Symptoms of a dead CMOS battery

  • Date and time reset to a past date (often 1/1/2009 or similar) every time you power off
  • BIOS settings reset after each shutdown
  • Boot warnings about "CMOS checksum error" or "CMOS settings wrong"

The battery

Most motherboards use a CR2032 coin cell — $1–2 at any hardware or grocery store. Confirm by reading the battery on your board before purchasing.

Desktop replacement

  1. Power off, unplug from wall.
  2. Locate the silver coin cell on the motherboard — it snaps into a holder.
  3. Use a small flathead to gently pry the battery out of the clip.
  4. Insert the new battery with the positive (+) side facing up.
  5. Power on, enter BIOS, set the correct date/time, save and exit.

Laptop replacement

The CMOS battery is a small 2-wire connector inside the laptop — typically near the RAM or under the main battery. Use the iFixit guide for your model. Same CR2032 or flat pouch cell depending on model.