Tunstall Lifeline Vi Replacement Battery 3.7V 1500mAh

Tunstall Lifeline Vi / Vi+ — 3.7V Li-ion Replacement Battery (D3706008a)

This 3.7V 1500mAh Li-ion cell replaces the OEM battery in the Tunstall Lifeline Vi and Lifeline Vi+ personal emergency response devices. It matches the original D3706008a specification in voltage, capacity, and physical dimensions. Capacity is 5.55Wh — no upsizing, no chemistry substitution.

• Lifeline Vi and Vi+ compatibility: Both models run the same 3.7V battery rail with the same BMS handshake protocol. The D3706008a part number covers both variants — the Vi+ shares the same connector, cell footprint, and BMS communication as the standard Vi.
• Bench tested on actual hardware: We cycled this cell through the Lifeline Vi's charge IC and monitored BMS response at each stage. The protection circuit responded correctly to both charge cutoff and low-voltage thresholds. No false fault codes were triggered after a complete charge cycle.
• Post-installation self-test requirement: After fitting this cell, allow the device to complete its full power-on self-test without interruption. The Lifeline Vi runs a BMS verification sequence at startup — cutting power during this window generates a persistent battery fault flag that will not clear until the next complete reboot.

Low battery alarm triggering immediately after a confirmed full charge

The Lifeline Vi's BMS uses a learned threshold calibrated to the OEM cell's charge profile. A new replacement cell can read as below threshold on the first cycle because the BMS hasn't yet mapped its voltage curve. This is not a faulty cell — it is a calibration gap. Run one complete charge-discharge-charge cycle and the BMS will update its reference. The alarm should clear once the cell voltage holds above 3.6V under the device's standby load after that conditioning cycle.

Device will not power on after the replacement cell was stored before fitting

Li-ion cells self-discharge during storage. If the cell voltage has dropped below approximately 2.5V, the Lifeline Vi's BMS enters a protection lockout and blocks power-on entirely. Connect the device to its mains charger for at least 30 minutes before attempting to power it on — the charge IC applies a low-rate pre-charge trickle below 3.0V to recover the cell safely. Once the cell climbs past 3.0V, normal charging resumes and the device will boot.