A typical low-cost UPS for home use will put out 600VA at 230V and will contain an internal 12V 7Ah sealed lead-acid "maintenance-free" battery. You can improve backup time by replacing it with an external 12V car battery. A common car battery is the NS60SL which puts out 12V at a whopping 45Ah, an improvement in endurance of over 600%.
Note: car batteries are built for a few seconds of very high current output and do not like the way a UPS may require a sustained discharge over many minutes or hours. They do not last as long as a UPS battery which tend to be of the "deep-cycle" variety. Still, a wet lead-acid car battery is cheap and cheerful, and rather than buying a new battery for your UPS, you might want to use your old/spare car battery ...
Picked up some Teletron P600 UPS(now discontinued) at a sale for RM20(USD5) each. The plastic casing is too weak for its weight, and the AC power input socket is prone to dry solder joints but it packs a hefty transformer. This makes it quite tolerant of overloading and abusive loads like small freezers and refrigerators. It is also easily repaired and replacement parts are cheap.
Just find the battery cables and extend them out of the casing. In the picture the thick black and red cables are the battery cables
Attach car battery connectors. You now need to improve cooling by adding a fan. An old Intel Pentium MMX/AMD K6-2 (Socket 7) fan should do. This is because some low-cost UPS do not have adequate cooling in backup mode (ie no mains voltage). They are counting on the 7Ah battery to run out before the UPS overheats. Connect the fan directly to the 12V battery.
Next drill some holes in the front and back so it is easier for the fan to force the hot air out.
The completed hack looks like this. The new battery is a wet NS60SL for 120 minutes of backup time
The battery now takes up to a week to get fully charged. If you connect your CRT monitor to the UPS as well, make sure you do not power up the CRT when the UPS is in backup mode. Make sure the mains is on and connected when you turn on the CRT. This is because many CRT monitors require a high starting current (mostly used by the degaussing circuit), and will damage the UPS. Once the CRT is on, the UPS can easily support its power demand. With your UPS in mains mode, the CRT starts up on mains AC power.
If you connect AC motors to the UPS, be warned - many UPS's produce square waves instead of sine-wave AC output. This may cause you motors to run hotter. When powering motors, first ensure your VA (Voltage times Ampere) rating is enough after increasing the motor rating 30-40%. This is to allow for the power factor. The equation is:
Power required = Voltage x Current x Power Factor
The Teletron P600 with a VA rating of 600 should be able to support a 360 VA motor, or 230V at 1.5A.
