Mitsubishi Electric MELSERVO-J4 Series Instruction Manual page 390

Replacement procedure of the built-in battery
When the MR-BAT6V1SET-A reaches the end of its life, replace the MR-BAT6V1 battery in the MR-BAT6V1SET-A.
Cover
Projection
(four places)
11 OPTIONS AND PERIPHERAL EQUIPMENT
388
11.8 Battery
1.
While pressing the locking part, open the cover.
Tab
2.
Replace the battery with a new MR-BAT6V1 battery.
3.
Press the cover until it is fixed with the projection of the locking
part to close the cover.