Dual 80Ah 12v Batteries in Series to power a Battery Backup UPS

Thread Starter


Joined Mar 1, 2017
I have two 80Ah AGM batteries in series utilizing an APC battery backup as it's charging source.
I have had this setup running for 7 years now but one of the batteries is finally giving up the ghost.

The other battery still seems to be working and charging but I'm not sure of the capacity of the working battery and therein lies my dilemma.

There are actually two dilemmas.....

I need 24v for the battery backup device so I need two batteries (because I don't have a 24v battery). And since they will be in series, the capacity needs to be similar if not exactly matched. If, for example, the still good battery can now only supply 30Ah of capacity, then what I'd like to do is get another 30Ah battery so that I can use the existing battery as long as it will work. Is that ok to do that?

But how do I find the remaining capacity? I basically know that I need to fully charge it then discharge it with a known load and measure the time it takes to drop below a certain voltage. That will give me the capacity. The average load on it in practical use is about 120watts. So can I assume that connecting a 120watt load to it is what I need to do?

Also, does it matter if one battery is AGM and one is Lead Acid as long as they both have equal capacities?

What cut-off voltage should I use? 12.4v ?

Thanks for any help.


Joined Jun 22, 2012
12V batteries should be charged at 10.8 to 11V minimum, upto 13.8V, for capacity monitoring try running a constant current load ideally of 1amp in hours, (or 10 amps for faster calculation) for the time it takes to get to 11V should give you its capacity.


Joined Nov 4, 2013
Unfortunately the reality is that after 7 years the other battery is likely right behind the other and will fail soon. That's why in multi battery systems that have a fair amount of age and usage on them it is proper to replace all the batteries together as a matched set.

Trying to be cheap and use mixed unmatched batteries is just going to cause you more problems and coast you more money than had you did it right first time.

It's like the tires on your vehicle. When all of them have 70,000 mile on them and all are worn you just change the one that has the belting coming through because the other three are not quite there yet.


Joined Jan 15, 2015
The reality is your best bet would be replace both batteries. I have several older UPS units which run 24 volts DC using a pair of 12 volt 7.5 AH SLA batteries. I would never think about just replacing one battery and as mentioned, you replace one and a few weeks later you will likely be replacing the other.


Thread Starter


Joined Mar 1, 2017
These are Werker AGM's. There were close to $200 each.

MAN I hate seeing them go.

Gonna replace them with Sealed Lead Acid Deep Cycle batteries. Much cheaper