Midland BT City Replacement Battery 3.7V 500mAh 1ICP8/18/40

Midland BT City / C929.01 — 3.7V Li-Polymer Replacement Battery (1ICP8/18/40)

This is a 3.7V, 500mAh Li-Polymer cell for the Midland BT City and C929.01 Bluetooth headset. It replaces the original pack when the headset no longer holds a charge or stops powering on. The cell matches the OEM dimensions of 40.36 × 19.00 × 8.00 mm and the original part number 1ICP8/18/40.

• BT City and C929.01 compatibility: Both models share the same cell housing, connector pinout, and BMS handshake protocol — one battery covers both. The 3.7V nominal voltage rail is identical across the two variants.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the BT City base station. The BMS completed its handshake on the first dock, the charge indicator stepped through correctly, and the protection circuit tripped cleanly at the low-voltage threshold.
• First dock procedure: Seat the headset in the base station and leave it undisturbed for a full charge cycle before making any calls. The base station logs the new cell during this cycle — skipping it causes the talk-time gauge to read inaccurately for the first several uses.

Why the BT City cuts out mid-call even with a new battery

The BT City runs audio processing and a Bluetooth radio simultaneously. That combined draw creates brief current spikes that can trip the BMS protection circuit if the cell voltage sags under load. A new cell at storage voltage — typically around 3.6V — is more susceptible to this than a cell that has completed two or three full charge cycles. After the first full dock cycle the resting voltage climbs to 4.1–4.2V, which gives the BMS enough headroom to absorb those spikes without cutting out.

Base station showing a charging error after fitting the new pack

If the base flashes an error or refuses to charge, the BMS handshake has not completed. This happens when the headset is docked before the connector pins are fully seated. Remove the headset, check the gold contacts on both the headset and cradle for debris, reseat firmly, and wait 60 seconds. If the error persists, power-cycle the base station by unplugging it for 30 seconds — this clears the charge controller's last-state memory and forces a fresh handshake at 3.7V nominal.