Lenovo MT 1871 Hardware Maintenance Manual page 44

Checkout guide
4. Remove the backup battery (see "Checking the battery
pack" on page 38).
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.
40
R50/R50p, R51 Series
Wire Voltage (V dc)
Red +2.5 to +3.2
Black Ground