How to read out VBAT i.e. voltage of CMOS battery

Any ideas? I thought it is prudent to monitor it as is causing BIOS resets. Thanks!

1 Like

That’s an interesting question. My mind went fast to the 10 years pseudo replacement that we were doing in the field. Which I think might not be very scientific and also may cause waste.

So I opened my book for the A+ certification of PC hardware to get good information.

On page 155 of " Mike Meyer’s CompTIA A+ guide ::: Managing and Troubleshooting PCs (2019) "

Losing CMOS RTC Settings
As mentioned before, your CMOS RAM needs a continuous trickle charge to keep the internal clock running and remember its settings. Motherboards use some type of battery, usually a 3 volts lithium-ion coin battery, to give the CMOS RAM the charge it needs when the computer is turned off. This is called the CMOS battery. Typical systems use a CR2032 battery.

If some mishap suddently erases the information on the CMOS RAM, the computer might not boot or you’ll get nasty-looking errors at boot. Any PC will boot to factory defaults if the CMOS clears, so the chances of not booting are slim - but you’ll still get errors at boot. Here are a few examples of errors that point to a lost CMOS information scenario:

  • CMOS configuration mismatch
  • CMOS date / time not set
  • BIOS time and setting reset
  • No boot device available
  • CMOS battery state low

Here are some of the more common reasons for losing CMOS data:

  • Pulling and inserting cards
  • Touching the motherboard
  • Dropping something on the motherboard
  • Dirt on the motherboard
  • Faulty power supplies
  • Electrical surges

If you run into any of these scenarios, or if the clock in Windows resets itself to January 1st every time you reboot the system, the battery on the motherboard is losing its charge and needs to be replaced …

… To retain your CMOS settings while replacing the battery, simply leave your PC plugged into an AC outlet. The 5 volts soft power on all modern motherboards provides enough electricity to keep the CMOS charged and the data secure.
So, I’m going to change my mindset of throwing good batteries out of the motherboard until I see the symptoms enumerated in this book.

Unless It’s a live server with important live computing that is going back to do good services, then I would change it at 10 years.

I would not use a multi-meter on each batteries to determine let’s say " it’s at 80% of 3 volts … let’s change it … " because the environment of each machines for all the variation in tasks and reboots would be to complicated :wink:


The (fairly new) power supply consumes 3-4 Watts when left ON plus packets are delivered the WS seen by blinking LED. I prefer to switch it OFF when not used and let the CMOS battery do the job. It should be straightforward to record its voltage given all the advances in technology.

Much appreciate the read of the article from the book, though, which is very good.

There seems to be a way by loading the right kernel model for the chipset while running sensors-detect:

1 Like