About this replacement
Bang & Olufsen BeoPlay A1 — 7.4V Li-ion Replacement Battery (C129D3)
This 7.4V 2600mAh Li-ion battery replaces the original C129D3 cell in the Bang & Olufsen BeoPlay A1 portable Bluetooth speaker. It restores wireless audio playback when the original pack has degraded past useful capacity. Dimensions are 68.56 × 38.10 × 19.90mm — check these against your existing cell before fitting.
- BeoPlay A1 fit: The A1 uses a single-cell 7.4V pack with a specific connector orientation and BMS communication line. This replacement matches that connector and communicates correctly with the A1's onboard charge controller so the indicator LEDs report accurate charge state.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on the BeoPlay A1 platform. The BMS handled charge termination cleanly and the protection circuit tripped at the correct low-voltage threshold without locking out the speaker's power logic.
- Fuel gauge reset after swap: After fitting this battery, run the A1 down to the low-battery warning before charging it fully. The speaker's fuel gauge calibrates against a known full cycle — skipping this step causes the charge indicator to misread remaining capacity from the first use.
Why the BeoPlay A1 reads full charge but audio cuts out after extended play
The A1's charge indicator samples cell voltage at low draw — it can show three LEDs while the cell is too weak to sustain the amplifier under real audio load. As the amp pulls current during playback, terminal voltage sags below the BMS protection threshold and the speaker shuts down. A degraded original cell accelerates this: internal resistance rises with age, so voltage sag under amp current becomes severe even when resting voltage looks healthy. A fresh cell with low internal resistance holds terminal voltage steady through the amplifier's current peaks.
BeoPlay A1 not waking from USB-C charge after full discharge
If the A1 was left discharged for weeks, the cell voltage may have dropped below the USB-C PD minimum acceptance threshold — typically under 2.5V per cell — and the charger won't negotiate power delivery to a pack that low. Connect the speaker to a 5V USB source rather than a high-wattage PD charger and leave it for 15–20 minutes; the onboard controller uses a trickle pre-charge path at 5V to recover the cell voltage to a level where full charging can resume. If the speaker still shows no response after that window, check the USB-C port for debris before assuming cell failure. Target cell recovery voltage before normal charging resumes is approximately 3.0V per cell.