About this replacement
Marshall Kilburn II / Kilburn III — 7.4V Li-ion Replacement Battery (TF18650-3200-4S2PA)
This 7.4V 6400mAh (47.36Wh) Li-ion battery fits the Marshall Kilburn II, Kilburn II V2, and Kilburn III portable Bluetooth speakers. It replaces the original pack when the speaker no longer holds a useful charge away from the wall. OEM part numbers TF18650-3200-4S2PA, C196A1, 7252-XML-SP, and C129C3 all cross to this cell configuration.
- Kilburn II, II V2, and III compatibility: All three models share the same 7.4V two-cell series architecture, the same physical housing dimensions, and the same BMS handshake protocol — so one pack covers the entire Kilburn line without connector or firmware conflict.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on a Kilburn II unit. The BMS engaged cell balancing correctly at 8.4V full charge, and the low-voltage cutoff triggered cleanly at 6.0V — no erratic shutdowns or charge-refusal events observed.
- Monthly discharge cycle for Kilburn owners: If the speaker lives on a desk and gets topped off constantly, let it run down below 20% at least once a month before plugging in — continual shallow cycling causes fuel gauge drift and accelerates capacity fade on these 18650 cells.
Bluetooth dropping at high volume on the Kilburn II
At maximum volume the Kilburn II amplifier draws a sharp current spike — stacked on top of the Bluetooth radio's steady draw, total load can momentarily sag cell voltage below the BMS protection threshold. An aged or partially depleted pack with higher internal resistance sags further and faster under this combined load. The BMS reads the sag as an undervoltage condition and trips, cutting the output mid-track. A fresh pack with low internal resistance sustains the combined draw without hitting that threshold, keeping the radio and amp running cleanly through loud passages.
Audio distorting before the battery indicator shows empty
The Kilburn II's indicator reads state of charge from cell voltage at rest, not under load. As a pack ages, the gap between resting voltage and loaded voltage widens — the indicator shows two bars while the amplifier is already seeing a sagged rail too low to deliver clean output. The amp clips under that voltage sag before the indicator ever reaches one bar, which is why distortion appears while the display still looks healthy. Charging the pack fully to 8.4V and checking whether distortion moves later in the discharge curve will confirm whether the old cell or the new one is responsible.