Checking The Backup Battery - IBM ThinkPad T21 Hardware Maintenance Manual

Note: Recharging will take at least 3 hours, even if the
indicator does not turn on.
If the voltage is still less than +11.0 V dc after
recharging, replace the battery.
4. If the voltage is more than +11.0 V dc, measure the
resistance between battery terminals 4 and 5. The
resistance must be 4 to 30 K .
If the resistance is not correct, replace the battery
pack. If the resistance is correct, replace the system
board.

Checking the backup battery

Do the following:
1. Power off the computer, and unplug the ac adapter
from it.
2. Turn the computer upside down.
3. Remove the battery pack (see "1010 Battery pack" on
page 60.)
4. Remove the backup battery (see "Checking the battery
pack" on page 36.)
5. Measure the voltage of the backup battery. See the
following figure.
Red (+)
Black (-)
v If the voltage is correct, replace the system board.
v If the voltage is not correct, replace the backup battery.
v If the backup battery discharges quickly after
replacement, replace the system board.
Wire Voltage (V dc)
Red +2.5 to +3.2
Black Ground
General descriptions
Checkout guide
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