[Refurbishing] Test the CMOS battery

  • Posted on: 16 August 2018
  • By: charm
Checking a CMOS battery with a multimeter

One of the steps in our refurbishing process at The Working Centre's Computer Recycling Project involves testing the CMOS battery inside each desktop PC we refurbish. Testing the CMOS battery is a relatively simple process and you can do it a number of ways. The simplest is with a cheap battery tester. You can sometimes find these at dollar or hardware stores. But battery testers can't be found everywhere so the next simplest method is with a multimeter.

All the desktop CMOS batteries we've scene tend to be the CR 2032 3-volt button-style battery. You'll need to take the battery out of the system as we're going to measure the positive and negative sides of the battery using the red and black multimeter probes. Set your multimeter to the DC voltage above the battery (so more than 3 volts). Make sure you're set to DC voltage. In our case the next step up from 2 volts was 20 volts, so this is what we set our meter to (20v DC). The black (negative) probe goes on the bottom of the battery and the red (positive) probe goes on the top of the battery (the top of most CR 2032 batteries tends to have a + sign etched into it to show positive).

We reject anything below 3 volts. Although computers will run with CMOS batteries that are 2.8 or 2.9 volts as a matter of principle when we're rebuilding a PC we want to make it as new as possible, and one of the inexpensive ways we do this is replacing CMOS batteries with good new batteries. A new CMOS battery often measures are 3.17 - 3.29 volts, so don't be surprised to see over 3 volts.

On some systems a bad CMOS battery can cause a system not to POST (power on self test), so it's worth checking if you're having POST issues with a system you're building.