Cefar Myo 4 Replacement Battery 7.2V 2000mAh Ni-MH

Cefar Myo 4 / Activ 4 / Rehab 4 Pro — 7.2V Ni-MH Replacement Battery (2651)

This 7.2V 2000mAh Ni-MH battery replaces the original cell in the Cefar Myo 4, Activ 4, Rehab 4 Pro, and Bodymax Trainer (NGSF4) electrotherapy units. All four devices share the same battery bay, connector, and BMS handshake protocol. Capacity matches the OEM specification — 14.4Wh.

• Shared platform — Myo 4, Activ 4, Rehab 4 Pro, Bodymax Trainer: These units run the same 7.2V power architecture, identical connector pinout, and the same BMS communication logic. One cell fits all four without modification.
• Bench tested on actual hardware: We cycled this cell through charge, load, and self-test sequences on the Cefar platform. The BMS accepted the new cell without faulting, completed its chemistry verification pass, and held voltage within spec under simulated stimulation load.
• Power-on self-test — do not interrupt: After inserting this battery, let the device complete its full startup sequence before beginning any session. The Cefar BMS runs a cell-verification check at boot. Interrupting it locks in a false battery fault that won't clear until the next full cold reboot.

Low battery alarm on the Cefar Myo 4 immediately after a confirmed full charge

The Myo 4's BMS stores a chemistry profile calibrated against the OEM cell's internal resistance curve. A new replacement cell has a slightly different resistance signature until it beds in over the first few cycles. The BMS reads this as a weaker cell than expected and triggers the low-battery threshold early. Run one complete charge-discharge cycle before clinical use — after that, the BMS recalibrates its reference point and the alarm clears. If the alarm persists past cycle three, check resting voltage with a multimeter: a healthy charged cell should read 8.4–8.6V.

Cefar device won't power on after the replacement battery sat in storage

Ni-MH cells self-discharge at roughly 1–2% per day in storage. If this battery shipped or sat unused for several weeks, its resting voltage may have dropped below the Cefar BMS recovery threshold — typically around 5.4V for a 7.2V pack. The BMS interprets this as a damaged or absent cell and refuses to boot. Connect the charger and leave it for a minimum of four hours before attempting power-on; the charge IC will trickle-charge the pack back above the recovery floor before switching to full current. Do not attempt to force a boot during this phase.