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

@woozle @rysiek Avoiding overcharging means a tolerably decent multi-stage battery charger that's configured correctly. E.g., running an equalize cycle every 6 months (or as recommended by mfg) can greatly increase the life of a flooded battery. But you must never configure your charger to run an equalize cycle on a sealed battery (it'll boil off electrolyte that you can't replace!). Etc.

But, in the end, a decent three-stage battery charger and a decent AGM lead-acid is simple and reliable.

@jond
All this discussion of DoD brings up another question: how does one measure that? I gather it involves a certain amount of modeling, and that the algorithms may vary depending on battery chemistry and configuration -- is there a good reference resource anywhere for best practices?

cc @rysiek

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@woozle @rysiek Good question! DoD looks tough but the math ends up being pretty simple; it's just difficult at first to grasp the terms, etc. "days" as an example time unit here:

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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