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ITFrequently Asked Questions · Batteries
Can I use car batteries in a UPS?
No. Car batteries are designed for a brief, powerful start, not for steady discharge.
Car starter batteries have many thin plates so they can deliver a very high current for a few seconds and then be recharged immediately by the alternator. A UPS, however, discharges them continuously for minutes or hours. In that kind of use the plates wear quickly and the battery loses capacity within a few cycles.
There are safety issues too. Many car batteries are vented and release gas while charging, which is unsuitable for an office, a home or a closed cabinet. In addition, the UPS charging voltage is set for sealed VRLA batteries, so a different battery may be overcharged or not charged properly. And since it does not fit inside the UPS, it ends up connected with improvised cables.
A common example is connecting a large car battery to a small UPS for "more backup time". The charger of a small UPS is not designed for that much capacity, and its inverter is usually not intended for continuous operation over many hours.
If you need longer backup time, the right solution is a UPS that supports external batteries, using the VRLA batteries specified by the manufacturer.
