@woozle ideally the batteries of an UPS do less than a dozen circles in theire entire life, maybe it's that.
In case you are going to buy lead-acid batteries: the common car batteries are not good for UPSes because they are optimized for lots of amps for only a short time.
@Gregor Right, that's what I'm trying to avoid. I want something that can be discharged most of the way down more than a few dozen times -- equivalent to a laptop or cellphone battery.
I was aware that lead-acid batteries are bad for this, but that led me to wonder why they use them in UPSs if so.
I guess most people don't have as many power-outages as we do?
@woozle Deep-cycle lead acid typically means something like 1500+ cycles to 50% depth-of-discharge. Thicker lead plates, wider plate spacing, putting some Sb or Ca in the lead mix, etc. They have lower peak current compared to a car battery but can survive *much* more use.
Lifetime num cycles tends to go linearly with DoD to a point: draining 20% of capacity before recharging might yield 4000 cycles; 40%, 2000 cycles, but beyond 50% DoD the battery typ wears out much faster.
So people say "discharge cycles are bad" but that's a generalization. Buy a good battery and size it correctly so, in your application, you keep your cycles from 0% DoD to no more than 50% DoD and you can get many many years out of a battery, etc.
You can still use that last 50% capacity in an emergency, but plan to not use it during normal operation.
@woozle @rysiek E.g., if you are running a daily charge-discharge cycle that uses up 100 Ah of capacity, put 200 Ah or more of good deep-cycle lead acid in there and it should be good for 4 or 5 years. Etc. A car battery (optimized for cost and peak current) would prob be toast within a year.
Things that make LA batteries mad: overcharging, repeated very deep discharge, prolonged extreme heat, freezing the electrolyte
load_Ah_per_day = load_amps * load_on_hours_per_day
battery_Ah = load_Ah_per_day * num_days_without_charging / (1 - max_DoD)
Then, as you suspect, you may add a small multiplier if you're in a cold environ, want to factor in performance at year 5, etc. Battery mfg data sheets will have clear curves for that.
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