it's reasonable to say that the project failed in my particular case. However, I do believe it would still be fine if I could capture the sun during the whole day (if my balcony wasn't in such a bad spot, the solar panel would be able to keep up).

As the batteries were draining I decided to implement a backup power solution, to protect the batteries. It's bad for lead acid batteries to be in a discharged state for a long time.

Therefore, I'm now using a battery charger that is connected to a relais that my software is controlling. If the voltage drops below 12.00 volt, it will start charging the batteries for 24 hours.

Upgraded Raspberry Pi 3b+ to a Raspberry Pi 4B (May 2022)

The idle power usage of the 4B is almost the same as the 3b+ model, although it requires some tweaking to reduce power usage.

The old 3b+ continuously complained about under voltage (detected), but the Pi4 seems to be less picky and it works fine with the XY-3606 power converter (12V to 5V).

the position of the panel is not optimal, so I will never get the panel's full potential. ↩
You don't have to buy the cable supplied by Victron, it's possible to create your own. The cable is not proprietary. ↩
It failed. Please read the update at the bottom of this article. ↩