JBL Synchros E50BT Compatible Battery 3.7V 570mAh Li-Polymer

JBL Synchros E50BT — 3.7V Li-Polymer Replacement Battery (P433638)

This 3.7V, 570mAh Li-Polymer cell replaces the original P433638 battery inside the JBL Synchros E50BT over-ear wireless headphones. It restores Bluetooth audio playback, active noise cancellation, and onboard controls to full function. Dimensions are 46.00 x 40.20 x 4.24mm — physically matched to the original cell cavity.

• Synchros E50BT fit: The E50BT runs a single-cell Li-Polymer pack at 3.7V nominal, shared across Bluetooth radio, ANC circuit, and playback. This replacement matches that voltage rail and the original connector orientation, so the headset's onboard BMS accepts the cell without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the E50BT platform and confirmed the BMS completed its handshake without fault codes. Charge acceptance was clean from storage voltage through to termination.
• ANC draw on first charge: The E50BT's ANC circuit pulls current alongside the Bluetooth radio simultaneously. On first use, charge the headphones to full termination before enabling ANC — this lets the BMS log the cell's actual capacity ceiling before high-draw operation begins.

Why the E50BT cuts out mid-track after a battery swap

A freshly installed cell often sits at storage voltage — around 3.6V — not full charge. When the ANC circuit and Bluetooth radio both draw simultaneously, that marginal voltage drops below the BMS cutoff threshold and the headset shuts down. This is not a faulty battery. It is the BMS protecting a cell that was never brought to full charge first. Charge the headset fully before first use — the cutout stops once the cell reaches 4.2V termination.

Talk time shorter than expected across the first few uses

Li-Polymer cells do not deliver peak capacity on cycle one. The E50BT's combined ANC and Bluetooth draw makes this gap more noticeable than it would be on a simpler device. Capacity improves progressively across three to five full charge-discharge cycles as the electrolyte fully wets the electrode surfaces. By cycle five, measured output should be at or near the rated 2.11Wh.