Serious question:

If lead-acid batteries aren't good for deep-cycle use, then why do most UPSes use them? Do they use a special lead-acid design that's actually ok with deep discharges?

(I'm trying to figure out how to put together a battery-driven portable power-system, at least in theory.)

@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.

@woozle ooops, forgot tag in @rysiek as you had in your DM.

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

I'd like to add one bit from the UPS point of view, really good temperature management can extend lifetime a couple of years. Replacing batteries every 6-7 years instead of 4-5 is probably a big enough cost saving worth keeping the UPS room at 21° instead of letting it wobble up towards 25-27°.
@woozle @rysiek


Do they prefer colder temperatures, generally, or is there a "sweet spot"?

@jond @rysiek

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@woozle @maswan @rysiek Colder temps reduce available capacity but improve lifetime*. Good rule of thumb is every +10 C halves a battery's life. Specs are usually given at 20 C.

Batteries do not like freezing, but a charged battery has a lower freezing point (more solute in the water!) than a dead battery, so LA mfgrs will say, e.g., "below 0 C, keep state of charge > 50%"

* if the reduced capacity means you have to go to higher DoD, lifetime gains can easily be nullified by the harder use!

@woozle @maswan @rysiek For some clear curves, see something like the "SAGM 12 135" PDF link here:

that shows % capacity available over temperature, etc

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