Connecting a larger external battery to a UPS:
techenclave.com/ forum comments:

Small UPS are usually made to run only half an hour to 1 hour depending on the load and the internal components heat up after that time and also charger is made to charge small battery.

Better go for a sinewave inverter which can run computers with large battery.

i am using it presently my ups bty died got myself a external bty my laptop runs for 5 hrs on it.


Links:
Batteries in Home & Garden.
Charging at Deep Cycle FAQ
Charging the lead-acid battery – Battery University
12 V power for ham radio
The charge of the mobile phone brigade by Graeme Philipson
Hand Crank flashlights and Mobil Phone Chargers
Solar Panels
Power requirements
Surge Protectors
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last updated 14 Apr 2008