My mother's car battery is about five years old and is starting to fail (doesn't hold charge for long). I was wondering whether it would be feasible to add it in parallel to the battery inside my computer UPS to increase the duration that the UPS would last on the rare occasion that it gets used. The fact that it doesn't hold charge well shouldn't matter, I suppose, if it's constantly getting topped up.

 

Sure it's possible but a retail computer UPS is not designed to be run at 100% capacity for long periods. Extending the run-time with external batteries could cause a fire as the internal UPS transformer will be very hot after the normal run-time of the internal battery.

 

Not to mention the extra load on the charging circuit.

 

The fact that it doesn't hold charge well shouldn't matter, I suppose, if it's constantly getting topped up.

Putting a bad battery in parallel with a good battery isn't a good idea. The bad battery could prevent the good one from charging fully and cause it (good) to discharge faster.