Philips Heartstart MRx M3538A Replacement Battery 14.8V

Philips HeartStart MRx — 14.8V Li-ion Replacement Battery (M3538A)

This 14.8V, 6000mAh Li-ion battery replaces the Philips M3538A and fits the HeartStart MRx defibrillator, HeartStart MRx Monitor, and Laerdal Monitor. It also cross-references OEM part numbers M3536A, M3535A, 989803129011, and M5055. Voltage and capacity match OEM specifications exactly.

• HeartStart MRx and Laerdal Monitor compatibility: These units share the same 14.8V battery bay, BMS handshake protocol, and connector pinout. One cell pack covers all listed models without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the HeartStart MRx platform. The BMS completed its handshake, accepted the charge cycle without fault, and reported correct state-of-charge after the first full conditioning cycle.
• Post-swap self-test protocol: After installing this battery, let the HeartStart MRx complete its full power-on self-test without interruption. The device runs BMS verification at startup — cutting power during this sequence triggers a persistent battery fault that won't clear until the next clean reboot.

Device not completing boot sequence after battery swap

The HeartStart MRx runs a multi-stage BMS learn cycle on first power-up with a new cell. If the cell voltage sits below roughly 13.5V after storage, the device may stall mid-boot or display a battery fault before the self-test completes. Connect the device to the AC adapter before powering on — this lets the charger raise cell voltage to an acceptable threshold while the BMS initialises. Do not disconnect AC power until the self-test finishes and the device shows a green ready status.

Charge indicator not reaching 100% on the first charge cycle

The HeartStart MRx charge IC applies a conservative upper voltage limit on cells it hasn't profiled yet. On the first charge, the display may plateau at 85–95% and hold there. This is the charge IC capping current while it maps the new cell's internal resistance. Run one full charge-to-discharge-to-charge cycle and the charge IC recalibrates — subsequent charges will reach 100% and hold correctly.