@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?
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.
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°.
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!
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