Serene SA-40 HearAll Replacement Battery 3.7V 900mAh

Serene SA-40 HearAll — 3.7V Li-ion Replacement Battery

This is a 3.7V, 900mAh (3.33Wh) lithium-ion replacement battery for the Serene SA-40 HearAll hearing aid. It slots into the SA-40 HearAll when the original cell has degraded and can no longer hold a consistent charge. Dimensions are 46.20 × 34.14 × 5.68mm — verify these against your existing cell before installing.

• SA-40 HearAll cell fit: The SA-40 HearAll uses a compact Li-ion cell at 3.7V nominal to power its amplification and digital sound processing circuits. The BMS on this device monitors cell voltage closely — a degraded cell that sags below threshold will cause the hearing aid to cut out or shut down unexpectedly.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted the cell without fault codes, and the protection circuit responded correctly to both low-voltage cutoff and overcharge conditions.
• Post-swap recalibration tip: After installation, run one full discharge from 100% to automatic shutoff before recharging completely. The SA-40 HearAll's fuel gauge will recalibrate its capacity estimate against the new cell's actual discharge curve, giving accurate charge readings from the second cycle onward.

Why the SA-40 HearAll shuts down before the battery indicator reaches empty

The SA-40 HearAll's protection circuit cuts power when cell voltage drops below a safe floor — typically around 3.0V under load. An aged cell has higher internal resistance, so voltage sags faster under the current draw of active amplification and DSP processing. The device reads this voltage sag as a hard cutoff condition and shuts down, even though the fuel gauge still showed remaining charge. A new cell with lower internal resistance sustains voltage under load longer, so the indicator and actual runtime align again.

Charge indicator reading incorrectly after fitting the new cell

The SA-40 HearAll tracks remaining charge using a stored discharge curve mapped to the original cell. A new cell has a slightly different voltage-to-capacity profile, so the device misreads the state of charge until it learns the new curve. This shows up as the indicator jumping, pegging high, or dropping suddenly in the last portion of discharge. One complete discharge-to-shutoff followed by a full uninterrupted charge resets the calibration — after that cycle, the reading stabilises.