Louwrentius

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.

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 system⁹. 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 cabling⁸.

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.

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

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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.

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.

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 weld². 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 systems¹ 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 battery⁸.

Lead acid batteries need a specific 3-stage charge process⁶ in order to preserve their condition.

In practice, if you don't discharge a battery beyond 50%, it takes less time to recharge the battery⁷.

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.