CMOS, Circuit Board, PC Mount Batteries

What are CMOS or clock batteries?

CMOS & Clock Backup batteries perform the same function in desktop and laptop computers: when the computer is turned off, the battery maintains the time and date, thus insuring their accuracy when the system is once again restarted. More importantly, the battery saves the computers BIOS setup configuration, which allows the system to efficiently reboot once it is restarted. The computer knows what type of hard drive it is dealing with, etc. Not surprisingly, these batteries are known alternatively as CMOS batteries, Real Time Clock (RTC) batteries, or simply internal batteries.

The most common CMOS battery chemistries are Lithium, Nickel Cadmium (NiCad) and alkaline. They are usually somewhere in the 3 to 7.2 volt range and either solder onto the motherboard or plug in via a snap-in connector (depending upon the computer manufacturers design).

In most cases, replacement of the CMOS battery is an easy task. It is simply a matter of locating the battery on the computer's motherboard, removing it and plugging in a new one. As a rule, internal batteries should be replaced by the same type of battery which was originally used in the machine or according to the manufacturers specifications. The major exception to this rule are older PCs which were manufactured with a NiCad battery soldered onto the motherboard. These computers usually have a three or four pin male plug, with two of the pins connected via a jumper (this is generally found in the same area of the motherboard as the original battery). This plug gives you the option of leaving the soldered battery in place and replacing it with a plug-in lithium or alkaline battery. Removing the jumper tells the computer to ignore the soldered battery and to look to the pins for its power source. If the motherboard has this provision, you can install a standard PC plug-in battery instead of removing the soldered battery and re-soldering a new one (the standard PC plug-in battery is Electro Battery part number BAT 2005 (lithium) or BAT B40 (alkaline). These batteries are interchangeable).

A word of warning: some computers have 4 pins on the motherboard, whereas today's plug-in batteries come with a 3 pin connector (one of the pinholes is closed in order to prevent the user from inadvertently plugging the battery in with reversed polarity). If this is the case, you should clip the pin from the motherboard that corresponds to the sealed pinhole on the battery plug. That pin is nonfunctional and by clipping it you ensure that future batteries will not be installed on the motherboard with reversed polarity.