Checking The Backup Battery - Lenovo THINKPAD L420 Hardware Maintenance Manual

1(+)
3. If the voltage is less than +11.0 V dc, the battery pack has been discharged.
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 5 and 7.
The resistance must be 4 to 30 K Ω.
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 power adapter from it.
2. Turn the computer upside down.
3. Remove the battery pack (see "1010 Battery pack" on page 57).
4. Remove the backup battery (see "1140 Backup battery" on page 80).
5. Measure the voltage of the backup battery. See the following figure.
• If the voltage is correct, replace the system board.
• If the voltage is not correct, replace the backup battery.
• If the backup battery discharges quickly after replacement, replace the system board.
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Hardware Maintenance Manual
7(-)
6(-)
5
3 4
2(+)
Terminal Voltage (V dc)
1 + 0 to + 12.6
7
Ground (-)
Wire Voltage (V dc)
Red +2.5 to +3.2
Black Ground