MR-BAT is the absolute encoder backup battery unit for Mitsubishi Electric’s MELSERVO MR-J2S and MR-J2M series servo amplifiers, providing continuous power to the absolute position encoder during main power-off periods to retain multi-turn counter data without requiring a homing cycle on every startup. The MR-BAT is a 3.6 V lithium thionyl chloride (Li-SOCl₂) primary battery housed in a dedicated connector assembly, designed for direct insertion into the battery holder on compatible MR-J2S amplifier front panels. Available now at Atlantech Drives with worldwide express delivery and 12-month warranty — complete our quote form for pricing and availability.
What Is the MR-BAT?
The MR-BAT is a consumable maintenance component within the MELSERVO-J2S and J2M servo drive ecosystem. When a servo amplifier is powered off, the absolute multi-turn encoder fitted to the servo motor loses its position count unless a battery maintains the encoder IC’s supply voltage. The MR-BAT provides this standby power, allowing the controller to resume from the last known absolute position without executing a reference return (zero point) operation — a critical requirement in applications such as CNC machine axes, vertical load-bearing axes, and multi-axis robots where an uncontrolled homing cycle would be mechanically unsafe or operationally disruptive. The battery connects to a dedicated BAT connector on the front of the MR-J2S amplifier and is monitored continuously by the amplifier firmware; when battery voltage drops below the low-voltage threshold, the amplifier raises an AL.92 warning (Battery Warning) giving the maintenance engineer time to replace the unit before AL.9F (Battery Alarm) triggers a fault that locks out servo operation.
Key Technical Specifications
- Battery Chemistry: Lithium thionyl chloride (Li-SOCl₂)
- Nominal Voltage: 3.6 V DC
- Typical Capacity: 1,800 mAh
- Compatible Amplifiers: MR-J2S-□A, MR-J2S-□B, MR-J2M series
- NOT compatible with: MR-J3, MR-J4, MR-J5 series (these use MR-BAT6V1SET)
- Battery Life (standby): Approximately 3 years at 20 °C ambient
- Storage Life: Up to 5 years from date of manufacture (sealed)
- Operating Temperature: -40 °C to +85 °C
- Housing: Integrated connector assembly for tool-free insertion
- Hazardous Material: Contains lithium — subject to transport regulations (IATA PI 968/969/970)
Compatibility & System Integration
The MR-BAT is specifically designed for and limited to the MR-J2S and MR-J2M servo amplifier generations. It is not interchangeable with batteries used in newer MELSERVO generations: MR-J3 amplifiers use the MR-J3BAT (3.6 V, different connector), and MR-J4 and MR-J5 amplifiers use the MR-BAT6V1SET (6 V lithium battery pack with different housing). Installing an incorrect battery type — even if it physically fits — will result in incorrect voltage being applied to the encoder backup circuit, causing either immediate AL.92/AL.9F faults or, in worst cases, encoder data corruption. In multi-axis MR-J2S systems, each amplifier requires its own individual MR-BAT; there is no shared battery bus across amplifiers. For systems using the MR-J2S-□B (SSCNET) variant, the battery replacement procedure requires the SSCNET optical fiber link to remain connected during replacement to prevent the controller from losing axis synchronisation — refer to section 5.3 of the MR-J2S-B Servo Amplifier Instruction Manual for the correct hot-swap sequence.
Replacement & Upgrade Notes
Battery replacement should be performed before the AL.92 warning escalates to AL.9F, as the AL.9F alarm locks servo operation and requires a manual alarm reset after battery replacement, followed by a full absolute position re-establishment (reference return) — negating the primary operational benefit of the absolute encoder system. Mitsubishi Electric recommends replacing the MR-BAT every 3 years regardless of alarm status, as battery capacity degrades gradually and the remaining life indicator (visible in MR Configurator or the amplifier parameter monitor) becomes unreliable below 30% remaining capacity. When replacing the MR-BAT, the amplifier must remain powered on — replacing the battery with the amplifier powered off will cause the encoder to lose its multi-turn position data, requiring a reference return after power-up. Keep a minimum of one spare MR-BAT per axis in maintenance inventory for sites running MR-J2S-based machinery, as lead times for consumable items can extend during peak demand periods. If a site is planning a servo amplifier upgrade from MR-J2S to MR-J4, note that the MR-J4 platform uses the MR-BAT6V1SET and the absolute encoder data from the MR-J2S system cannot be transferred — a full reference return on all axes will be required after amplifier replacement.
Frequently Asked Questions
Q: Can the MR-BAT be replaced while the MR-J2S amplifier is powered on?
A: Yes — and it must be. Replacing the MR-BAT with the amplifier powered off will immediately cause the absolute encoder to lose its multi-turn position data, requiring a reference return (zero point restoration) after power-up. With the amplifier powered on, the encoder IC receives continuous power from the main supply and the battery can be swapped without position data loss. The replacement window once the battery is disconnected from the connector is approximately 30 seconds before the encoder capacitor discharges — work quickly and have the new battery ready before disconnecting the old one.
Q: How do I know when the MR-BAT needs replacing on an MR-J2S system?
A: The MR-J2S amplifier monitors battery voltage continuously and raises AL.92 (Battery Warning) when voltage drops to the low threshold. This warning appears on the amplifier’s 7-segment display and is logged in the alarm history accessible via MR Configurator. Additionally, parameter Pr.PC13 can be set to define a custom battery warning output on a digital output terminal, allowing the PLC to generate a maintenance alert before the alarm escalates. Do not wait for AL.9F before replacing — at that stage, servo operation is locked.
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