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Unusual Motherboard Behaviour on Battery Failure

First published: 01st February 2011

This note describes symptoms encountered when the CMOS backup battery on an AW1 motherboard failed. The symptoms were not typical of battery failure, and wasted time because the problem was initially attributed to other causes.

Expected symptoms for a CMOS backup battery failure are loss of system time and/or a CMOS Checksum Failure warning at boot time.

System Details

The system is an AW1 Appliance Series Mini System format. BIOS details are:

AwardBIOS v6.00PG
XP4/46281A (Jan.29.2003)


The computer would occasionally fail to boot. Over a period of weeks, the failure became more frequent. After multiple restart tries, and other attempts to diagnose the problem, the computer would start booting again.

On boot, the AW1 displays a splash screen that obscures the POST memory test progress and disk detection. At this point, either Tab or Del may be pressed to display the POST progress, or enter the BIOS setup utility. When the startup problem occurred, pressing Tab shortly after the power-on beep would result in the display of the POST, but pressing Del, or waiting before pressing Tab, would have no effect, the computer would stay displaying the splash screen.

If Tab was pressed and the POST screen displayed, the memory test could be observed progressing normally, then the CPU detection message, and then the message Detecting IDE Drives.... No further message was displayed. Eventually, the hard disc activity light would go out.

Naturally, a disc problem was suspected, and extensive tests were made... without the DVD drive, with different hard discs, with no discs. The system's hard disc was also tested on other systems, with no problems found. Other potential causes checked were connector problems, CPU, chassis and power supply fan operation, and memory. The intermittent nature of the problem led to incorrect diagnoses that these were the cause.

Careful observation during the memory test revealed that, sometimes, a single character was displayed on the line following the memory test progress text. The character varied, Z and C were seen. The character was overwritten by the normal text at completion of the memory test.

When the CMOS backup battery was replaced, the system booted OK, and the old battery was tested and found to be weak. The battery required for this motherboard is lithium type CR2025.


Consider the possibility of a CMOS battery backup failure, even if expected symptoms are not present. A replacement battery is cheap, and could save a lot of time.

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