Can I mix Amp Hours (Ah) batteries in a UPS?

Thread Starter

JesseInSTL

Joined Sep 15, 2021
1
I have a Trip Lite UPS that has two 12-volt 9 Ah batteries in series that I need to replace. I have an 8Ah and a 9Ah batteries on the shelf. What would happen if I were to connect these in series?
 

ericgibbs

Joined Jan 29, 2010
13,825
hi Jesse.
Welcome to AAC.
The 8Ah would determine the longest loaded time.
Assuming that they were both fully charged and in good condition.

E
 

LowQCab

Joined Nov 6, 2012
1,183
Charging could be an issue,
but since they are only on a "Float Charge" maybe not.

It's important that both Batteries are basically brand-new.

It's generally bad practice to "mix-n-match" Batteries.
.
.
.
 

crutschow

Joined Mar 14, 2008
27,895
I agree with LQC, that mixing battery size and/or brands is not a good idea.

But It shouldn't hurt to try, if the batteries are available and you have no other use for them.
However, you should fully charge them both separately before you put them in the UPS.
 

Ian0

Joined Aug 7, 2020
3,463
Recharging after a deep discharge could be the biggest problem. The 8Ah battery will become fully charged first, and probably get overcharged as the 9Ah charges. However, if you stick to the rule of only discharging to 50%, then each battery will need the same amount of recharge current, for the same amount of time and so they should keep in sync.
 

ericgibbs

Joined Jan 29, 2010
13,825
hi Ian,
I would not charge two different Ahr batteries in series.

I think the TS just wants to use what he has on the shelf.
Assuming that they were both fully charged and in good condition.

E
 

Ian0

Joined Aug 7, 2020
3,463
I would not charge two different Ahr batteries in series.
I wouldn't either, if I could possibly avoid it! But it wouldn't do any harm provided that the voltage of either battery doesn't exceed its boost charge voltage, and the current doesn't exceed C/5 of the smaller one.
 

ericgibbs

Joined Jan 29, 2010
13,825
hi Ian,
LTS shows an interesting 'side effect', if the nominal 24V is discharged down to 21.6v, the 8Ahr battery is discharged slightly lower than the recommend 10.8V.
So when the recharge cycle starts the 8Ahr versus the 9Ahr, the slightly lower discharged state of the 8Ahr, results a fairly even recharge for batteries.
Run it for 3 Dis/Charge cycles,
As the two batteries are so close in Ahr, it appears that in this case there should not be a problem.

Balances out close 8.5Ahr.

E

ESP_ 817 Sep. 18 14.15.png
 

Ian0

Joined Aug 7, 2020
3,463
I'm intrigued to learn that there are SPICE models for lead-acid batteries. How good are they? Do the include such things as Peukert effect?
 
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