Bang & Olufsen BeoPlay A1 Replacement Battery 7.4V 3400mAh

Bang & Olufsen BeoPlay A1 — 7.4V Li-ion Replacement Battery (C129D3)

This is a 7.4V, 3400mAh (25.16Wh) Li-ion replacement battery for the Bang & Olufsen BeoPlay A1 portable Bluetooth speaker. It matches the OEM part number C129D3 and fits directly into the speaker's internal battery bay. Swap it in when the original pack no longer holds a charge or cuts out during playback.

• BeoPlay A1 fit: The A1 runs a dual-cell 7.4V Li-ion configuration that communicates charge state back to the speaker's onboard fuel gauge. This replacement carries the same cell arrangement and BMS signalling, so the battery indicator reads accurately after a full charge cycle.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on a BeoPlay A1 unit. The BMS balanced both cells correctly, protection circuitry tripped at the expected low-voltage cutoff, and the speaker's charge LED tracked state-of-charge without error flags.
• Fuel gauge reset after install: After fitting this battery, run the A1 from full charge down to auto-shutoff once before recharging. Skipping this step can leave the fuel gauge reading from stale calibration data, causing the indicator to show empty well before the cell is actually depleted.

Capacity fade on the BeoPlay A1 from constant top-off charging

The A1 is typically left on a desk and plugged in whenever it dips below 80%, which is one of the fastest ways to degrade a Li-ion cell. Shallow cycling keeps the cell in a high-voltage state for extended periods, accelerating electrolyte oxidation and reducing usable capacity over months. The speaker's fuel gauge compounds this — it recalibrates against actual measured discharge, so if it never sees a full discharge, it starts trimming its reported capacity estimate. The fix is a full discharge to auto-shutoff at least once a month before recharging to 100%.

Audio distorting before the battery indicator reaches empty

At high volume, the A1's amplifier draws a sharp current spike that the aging cell can't sustain without voltage sag. When cell voltage sags under combined amplifier and Bluetooth radio load, the amp clips before the battery indicator has dropped to the low-battery threshold — so the speaker sounds distorted but the LED still shows charge remaining. This is a cell condition issue, not a speaker fault. Replacing the battery restores the cell's internal resistance to spec, eliminating the sag that causes clipping. After fitting a new pack, verify the distortion clears at full volume with the cell above 7.0V under load.