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.
---------------
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".
--------
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.