Connecting Two Batteries (Old and New) with UPS

The manufacturer's instructions for the UPS will probably require ordering more/replacement batteries from themselves if the product warranty is to be sustained.

If the either battery were to fail for whatever reason, then the other battery might discharge into it causing untold mayhem that might make the user unpopular with his/her fire indemnity insurers. If no other option were available, then it would be worthwhile taking the precaution of isolating the batteries' charge and discharge circuitry with suitable rectifiers so that this potential tragedy could not happen.

The other option is to cascade-out the 80AH battery for a larger, purpose-built, single unit.

This is a two sided question. People who are not skilled in electrical maintenance of batteries and UPS technology should not be experimenting with said systems. On the other hand, large UPS systems such as a 500 KVA UPS, batteries are added for time and duration. If a battery goes bad, it is replaced but you don't replace the entire battery bank.

I hope this helps, good luck.

Good Answer GHOG,

Many UPS systems have connection points for adding a secondary larger battery bank. Since the OP asked if this could be done, no external battery connection exists in the OP's system. Adding steering diodes to correct the possible problem of one failed battery bank prematurely discharging the good battery bank will cause a bigger problem, one or both battery banks will never be recharged by utility power.

So to get longer operation during a loss of power the OP has two choices. Either reduce the load on the UPS circuit by using more efficient loads (fluorescent or LED lighting instead of filament) and removing unnecessary loads from the UPS circuit. Do you really need fans running when power is lost? Most UPS have some load monitoring capability so a systematic approach to identify improvements can be done. Or the OP must increase the amount of reserve power. This can be done by either replacing the existing UPS or adding a second UPS and moving some of the loads to the other UPS circuit.