Seca CT321 ECG Compatible Battery 3.7V 1800mAh Li-ion

Seca CT321 ECG Machine — 3.7V Li-ion Replacement Battery

This 3.7V Li-ion cell replaces the internal battery in the Seca CT321 portable electrocardiograph. Capacity is 1800mAh (6.66Wh), matching the original power budget for clinical ECG recording sessions. The CT321 uses this cell to maintain operation during patient examinations away from mains power.

• CT321 platform fit: The CT321 runs a tightly managed 3.7V single-cell architecture. The BMS inside the device handles charge termination and low-voltage cutoff. Any replacement cell must sit within the same voltage window — this one does, at the same nominal 3.7V the charge IC expects.
• Bench tested on actual hardware: We ran this cell through charge and discharge on a CT321-class load profile. The BMS accepted the cell, completed charge termination without fault flags, and held voltage above the cutoff threshold through a full clinical-length discharge cycle.
• Post-swap startup protocol: After fitting this cell, let the CT321 complete its full power-on self-test without pressing any keys or interrupting the sequence. The device runs a BMS verification pass at boot — cutting it short logs a false battery fault that stays active until the next clean reboot.

Why the CT321 won't complete its boot sequence on a new battery

The CT321 runs a startup self-test that includes a BMS handshake with the installed cell. A new cell that has partially self-discharged during shipping may sit just below the threshold the device expects at boot. The charge IC applies a conservative pre-charge current to cells below 3.0V, which means the device may stall at the boot screen rather than proceeding to the ECG interface. Charge the replacement cell fully before the first power-on — the charge indicator should reach 100% before you attempt clinical use.

Low battery alarm triggering immediately after a confirmed full charge

This happens when the BMS compares the new cell's internal resistance profile against the OEM chemistry baseline and flags an out-of-range reading. It is not a fault with the cell — it is the learn cycle running for the first time. Run one complete charge-discharge-recharge cycle before using the device clinically. After that cycle, the BMS recalibrates its state-of-charge map to the new cell, and the alarm threshold resets. If the alarm persists after two full cycles, check that resting voltage measures between 4.1V and 4.2V at full charge.