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Comment by amatecha | Hacker News Reader

Tip to OP (I think they already have this idea, based on the post): when that lead-acid battery dies, replace it with a LiFePo4, which is substantially more efficient, lasts 3x as long, has a higher energy density, provides a usable voltage far further into its discharging %, etc. etc... A couple pages that describe the differences linked below...

tl;dr if you are starting a solar battery project, LiFePo4 is the clear choice by far, unless you're going for the absolute cheapest possible build due to immediate budget constraints.

https://www.canbat.com/a-detailed-comparison-of-lead-acid-ba...

https://www.power-sonic.com/blog/lithium-vs-lead-acid-batter...

View on HN

I've addressed LiFePo4 in my blog and why I'm not yet running this.

I'm trying to respond to some comments, but since this is not my thread, I'm rate-limited.

> 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. I bought the battery 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 charge. 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."

I think the rate limiting is not based on which thread you're active in, but whether the mods have placed a rate limit on you. It's hard to say since it's not very transparent, but it seems the mods here are pretty liberal about dispensing permanent rate limit penalties.

Thanks, the limit seems removed now. With my own submissions I’m never rate limited.

By far… not so. Just, marginally, as of the last year, perhaps - but the total cost over the lifetime per Wh of storage is still on a par with lead, because lead acid batteries are cheap as chips, require little or no maintenance if you’re sealed, and if you get something like OPzS cells and treat them well, you will get 20 years of life from them. LiFePo4, you’re looking at a decade, maximum, before they’re trash.

I set us up here, totally off grid, three years ago, using a bank of OPzS cells, giving us 42kWh of usable storage. I rarely take them below 70%. Their performance has barely budged since installation, and with the condenser caps, they have required zero maintenance - I check the specific gravity of the acid once a year, and go “yup”, and that’s it.

LiFePo4 I am considering for a booster bank at our cabin, which is 500m from our power shed, as the cable limits us to ~2.5kW here before we see significant voltage drop - it’s either that or step-up/step-down at either end, but transformers are surprisingly expensive. Main reason I’m leaning towards them for this application is mass, as our only access is either by foot or an aerial cableway - our lead bank weighs about 2000kg, equivalent LiFePo4 is about 450kg.

So - LiFePo4 is a decent choice, but depending on your application, a pile of big lead acid cells can be better. See, for instance, storage at various solar projects - it’s virtually all OPzS cells, as the amortised cost is still favourable.

Idk. For a ten year battery in a nightly discharge application you get 0.28 AH for each amp he purchased even with the OPzS cells. First you have to diéntate to .7 for a nightly discharge rate, then to reach 10 years of life span you have to de rate to 0.4 depth of discharge .

So if for example, you need 1600 AH, you would spend $1500 for a 2000 AH lLiFe02 bank, plus 150 for a decent BMS, so $1800 US including shipping. (Just bought our 5th and 6th banks)

For the OPzS, you would need to buy 5714 AH to get the same cycle life and longevity, plus you have to write off 38 percent of your charging energy due to voltage and discharge rate discrepancies vs about 10 percent for the LiFeO2.

I seriously doubt you could obtain 5700 AH, shipped to your location, for less than $1800.. and then the size, maintenance, and corrosion issues (some of that goes away with the considerably more expensive sealed designs) also add to the headache.

If the battery magazine will be subject to below freezing temperatures, both types incur additional risks and energy costs for the lithium, you must heat it prior to charging. For the lead, you will need to add another 40 to 50 percent capacity to meet your target power capacity.

TIL. For non native speakers: "specific gravity" means "density, typically relative to max density liquid water", which does not make sense to me at all.

It is a common way to measure battery charge through a hydrometer in automobiles batteries (not the sealed type):

https://en.wikipedia.org/wiki/Hydrometer

>Battery hydrometer

The state of charge of a lead-acid battery can be estimated from the density of the sulfuric acid solution used as electrolyte. A hydrometer calibrated to read specific gravity relative to water at 60 °F (16 °C) is a standard tool for servicing automobile batteries.

It's a simple way to check changes in concentration of a chemical in water when you're not worried about external contaminants (i.e. just water vapor or anticipated chemical reactions).

For home brewers, measuring specific gravity at the start and end of your brew process is the simplest way to know how much alcohol is in your creation. For this you'd use a hydrometer: https://en.wikipedia.org/wiki/Hydrometer

> LiFePo4, you’re looking at a decade, maximum, before they’re trash.

I think they will be at 80% after a decade, thus not trash.

> if you’re sealed, and if you get something like OPzS cells and treat them well, you will get 20 years of life from them.

If you have the space, 6V or even 2V OPzS cells can be huge.

Mine are 2v 660Ah cells - 24 of them take up about 3m^2 of floor, and they’re about 80kg each. Power density, they do not have.

Thank for sharing, almost 2 ton in weight. But I believe that can work.

Isn't charging efficiency for lead acid lower tho ? Then again if you save on batteries might as well get that extra panel or two

Yes, LiFePO charges much faster and in solar setups can "use more of the input" when it does shine. Lead-acid can compete there mostly by building a much bigger battery pack.

I'd personally not want a lead-acid battery burping flammable gas inside an apartment. They're supposed to be installed in ventilated spaces with drainage holes for any acid that spills out.

Or you have recombiner caps - haven’t had to top up at all, as they turn the escaping O/H back into water.

I don't disagree with you but lifepo4 have a high upfront one time purchase cost for something equivalent in capacity to a 12V 100Ah AGM, like easily $450-500 per unit or more. They do pay for themselves over time with their much greater longevity.

Might be more than the blog owner wants to spend for what is just a fun hobby project.

There are some 3U rackmount size nominal 48vdc 50Ah lifepo4 (approx 4.8kWh capacity) with built in bms and LVD you can get from china now.

With lead acids you just have to make sure you don't go below 40% charge. They don't tend to like that. The deep cycle ones are a bit better but I've experienced them deteriorating quickly too once you go too low. The other disadvantage is that they are quite heavy and might end up taking up lots of space. I'm personally still using lead acid, but with plans to go LiFePo4.

If you can't go below 40%, it might make sense to reduce the capacity by 40% accordingly when making comparison calculations against tech that can do that no problem. How does bang for buck look then with SLA versus LiFePO4?

I don't have the exact numbers but even then it's cheaper. The only issue is - can you trust yourself (or your planning) to ensure you don't go below 40%? Real life is hard to plan.

What happens if you do let it go below 40% (either as a brief dip to high 30s, or letting it go to 0)? Is it closer to a case of "oops you hit 39%, this battery is now worthless" or "oops you went down to 0%, better not do that every day as statistically the battery will last 0.001% less long"?

edit: I pasted my comment (prefaced with "context = lead-acid batteries") into ChatGPT and got this answer, maybe someone who knows could, instead of having to explain it all to me, just confirm GPT-4 got it right, or if not the correct it / add detail?

> If a lead-acid battery is allowed to go below 40% state of charge (SOC), it can cause some issues, but the effects are not necessarily immediate or catastrophic. The depth of discharge (DoD) and frequency of these deep discharges can impact the battery's overall lifespan and performance.

> 1. Brief dip to high 30s: If the battery's SOC occasionally dips into the high 30s, it may not cause significant harm. However, consistently discharging to that level can gradually decrease the battery's overall lifespan.

> 2. Discharge to 0%: If a lead-acid battery is allowed to discharge to 0%, it can cause more serious damage. Repeatedly discharging the battery to this level will dramatically shorten its lifespan and could potentially cause it to fail prematurely.

> The relationship between DoD and battery life is not linear. For example, discharging a battery to 50% DoD might give you around 1,000 to 1,200 cycles, while discharging it to 80% DoD might give you only 300 to 500 cycles. It is generally recommended to keep lead-acid batteries at a higher state of charge to maximize their lifespan and maintain optimal performance.

> In summary, allowing a lead-acid battery to occasionally dip below 40% SOC may not cause immediate damage, but consistently doing so can reduce its lifespan. Discharging the battery to 0% can cause even more significant harm, especially if done repeatedly. It is best to keep lead-acid batteries at a higher state of charge to ensure their longevity and maintain optimal performance.

ChatGPT is not wrong... but in my personal experience, once you go below 40% it's all downhill from there. At least if you've done it 2-3 times. Batteries are not always exactly the same, and temperature conditions vary as well. I've been nursing batteries for years, and this is just my anecdotal comments on that.

One thing that we can all agree on though is that you must never ever use car batteries. Lead acid need to be the deep cycle kind.

More deets from ChatGPT

> please provide an example data to show non-linar relationship between DoD and lifespan in 5% increments.

Depth of Discharge (DoD) Approximate Number of Cycles 05% 3,500 - 4,000 10% 3,000 - 3,500 15% 2,500 - 3,000 20% 2,000 - 2,500 25% 1,800 - 2,200 30% 1,600 - 2,000 35% 1,400 - 1,800 40% 1,200 - 1,600 45% 1,100 - 1,400 50% 1,000 - 1,200 55% 900 - 1,100 60% 800 - 1,000 65% 700 - 900 70% 600 - 800 75% 500 - 700 80% 300 - 500 85% 200 - 400 90% 100 - 300

> thinking through the following steps please create an asci bar chart for this table, dod given in 5% increments:

1. Calculate the number of cycles for the first increment 2. Extract the maximum number 3. Calculate the number of stars with 1 start = 500 cycles 4. Put number of starts into a table 5. iterate over the table

Once the table has been completed create the asci bar chart plotting DoD vs Cycles

Example: 1. 5% DoD = 3,500 - 4,000 2. 4000 3. 8 stars

DoD Cycles

05% ********

10% *******

15% ******

20% *****

25% ****

30% ****

35% ***

40% ***

45% **

50% **

55% **

60% **

65% *

70% *

75% *

80% *

85% *

90% *

Lol, the way this came out to sit perfectly with OP's intuition. Even down to the drop from 2-stars to 1-star!

Yeah, there's def the upfront cost, which not everyone can justify. Though, at the same time, a lower-amp-hour battery can provide as much actually-usable energy as a higher-amp-hour-rated SLA battery (so you could purchase a LiFePo4 battery with a lower amp-hour rating than the equivalent SLA battery but still get the same run time). I wouldn't know how much lower you can go though offhand.

48v 100ah LifePO4 is under 1000$ in China

It is but you have to make very sure that you actually get what you ordered and not some substitute with lower grade cells than what's advertised.

Are there easy(/cheap) ways to verify this if one does buy, as an individual, a cheap battery like that? Or do you need to either rely on brand reputation or have access to expensive equipment for testing?

I think vendor reputation is almost as important (or maybe even more important) as brand reputation. In many cases a brand is just a sticker, and your vendor may well have a longer relationship with their customer.

For instance, telling a brand name cell from a fake cell can be extremely hard, but your vendor knows exactly where they bought them. I buy cells from nkon.nl, who are a high rep local vendor and I've yet to be disappointed but buying a pack that is sealed from a low rep vendor may well incur a lot of risk. You'd almost have to split open the shrink wrap to make sure you got what you paid for. One quick and dirty way to check is to weigh the pack and to compare the weight to the manufacturers spec for the cells. It's obviously not perfect but many fake cells have weights that are wildly different from the cells they pretend to copy (usually due to much lower actual capacity).

Discharge & record the curve. All it takes is one poor-performing cell & your whole pack is going to be hurting bad. Discharge down to like 2.85V * number of cells. The curve should be nice & even. If there are places where the voltage drops before the big drop at the end, it's a sign that one cell isn't performing.

If your has equalization, try to discharge & equalize charge it 2-3 times first.

If you can actually get access to the cells, measuring cell resistance is not expensive. Anomolies will show themselves very quickly. Everything should be pretty clumped together in internal-cell-resistance. If you have a pack equalizer, it can be used to help you read this out. My <$100 charger has a cell resistance test mode.

> All it takes is one poor-performing cell & your whole pack is going to be hurting bad.

Yes, that can't be stressed enough. And depending on the fault and how the BMS handles it a more dangerous condition could easily develop. For instance an undetected short would cause the other batteries to be overcharged to the point where they are much more likely to die themselves, and some of those may do so in pretty violent ways.

Cheap / missing BMS are a serious problem with cheap Lithium-Ion packs, they are the first line of defense and bad cells can even happen when supplied by good brands (but the chances are a lot higher with no-name stuff).

Another good test: charge the pack, balance it, measure the pack voltage and the individual cell voltages, then let it rest for a day or two and measure again. Any kind of deviation between cells is a sure sign something is not working well and is the earliest warning sign for trouble you can get. The rate of self discharge should be the same for all cells and it should be very low, too low to measure (and make sure you disable the balancer after the initial charge!). After a month or so it would be fine to see the pack drop a little bit, but not a lot.

Yet another quick test: use an IR camera to look at the pack both during charging and at rest. Any cells that are warmer than those around them are suspect.

You can do a capacity test quite easily, that will only tell if the capacity is the rated one. You will have to check for physical signs of bad cells, they enlarge (bloat), but that means you need to open the case (sometimes it will void any warranty) and check each cell.

I'm going to disagree with you, because decent deep-cycle lead-acid batteries can last very well, and they are very much cheaper than LiFePO4. https://youtube.com/watch?v=LPPUqLZOqCQ is the video that persuaded me, even if he does ramble on a bit.

LiFePO4 age by elapsed time more than cycle count, so after 10 years they will be reaching the end of their useful life regardless of how much you have used them. Lead-acid age by cycle count more than elapsed time, so you could have a battery last well longer than 10 years if you treat it well.

I'd note that lead-acid batteries are divided into deep-cycle batteries that have are designed for longevity and basic batteries that are optimised for peak supply current (for vehicle starter motors). Use a deep-cycle battery, otherwise yes it will die very quickly.

I'd argue that if you're in a part of the world that has unreliable sunlight, such that you want to store say 4 days of energy but rarely actually use it all, then lead-acid is a perfect fit, because you just buy lots of storage at a cheaper price than LiFePO4, and that regime automatically treats the batteries well.

If you're in a part of the world where sunlight is reliably every day and you only want to buy enough battery to last a single night, then I can see that LiFePO4 may be a better option. But I'd still be inclined to buy several days of lead-acid instead. If you want to charge your batteries from cheap early morning grid power and effectively cycle twice a day, then LiFePO4 is definitely the better option.

The other thing that is nice about lead-acid batteries is the recycling rate - they are very easily recyclable and very highly recycled. I don't think we're there with LiFePO4 yet.

Flooded lead acid needs to be topped up with distilled water, and shouldn't be kept near living quarters due to possible outgassing. If you run them dry, you will permanently damage the batteries. To recycle them, you have to drag them somewhere automotive. The premium you pay for LiFePO4 over lead acid is almost no maintenance and you can drop them off almost anywhere electronics are recycled to enter the recycling supply chain.

Batteries are going to improve rapidly over the next decade, and the battery you replace your current LiFePO4 with at end of life is likely to be much better when that time comes.

The video is ok for the spreadsheet-like comparison, but it claims that all batteries have "memory". Also claims that depth of discharge doesn't matter to lead acid (what happened to sulfation?).

One good point that was made is that lead acid starts to behave terribly after losing 20% of its capacity, to the point that they fail outright and can no longer hold a charge. Lithium can usually keep working way longer than that, just with a reduced runtime.

Those cycle life numbers look great, but lead acid batteries in practical applications tend not to last the lifespan advertised on brochures. We can gauge that by how confident the manufacturers are:

* The Trojan SIND 06 610 has a 2 year warranty.

* The NSB100FT - 3 years.

* The Battleborn BB10012: 10 years

The Trojan battery weights a whopping 220lbs and holds 600amp hours. It's not even in the same category as the other two. You need a minimum of two as they output 6v. The price for the amount of storage they provide is pretty good though.

> Lead-acid age by cycle count more than elapsed time, so you could have a battery last well longer than 10 years if you treat it well.

Have you ever seen a lead acid battery last this long? Was it able to hold a meaningful amount of charge?

Lead acid batteries may be cheaper but they are unlikely to survive for as long as claimed.

That said - if you want a battery backup that you basically never use, there is a chance that you'll get a good return on investment on deep cycle lead acid (specifically those industrial ones) – if they don't fail for whatever reason and are properly maintained. For the same amount of money you can buy a ridiculous amount of storage for a real emergency.

Good point about the recycle rate. Lithium is theoretically just as recyclable as lead acid, but the logistics are not there yet (but how will that look like in 10 years?).

About the video: This is the same guy that was crimping his cables with a vice and soldering them, with awful results. I bought a decent ‘crimping device’ for $30. I’ve seen his videos but I’m not buying it. Maybe OPzV stuff lasts 20 years but then you sacrifice a bedroom worth of floor space for a potentially dangerous maintenance burden that requires reinforcements of the floor. I’m exaggerating but still.

Second hand forklift batteries are a cheap source for lots of storage capacity. They tend to be deprecated well before the end of their service life simply because they can no longer make it to the end of a shift without recharging.

Do test the cells individually with an acid tester, a simple hydrometer one is easy to take along when you go shopping. One important thing to keep in mind when buying any kind of battery is that for some batteries there are pretty strict requirements as to how they can be installed, what kind of physical isolation you have to use and how you need to vent any gases that they produce. These rules can easily disqualify some battery chemistries or configurations from consideration.

How safe are LiFePo4 batteries these days? Can one use these in -20 to +60 degrees Celcius environments without a fire hazard?

I'm asking because I'm creating a similar setup to OP to control the climate in my glider's trailer.

LiFePo4 are the safest of the commercially available lithium batteries. They can still catch fire if you insist, but it is nothing like the kind of batteries typically found in cell phones that can readily turn into fireworks. Some models are rated from -20C to 60C.

Lead acid also have their own set of problems. Some types can produce hydrogen gas while charging, which can be an explosion hazard in an poorly ventilated room. Also, all types of battery are energy storage, and in case of a short circuit, they will release a lot of it as heat, and even if the battery itself can handle it, it can ignite flammable material in the vicinity.

Any lithium battery claiming to be rated for operation down to -20 C is likely lying. Lithium chemistries are sensitive to temperature and operate well in a narrow temperature range. Operation below freezing ambient temperatures requires a heater and heaters need energy - hence why they are uncommon and only add cost.

Au contraire, lead-acid batteries work pretty well at temperature extremes, needing only changes in charging (e:) voltage and being limited by discharge current.

Any lithium battery claiming to be rated for operation down to -20 C is likely lying.

Not necessarily lying but rather there are heating pads that use some of the energy to bring the battery up to safe operating temperature based on BMS readings. There are some LiFePo4 battery breakdowns on Will's YouTube channel that show the various heating pads used by each vendor. [1] So probably better to say they leave out some details that will affect capacity at lower temperatures. He has a HN account so maybe he will also comment.

[1] - https://www.youtube.com/c/WillProwse/videos

That’s exactly my point though - if you are aiming for robust operation at a wide temperature range, lithium batteries are actually a worse choice than lead acid batteries. Lead acid batteries require little to no temperature control, just some charging rate and voltage control depending on temperature.

I wasn't actually disagreeing with you, just pointing out that at worst the companies are lying by omission. That said I don't think there is a one size fits all. LiFePo4 are great where weight matters more than temperature such as boats, RV's or anything that is mobile. The power used by the heating pads in some cases is offset by the higher capacity and ability to maintain voltage at lower charge state.

For off-grid buildings lead/AGM is fine especially if not depending on solar and perhaps instead keeping a charge from a water turbine from a river or stream. AGM have significantly less charge cycles than LiFePo4 and the disparity is growing more every year. China went both Sodium and LifePo4 due to logistical and shipping issues with some of the materials required in Li-ion. Li-ion batteries also have heating pads. I can't see AGM being viable in solar farms due to the low charge cycle ratings. Either that or whoever the manufacture is will be very happy as those batteries will have to be cycled out quickly.

I have 4 AGM batteries for a few of my inverters that need higher surge capacity in a smaller space and the UPS driver remembers me for it. Now they make me get the batteries out of their truck. Since I keep the AGM's fully charged with commercial power the charge cycles are less of an issue for me and thus the TCO/ROI is acceptable. LiFePo4 current ratings are catching up though. Now I can get one that has a 200+ Amp discharge rating and I am told there will be one with a 300AH/320 Amp discharge rating soon.

I am most curious about what comes next. The 3D printed solid state batteries look very interesting. 50% of the weight for the same capacity, higher C rating, significantly safer. Mass production is just starting for those so time will tell I guess.

not disagreeing with you either :)

The killer feature of lead acid designs is cost and availability. In many applications, they are significantly cheaper. The chance cycle issue is real and i’m glad you brought that up. For some applications, they make sense. One is a low power draw solar-powered website. Lead acid batteries also outlast most lithium chemistries by several years. These batteries have been around for decades now are are absolutely everywhere and the voltages are standardized. that alone is a significant advantage.

Case in point - i run a UPS using my old 12 V car battery. Granted, i cannot keep this indoors but it works well.

Similar to what you said - off-grid or backup power is the mainstay of lead acid designs. almost every electrical substation out there uses lead acid for backup power.

That hasn't really been true for years. At the same temperature, FePO4 outperforms lead acid in voltage, discharge current, and capacity.

> Operation below freezing ambient temperatures requires a heater and heaters need energy.

No. Charging requires a heater, discharging does not. It's really not reasonable to say "operation" requires a heater. While you're discharging, you're worried about the capacity and duration, and running a heater is a huge problem. The slower your discharge, the more of an issue it is to run a heater.

While you're charging, it usually isn't. You're already losing plenty of power to the charger inefficiency, it just takes a little more power to keep it above ambient. If you're plugged in it's irrelevant.

Fair point that only charging requires a heater but how does that meaningfully address the issue of cold weather operation? A battery requires charging and discharging and being able do to one but no the other won’t cut it, surely? The loss of capacity to heat is significant - often 10-30% capacity but that figure depends.

Also, i’m not sure plenty of power is lost to charging inefficiency. most industrial inverters operate at nearly 90% efficiency. Besides, above ambient isn’t a good metric - lithium chemistries do well around 10-25 C. If ambient is - 5 C, a delta of +5 C doesn’t change my core point.

Even if you are plugged in, lithium batteries aren’t dissipating heat. Not sure what the charger inefficiency has to do with this because that heat often isn’t the batteries themselves.

> The loss of capacity to heat is significant - often 10-30% capacity but that figure depends.

But it isn't a loss of capacity (although there is also a loss of capacity due to cold, in any chemistry). The capacity is the same, it just takes more energy to put the same amount of energy in. If you are putting energy in, you usually have an oversupply, and heating the battery is not an issue.

> If ambient is - 5 C, a delta of +5 C doesn’t change my core point.

I used the phrase "above ambient" specifically because the battery does not need to be warm unless you need to charge it quickly. If it's above 0 Celsius, it can charge. The warmer you make it the faster you can charge it. If the wasted energy is an issue you can charge slower.

> most industrial inverters operate at nearly 90% efficiency.

An inverter is significantly simpler than a battery charger. FePO4 charging can start as low as 2.5 volts and go past 3.6 volts. Making a converter with a 50% swing in voltage is not particularly easy. This higher-end charger picked at random[1] drops to <75% efficiency at the end of charge.

[1]: https://www.analog.com/media/en/technical-documentation/data...

they're saying that when charging you likely have the spare current to warm up the batteries before you charge them.

GP said rated, not necessarily for operation. It could just mean that the battery will survive such low temperatures unharmed.

I have a summer cabin with a solar panel and a couple 12 V lead acid batteries. It would be great to be able to install some lithium type batteries instead, so long as I know they will survive winter.

Nope. IIRC typical one is safe to discharge at -10 but needs to go to 0 to start charging.

I wonder whether there is some off-the-shelf controller that can do something clever like run a heater instead of charging battery till it gets up to temperature

There are some specialty ones that allow lower temps but that comes back to cost again

There are some batteries that integrate temp sensors and heating pads: losing some energy to bring the battery up to operating temperature.

Yeah one of the vendors I mentioned above sells batteries with exactly that functionality -- if the temperature is too low, the BMS disconnects and the heating circuit is powered by the incoming energy (whatever the source might be) until the cells are warm enough: https://www.canbat.com/product/12v-100ah-cold-weather-lithiu...

Are these more or less a plug in replacement for lead acid, likely to work with our existing charge controller?

Most charge controllers made for serious applications allow for the selection of battery chemistry.

It seems not: https://www.power-sonic.com/blog/how-to-charge-lithium-iron-...

They may work for the the initial phases of charging but not for “float” I.e keeping the battery as close to 100% charge over a longer period.

Not all are, but the Simpliphi batteries are designed to be plug-in replacements more or less

> unless you're going for the absolute cheapest possible build due to immediate budget constraints.

Or, you know, you get limitless amounts of 24Ah 12V SLABs free for the hauling away because they test at a bawhair under factory spec after five years of having an easy life.

Unless you will be in cold temperatures, then LiFePo batteries will die if you charge them.

How cold is "cold" ? Here in the Netherlands we rarely get more than a couple of days per year with temperatures much below zero.

Below freezing seems to be the standard rule[1]. One typical solution is to use the charging current to heat up the battery instead until it's above freezing, then start the charging.

You could of course also use some of the charge from the battery itself to heat it up.

[1]: https://relionbattery.com/blog/lithium-battery-cold-weather

Anything below 0C is problematic for LiFePo4.

I have a small solar powered device (Pi Zero controlling an antenna switch) in my backyard that I use a little mechanical temperature switch to turn on and off a teeny tiny mat heater attached to the battery. It's all in an enclosure that's water tight, so I doubt it got below 0 during the day in there in the first place. We didn't get temperatures below 0C this winter in the NE USA (which is not ordinary), and even with some dips below 0 the battery seems fine.

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