Trying to pick charger for DIY UPS bank

Submitted by Valalvax t3_123d0st in DIY

I'm in the process of planning a diy UPS for my server ~550 watts

I have a variety of batteries from 7AH to 48AH, all 12V, I understand that mixing and matching will ruin your total capacity but I was thinking maybe adding relays in and using size A, then B, then C before finally gracefully shutting the server down

But anyway, my current problem is I'm not sure what kind of charger I should get, obviously I want one rated for constant use, but I don't want one that will mess up the batteries (or worse) from overcharging... Another possible obstacle is having multiple batteries and them being different capacities

Having trouble finding anything that isn't a car charger or for 18650s and the like, hoping someone has done a similar project in the past

1

Comments

You must log in or register to comment.

Hattix t1_jdumhy1 wrote

Mixing and matching shouldn't hurt the total capacity, so long as they're never in series.

2

Valalvax OP t1_jdv7jq3 wrote

You sure? I've always heard different and logically it makes sense...

Say a 7 and 14 AH both starting at full, the voltage would drop faster in the 7 thus drawing power from the 14, maybe all you'd lose is charging inefficiencies...

Guess it's something I could test to see how it responds

1

Stehlik-Alit t1_jdvgb1j wrote

Plenty of manufacturers mix capacity in different strings.

The higher capacity batteries have a lower internal resistance (due to more plate surface area) and thus will discharge roughly their % of the load in relation to their % of capacity.

As hattix said above, so long as theyre not in series, its fine.

1

Diligent_Nature t1_jdvixr6 wrote

That's not how it works. You can parallel 12V lead acid batteries of the same state of charge. The main concern is not overcharging the batteries. After the bulk charge (around 14.5V), the voltage should be reduced to a float charge level (around 13.7V). Many battery chargers can do one or the other but don't switch to float when the battery is charged. Look for smart chargers which give their voltage specification for each mode.

edit: AGM gel and flooded batteries may have slightly different voltages. It is best to use a charger designed for the chemistry of the battery.

1

Hattix t1_jdvnt5p wrote

Voltage can't drop faster on the 7 because you can't have (much of) a potential difference across a conductor. The 7 Ah battery would need to be at a lower voltage than the 14 Ah battery and... well, how would that happen? In practice, the batteries are discharged in a rate proportional to their capacity, so the 14 Ah battery would provide 2/3rds of all current, and the 7 Ah would give 1/3rd.

If they're in parallel, the capacities will add and they'll always be at the same state of charge.

There are exceptions to this in extreme cases (where internal resistance becomes an issue) but I can't see you hauling over a hundred amps out of these things if all they are is a UPS.

1

Valalvax OP t1_jdvxr67 wrote

Well crap, I'm glad I mentioned that because what I thought I knew was flat wrong, wonder where I got the idea from .. that definitely makes a huge difference in complexity, now all I need to do is figure out a charger and I'm good

1

x7q t1_jdvgut6 wrote

Probably better off finding an actual UPS for cheap. I got an old Tripp Lite rack mount (with expansion) for $100 with no batteries. Paid about $300 for new batteries and now my 300W rack can go over 10 hours without power.

Your DIY method seems needlessly complicated, but I do have something similar for my sump pump backup. 12V AGM battery hooked up to a transfer switch with an inverter. I just use a noco battery charger, but really any trickle charger will work.

2