My trusty APC XS 1300 UPS’s battery died and wouldn’t keep a charge anymore. I really like this UPS, so I wasn’t ready to give up on it yet.

**Original Battery**

I first set out to buy a replacement battery, APCRBC109, but at $109.00, it was almost as expensive as a whole new unit. The APCRBC109 is 2 SLA 12v 9Ah batteries, which means its 2 sealed lead acid 12 volt 9 amp hour batteries that are wired in series to product 24 volts.

An amp-hour in its basic idea would be you can get 1 amp for 9 hours or 9 amps for 1 hour at 12 volts each.

So, if we were to use the UPS at full power we can estimate how long the battery on the UPS would last. This would give a estimate to compare other batteries with. You can get the total rating from the specs, 780 watts. To figure out how many amps it will draw from the battery, use the watts formula:

Watts is voltage x amperage, w = v x a. Re-arrange it so a = w / v

`780w / 24v = 32.5 amps`

In a perfect condition (ignoring UPS electronics loss, heat, etc)

`9ah / 32.5a = 0.277 hours`

Or about 16 minutes run time at full power. Pretty good to protect against short power outages, but we can do better if we want.

**Replacement Battery**

I bought 2 AGM 12v 55ah batteries.

If I apply the same formula as above, I can calculate theoretical run-time.

`55ah / 32.5a = 1.69 hours`

Or about 101 minutes! That makes sense because our new batteries hold about 6.11 times more power (55ah / 9ah) then the old ones.

The batteries need to be hooked up in a series, or positive to negative, circuit to create 24 volts (voltage is added in series) in the same way the original batteries do. The batteries wont fit into the UPS, so I picked up 2 battery cases from Amazon and a resettable fuse. The fuse was tied in series with the batteries just in case too much power is being drawn.

**Conclusion**

You can run standard AGM batteries to increase run time.