About this replacement
Nihon Kohden Cardiolife 7621K — 12V Ni-MH Replacement Battery (MD-BY01)
This is a 12V, 2800mAh Ni-MH replacement battery for the Nihon Kohden Cardiolife defibrillator series. It fits the Cardiolife 7621K, 7631K, 7721K, and 7731K, plus over 50 additional models sharing the MD-BY01 footprint. Cross-references include NKB-301V, SB-551V, and X065.
- Cardiolife 7621K–7731K platform fit: These four models share a common battery bay geometry, 12V rail, and BMS handshake protocol. The MD-BY01 connector and cell chemistry match that shared spec exactly — no adapter or modification required.
- Bench tested on actual hardware: We ran this cell through charge-discharge cycles on the Cardiolife platform and observed the BMS stepping through its chemistry verification sequence on each cycle. Charge termination and voltage cutoff responded correctly under simulated defibrillator load profiles.
- Post-installation self-test protocol: After fitting this battery, allow the Cardiolife unit to complete its full power-on self-test cycle without interruption. The device BMS runs a chemistry verification at startup — cutting power mid-sequence flags a battery fault that persists until the next clean boot.
Why the Cardiolife logs a battery fault after a successful charge
The Cardiolife BMS was calibrated against the specific charge curve of OEM Ni-MH cells. A new replacement cell has a slightly different internal resistance profile in its first few cycles, so the BMS may log a fault even after a confirmed full charge. This is a threshold mismatch, not a defective battery. One complete charge-discharge cycle normalises the cell's impedance signature and brings it within the BMS acceptance window. After that cycle, the fault clears and does not return.
Cardiolife not completing boot sequence after battery swap
If the device stalls or shuts down during the post-swap startup sequence, the cell voltage is likely below the BMS recovery threshold from storage self-discharge. Ni-MH cells self-discharge at roughly 1–2% per day and can drop below 10.5V during extended shelf time. The Cardiolife BMS will not complete the boot sequence below that threshold and will not initiate a recovery charge from this state on its own. Connect to mains power and allow a full supervised charge cycle before powering the unit on — confirm open-circuit voltage reads at least 12V before proceeding.