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?

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@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.

@rysiek You're welcome. Hopefully this shows:
1. good system design always starts with sizing your loads.
2. any watts you can shave off your loads have a really outsized impact on your system size, cost, etc. 1 extra load Ah is 1 Ah of charging source, wire, breakers, etc., and at least 2 Ah of battery (more because you need ride-out capacity!)

If anyone has questions on figuring out system sizing, I'm happy to talk. I'm not an installer by trade but I'm neck-deep in this stuff all day.

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

@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.

@jond @woozle @rysiek it's worth checking out 'lead-carbon' batteries, which are still fundamentally lead-acid batteries, with some fancy graphite addition which gives really nice discharge/cycle characteristics. A solar farm near by me installed a couple shipping containers full of them 2 years ago, so they seem to be commercially available.

Alternatively, there is an endless supply of used 18650 Li-ion cells in thrown away battery packs where only one cell is bad.

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

@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

@jond @woozle do you have any such knowledge on Sodium Ion batteries?

@meena @woozle I've dealt with NIB very little. It's an interesting tech but, as always, the devils are in the implementation and uptake. :(

At a NABCEP conference in like 2014 (?) I was just a couple booths down from the Aquion folks. That was my first chance to talk with people on the manufacturing and shipping side of a NIB operation. They were up front about the pros and cons of NIB but their op ran out of money. The tech is a tough sell right now.

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