The battery is integral, rechargeable, and soldered to the main board of the module. The on-board battery is not user-serviceable.
The purpose of the battery on the MVI module is to maintain the BIOS settings which are checked during the boot up of the MVI module. The battery needs to maintain a voltage between 2.3 and 3.7 VDC in order for the BATT LED to be off. If the battery drops below 2.3 VDC, then the battery will be unable to maintain the BIOS settings for the module; but, when the module is booted up they will be restored by the application code.
If the battery voltage falls below 1.5 VDC, then during a boot up sequence, the module will fail to boot. As long as battery voltage is above the 1.5 VDC threshold, the module will continue to function correctly, even if the BATT LED is ON RED all the time. However, if the battery level is below 1.5 VDC and power is lost to the rack, the module will not pass the battery check and the OS of the module will stop the boot up process.
If the module has been left in an unpowered state for more than 2-4 weeks, the BATT LED may come on when the module is first powered up. If left in a powered-up chassis, the on-board battery will recharge and then the BATT LED will go out once the battery voltage exceeds 2.3 VDC. If you leave the module in a powered-up chassis for at least 24 hours and the BATT LED is still on, the module will need to be returned for repair or replacement.