About this replacement
Nihon Kohden Cardiofax G ECG-2550 Monitor — 14.4V Ni-MH Replacement Battery (SB-201D)
This 14.4V, 3500mAh Ni-MH battery replaces the SB-201D in the Nihon Kohden Cardiofax G ECG-2550 electrocardiograph. It fits the ECG-2550 Monitor used in hospitals and diagnostic facilities for portable cardiac recording. Capacity is 3500mAh (50.4Wh), matching the original cell specification.
- ECG-2550 platform fit: The Cardiofax G ECG-2550 uses a fixed 14.4V Ni-MH architecture with BMS communication tied to the SB-201D part designation. The connector pinout and cell count are specific to this monitor line — no cross-compatibility with other Nihon Kohden ECG models.
- Bench tested on actual hardware: We ran full charge-discharge cycles on the ECG-2550 platform and confirmed the BMS handshake completes correctly. The charge IC accepted the new cell without flagging a fault, and the protection circuit tripped at the correct low-voltage threshold during discharge.
- Post-installation self-test cycle: After fitting this battery, let the ECG-2550 complete its full power-on self-test without interruption. The device runs a BMS verification sequence at startup — cutting power during this sequence triggers a false battery fault that persists until the next full reboot.
ECG-2550 not completing boot sequence on a new battery
The ECG-2550 runs a firmware-level power check during boot that compares reported cell voltage against a stored OEM threshold. A new Ni-MH cell sitting at partial charge from shipping can read below that threshold, causing the boot sequence to stall or abort. This is not a faulty battery — it is the BMS rejecting an unverified cell. Charge the battery fully before the first power-on, then allow the device to boot without interruption.
Low battery alarm triggering immediately after a confirmed full charge
This happens because the ECG-2550's charge IC uses internal resistance and voltage slope data to assess cell state — data it does not yet have for a new cell. Until the battery completes one full charge-discharge cycle, the BMS applies conservative thresholds that cause premature low-battery alerts. The cell is not defective. Run one complete cycle — full charge, then full discharge under normal device load — before relying on the battery indicator for clinical use. After that cycle, alarm behaviour normalises.