Mitsubishi Electric MELSERVO-J3 Series Handbook page 533

Part 8: Review on Replacement of Optional Peripheral Equipment
(3) Battery replacement procedure
(a) Installation procedure
Insert the battery along the rails.
(b) Removal procedure
CAUTION
Pulling out the connector of the battery without the lock release lever pressed
may damage the CN4 connector of the servo amplifier or the connector of the
battery.
While pressing the lock release
lever, pull out the connector.
While pressing the lock release lever,
slide the battery case toward you.
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Insert the connector of the battery
into CN4.