Re: UPS question!
In normal circumstances battery inside a UPS unit is charged with limited current till it is "full" and then the charging mode is switched from "charge" to "trickle".
Connecting directly external 80Ah battery will most likely not damage the UPS and you have good chances that this combination of internal and external betteries may work well.
However, I wouldn't excercise this option and connect this battery through a, say, 10A diode, so it is only used when there is no external power, and cherge it with a proper battery charger when it is required. In this way you will have this huge backup, but in the same time you will not see its presence.
Regards,
IanP