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I have an uninterruptible power supply which powers my PC, monitor, and WiFi router. My question is: can I add a second pair of batteries to the UPS to get more run time on batteries without damaging the UPS?

The second pair of batteries I plan to use are identical to the batteries that are inside the UPS. I would wire them in parallel to the internal batteries.

I saw in several other posts that wiring batteries in parallel can cause high current flows between batteries when the batteries are not charged equally. This could be the case when I initially add the new batteries.

Stephen Daddona
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Your description of the issue with unequally charged batteries is accurate. However, simply connect the batteries in parallel through a resistor until balanced.

A 1 or 2 ampere incandescent lamp of the appropriate voltage (or two 12 VDC car lamps in series for a 24 VDC source) makes an ideal balancing resistor:

  • At initial higher current, it's resistance is greater, slowing charging to a safe level.
  • As equalization progresses, the resistance drops, speeding final charge equalization.
  • It also functions as an indicator: until all visible glow is extinguished, batteries are still equalizing.

You could leave the lamp in place, with a shorting switch across it, for use next time a set of batteries needs to be replaced.

DrMoishe Pippik
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