About this replacement
Plantronics Discovery 665 / 655 / 645 / 640 — 3.7V Li-Polymer Replacement Battery (HS-DISC655)
This is a 3.7V, 80mAh lithium-polymer cell for the Plantronics Discovery 665, 655, 645, and 640 Bluetooth headsets. It replaces OEM part HS-DISC655 (also cross-referenced as AHB351045, 68400-61, and related suffixes). The cell matches the original 26.40 × 12.13 × 4.10mm profile and connector orientation inside the Discovery housing.
- Discovery 640–665 platform fit: These four headset models share the same internal cavity dimensions, connector pinout, and BMS communication protocol. Swapping between them requires no modification — the voltage rail and handshake signals are identical across the series.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the Discovery 665 base station. The BMS negotiated normally, the charge LED sequenced correctly, and voltage at full charge held steady at 4.18V with no cutoff errors logged.
- First-cycle base station registration: Place the headset in the base station and leave it for a complete charge cycle before making any call. The Discovery base logs the new cell during this first cycle — skipping it causes the talk-time indicator to misread the actual state of charge.
Talk time shorter than rated on the first few uses
A fresh lithium-polymer cell does not reach full capacity on cycle one. The electrolyte wetting process completes gradually, and usable capacity typically climbs over the first three to five full charge-discharge cycles. If the headset cuts off sooner than expected in the first week, run full cycles rather than topping up after short calls. By cycle five, resting voltage after a full charge should stabilise at 4.17–4.20V.
Headset cuts out mid-call even with a charged battery
The Discovery combines Bluetooth radio transmission and audio processing at the same moment — the combined current draw spikes sharply during active calls. An 80mAh cell with any internal resistance above spec cannot hold the voltage rail steady through that spike, and the BMS trips a low-voltage cutoff. This is not a base station fault. Confirm resting voltage reads above 3.85V after a full charge cycle; if it reads lower, the cell has not completed conditioning or has a resistance fault.