Mitsubishi Melservo comes in 4 main flavors; MR-JE (simplistic), MR-J2S (discontinued\unsupported). MR-J3 (in the process of phasing out) and MR-J4.

In most applications, a battery is not required to be purchased in addition to the amplifier. However, if absolute position control is required in the application, then a battery is required to keep the position value stored).

Note: RAM is a volatile memory, and unlike ROM, it requires a battery to keep the memory alive.