I have never seen a UPS that used NiCads. Constant current and constant voltage are ratings of power supplies, not batteries. Granted NiCads can handle low discharge currents better then SLA's for a longer period of time but as long as you charge them back up when the power comes on you will be fine.

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3 Car batteries and a power inverter will work fine... and if you get good quality batteries you could probably run for most of the day. If not then it won't hurt to add more batteries. This will not have automatic features as in a true UPS. In other words it won't start automatically when your power goes... you will have to manually switch it over. HAM is also very correct... when charging these batteries you need to be careful of the fumes that are created. Gel batteries would be safer but they would cost more (Gel batteries are like leadacid but they are used in things like emergency lighting, kids electric scooters...)

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Yuh mean there are folks who will do ANYthing to keep that computer on?! Dont they have a life outside? (tongue in cheek) Actually, we are too spoiled with technology. And when it goes down, we shut down too, dont know how to live.

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actually what i have learned so far that most UPS's (including mine) use sealed lead-acid 12V batteries so substituting this with ordinary car battery will not harm the circuitry of the UPS except for the charging power which may be much more.and a good car battery charger connected to the battery along with UPS wires will solve the problem.as for hydrogen gas, a ventilation will do the job.

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I repair UPS units for a living. I have seen people do this before and I would urge caution in a few areas (I am not recommending this -- especially if you aren't sure what you are doing.) First (and this has already been mentioned), make sure to have the batteries well ventilated area, the batteries can explode from the hydrogen gas given off during charging / discharging. GEL cell batteries still give off hydrogen and won't work well in UPS units because of their design (they are designed for deep cycling and won't provide as much inrush power where AGM batteries are designed for floating with a full charge and will supply a large inrush.) Second, make sure you provide a proper fuse to the larger batteries and insure you have the correct voltage (i.e., if you have UPS which uses a 12 volt setup, use a 12 volt setup -- if you have multiple batteries, they must be connected in parallel for 12 volts or series for higher voltages.) If something goes wrong with the UPS and shorts, the batteries can leak, explode or start fires. If you have multiple car batteries, I can't stress how important a proper fuse is -- without it, wires can overheat and start fires. Smaller UPS units don't necessarly have fuses because there is not as much of a risk of fire since the batteries are smaller. The one problem that most people run into when they modify UPS units is that the charger in the UPS unit is not designed for larger batteries and can burn out with larger batteries (this happens mostly after a deep discharge and the power returns.) You might consider buying a UPS that is designed for larger batteries. See APC's Smart-UPS XL series. www.uninterruptiblesolutions.com has some refurbished Smart-UPS XL units that are cheaper that might work for what you need.

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