Lead acid isn't that much cheaper. You can get LFP 12v 100AH batteries for $290, with a 10yr warranty. You also don't have to worry about discharging them too low.
So lead acid is okay at lower temps and with only occasional deep discharges.
Also, they are heavy. Not a problem in fixed install or vehicle but pain if need to move.
> Lead acid battery can only use 50% of rated capacity.
That's definitely wrong. Lead Acids lose durability when you deeply-discharge them, but all Lead-Acids have *hundreds* of deep-discharge cycles supported.
The question is one of frequency. Li-ion is great because it has high-durability through deep-discharge cycles. But is this the use case of a UPS or Solar Backup? I argue no, it isn't.
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The key advantages of Lead Acid are its cheapness and simplicity. If you can overlook its size and weight (okay, 15lbs is heavy if you're used to Li-ion... but is it really a *problem*? I argue no), and also know that deep-discharges would be rare (ie: UPS style "backup battery" usage), then LeadAcid suddenly rises to #1 choice due to gross simplicity you can have when designing circuits.
As I described earlier: two diodes and a constant 13.5V source are pretty much all you need to create a minimally viable Lead-Acid battery backup solution. None of this "LiFePo4 charge controller" crap or CC-CV charging circuits (Lead Acid _prefers CC-CV especially if you want to charge it in 20 hours or less, but it doesn't _need_ CC-CV circuits)
A long-term trickle charger over days (~72-hours of charge) of use is perhaps a non-obvious usecase. But that's called "Always plugged in UPS" and is IMO, the kind of use-case that's best suited for "SolarPi".
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You might laugh at "hundreds" of deep discharge cycles or counter with the "thousands" that LiFePo4 can support. But seriously, think about it.
A 240 W-hr battery (12V and 20Amp-hours) hooked up to a 3W Rasp. Pi will give you 80-HOURS of life per cycle, that's over 3 days.
Assuming ~500 Deep Cycles of durability (which is in-line with Lead Acid specs), that's 1500 days of worst-case deep-cycles, or 4 YEARS of life.
And that's deep-cycling every 3 days, then fully charging on the 4th day instantly (it takes 20 hours to charge Lead-Acid in practice), so really you're looking at closer to 2000+ or 5+ years of worst-case life from something like the PS-12200.
Do you _really_ need the thousands of deep cycle charges? The damn chemistry is going to fail for other reasons (temperature / humidity issues) after 5 to 10 years, rather than from the cycles. Especially in a UPS situation where you've got oversized Solar Panels keeping the Lead-Acid at near-100% charge for most of its life.