My Bios Battery?

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

Moderator: The Mod Squad

My Bios Battery?

Postby Karlsweldt » Tue Jan 24, 2006 10:35 am

All modern computers have a small battery to maintain the settings in the Bios, when the PC is not powered. It must be replaced on at least a 3~4 year use basis, or the voltage level will drop to a point where the date/time become erratic, and desired settings for the system are lost. It may even present a no-start condition.
The proper term is CMOS battery, for Complimentary Metal-Oxide Semiconductor, the memory storage area that has the user-settable data for the system configuration.
The common type is the "coin" battery, so-called because it resembles a common US quarter. A common type is the CR-2032, so named due to its thickness, voltage and diameter.

It is easy to check.. but you must use a digital multimeter, not an analog type, as the output current of the battery is very low. An analog meter may give false readings. Simply pull the power plug, and use any black lead or a case ground point (the keyboard shield) for the (-) terminal. The top of the battery is the (+) terminal. A reading of 3.0 volts is nominal. Dropping to 2.7 volts or less indicates a weak battery.
Karlsweldt
Mobo-fu Master
Mobo-fu Master
 
Posts: 20671
Joined: Wed Nov 12, 2003 11:57 am
Location: 07438

Postby tedybear » Tue Feb 28, 2006 5:11 pm

As a side note to Karl's notes on the CMOS battery. Radio Shack and most office supply stores carry this very commen battery. Please do not trust a "watch battery tester" to verify in the store if the battery is good/bad. The "load" the tester will place on the battery is designed for a low-current-draw watch. The CMOS circuit's will draw a bit more current then the watch will. The tester has been known to give a false "good" when in fact the battery can/will be buckled under the slightly higher load.

I discovered this upon working with one of the arcade games that uses a cmos setup to retain settings. The Clerk at the local Radio Shack tested the battery as being "Good". however the machine did not retain any settings until I replaced the battery with a new one.

It's a very low cost item, and is cheap for piece of mind maintence!

Again---Well Done Karl on the excellent reference.

S-
tedybear
Black Belt 5th Degree
Black Belt 5th Degree
 
Posts: 7251
Joined: Sat Feb 14, 2004 12:55 am
Location: Fulton, New York, on Earth

Postby Karlsweldt » Wed Mar 01, 2006 6:30 am

Hard to explain in simple terms, the technology behind the BIOS and CMOS. But the BIOS means simply "Basic Input/Output System". The CMOS is defined previously.
Certain areas in the CMOS are reserved as ROM, or read-only memory that cannot be changed. These settings are dedicated to the mobo design. Other areas are EPROM or "flash" memory areas, that can be erased and reprogrammed, for specific features or operation. EPROM means Erasable Programmable Read-Only Memory. Most setups have 512 Kb or less total dedicated memory for these purposes, but some setups may have as much as 2 Mb of dedicated memory for these purposes. The ROM part is not bothered by battery voltage loss, but the EPROM sector may be. The CMOS setup also has a real-time clock feature, where a small circuit generates the proper time constants.. similar to a digital wrist-watch. These time constants are important to a PC, for proper program response. If the BIOS battery runs low on its potential, the clock feature becomes erratic. It's design is dependent on a specific voltage range, not on a "variable" voltage range. This is due to the design of the circuit, having a VDO or Voltage-Dependent Oscillator as its 'heart'. Crystal oscillators too may suffer from different voltage inputs, so the circuits on more-critical timing controls usually have their own regulation circuits. Newer PCs and other devices "refresh" the time settings by monitoring the GPS signals and the NBS (National Bureal of Standards) time signal broadcasts. However, we do not care about splitting a second into a ten-thousandths spectrum! Plus/minus one second per day is more than accurate enough for 99.9% of tasks.
There is another "clock" in a PC, that is the main system "clock".. that generates the frequency used by all devices, and is entirely independent of the CMOS settings and the real-time clock. The circuit design may be similar, however.
Karlsweldt
Mobo-fu Master
Mobo-fu Master
 
Posts: 20671
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