The CMOS or BIOS battery:

A place where the most common PC questions are already answered for you.

Moderator: The Mod Squad

The CMOS or BIOS battery:

Postby Karlsweldt » Sat Oct 06, 2007 5:15 am

The BIOS battery is also called the CMOS battery.
It is about the size of a quarter, commonly called a "coin" battery. It would normally be flat on the motherboard in a holder, sometimes standing on end.
It is a 3.0 volt Li-Ion cell, very similar to those used with watches or other electronic equipment. The typical one is the CR2032 model. The model # is on the top of the cell. The shelf-life is around 4 years maximum, and in use with a PC it ranges from 3~4 years, depending on how long the PC is not powered on. The cell maintains the memory structure for device status in the CMOS (Complimentary Metal-Oxide Semiconductor). BIOS stands for Binary (or basic) Input/Output System. It also powers the real-time clock, which maintains the date/time. If the voltage falls below about 2.7 volts, then the clock may not function.. and the CMOS memory may become corrupt.
Replacing the battery and then clearing the CMOS memory (only user-choice configurations), then resetting the page configurations in the BIOS will restore a system to proper operation. The surest way to reset the BIOS is to pull the power plug.. then remove the battery, and move the jumper for the CLRCMOS process from one set of pins to the other for about 5 seconds, then put it back to the 'normal' position. Then replace the battery, connect the power cord and power up, going directly into the BIOS pages to reset all preferences.
The battery voltage is easy to check. With the power cord pulled, use a decent digital multimeter set at 20 volts. (An analog meter would load down the voltage output, giving false readings.) Any bare chassis point or black lead terminal is the negative side of the battery, and the top is the positive. A reading between 2.98 volts and 3.1 volts is good.
The life of this battery isn't like the normal lead-zinc flashlight battery, which looses power gradually. This type of cell, including the NiCd type, has a very sharp drop-off when the life has ended. They are available in most stores, but best to obtain one where they are sold frequently.

The jumper for the CLRCMOS process normally is very close to the battery. On some motherboards, it may be farther away, or may only be two small solder dots in a small printed box, which you must short with a small screwdriver blade.
Karlsweldt
Mobo-fu Master
Mobo-fu Master
 
Posts: 20676
Joined: Wed Nov 12, 2003 11:57 am
Location: 07438

Return to MBHW Q&A

Who is online

Users browsing this forum: No registered users and 1 guest