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CMOS battery

Posted on 1999-01-23
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Last Modified: 2008-01-09
Upon startup I receive an error message saying that the  cmos battery has failed. I have very little documentation on the motherboard and I was wondering where would the battery be located on the motherboard and what would it look like? Looking inside at the motherboard, there is a black square chip with the name Houston Tech and the letters HT 12888B 9701A2  16870. Could this be the battery? If so, how easy is the removal and replacement of the battery. The computer that is malfunctioning is not a primary computer but one a friend was trying to keep as a backup. Any help would be appreciated
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Question by:birchenall
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by:kinross
ID: 1137951
A battery is usually something thike a watch battery or could be a box with two wires running to the motherboard.  I don't think it is the chip.
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by:birchenall
ID: 1137952
There doesn't seem to be anything like a watch battery on the motherboard. Could it be located on the backside of the motherboard?
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jhance earned 50 total points
ID: 1137953
>Houston Tech and the letters HT 12888B 9701A2  16870

Yes, this is a CMOS RAM/BATTERY combo device.  The battery is inside the plastic and it not replaceable except via cutting away of the plastic and somehow hooking a new one up.  If the device is socketed (hopefully) you can replace it.  Houston Tech is a small time supplier, I tried to find a rep/distributor for them without success.  These are most commonly made by DALLAS Semiconductor and you can find replacements at most electronic parts suppliers.  I.e. Jameco, JDR, Fry's, etc...  
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by:rmarotta
ID: 1137954
If there is a pin header on the motherboard (usually located near the device), you may be able to connect an external battery pack to it.

Regards,
Ralph
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by:birchenall
ID: 1137955
What would a pin header look like?
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by:rmarotta
ID: 1137956
It is a single row of four pins.  Usually pins 2-3 are covered with a jumper cap to use the onboard battery.
The external battery connects to pins 1-4, with the jumper removed.
Ralph
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by:birchenall
ID: 1137957
rmarotta, Thanks for your help. I have found something on the motherboard and in the small documentation that came with the computer that is called the CMOS RAM discharge jumper. It has only 3 pins. 1 is for the internal battery 2 is CMOS and 3 is ground. It lists the normal mode with jumper on 1 and 2. It lists Clear CMOS as covering 2 and 3. I assume I would connect the external battery to 1 and 3. Thanks again for your assistance.
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by:rmarotta
ID: 1137958
Sorry, but I'm afraid that won't work.
The jumper you refer to is for clearing the CMOS memory by shorting it to ground.  I think the other jumper position is simply a place to store the jumper cap when not in use. (not a connection for an external battery)
Ralph
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