Lenovo ThinkPad T420s Hardware Maintenance Manual page 51

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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Wire Voltage (V dc)
Red +2.5 to +3.2
Black Ground
.
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