About this replacement
Marshall Kilburn II / Kilburn III — 7.4V Li-ion Replacement Battery (TF18650-3200-4S2PA)
This is a 7.4V Li-ion replacement battery rated at 5200mAh (38.48Wh) for the Marshall Kilburn II, Kilburn II V2, and Kilburn III portable Bluetooth speakers. It replaces OEM part numbers TF18650-3200-4S2PA, C196A1, 7252-XML-SP, and C129C3. Dimensions are 69 × 37 × 37mm — confirm your pack matches before ordering.
- Kilburn II, Kilburn II V2, and Kilburn III compatibility: All three models share the same internal battery bay dimensions, connector pinout, and BMS communication protocol. The 7.4V nominal voltage rail feeds both the Class D amplifier board and the Bluetooth radio module, so a single cell spec covers all three generations without modification.
- Bench tested on actual hardware: We ran the pack through charge and discharge cycles on a Kilburn II unit and monitored BMS handshake, cell balancing across both parallel strings, and protection trip thresholds. The BMS engaged overcurrent cutoff correctly on a simulated high-draw event and recovered cleanly after a reset cycle.
- Fuel gauge calibration for daily-use speakers: Discharge below 20% at least once a month before recharging. The Kilburn's onboard fuel gauge drifts when the pack is constantly topped off from 50–80% — the gauge loses its low-end reference point and starts reporting inaccurate charge levels within a few months.
Why the Kilburn II voltage sags when the amplifier hits loud passages
The Kilburn II's Class D amplifier draws current in sharp spikes during loud audio peaks — the combined load of the amplifier and the Bluetooth radio can pull significantly more than the average idle draw in a fraction of a second. An aged or degraded cell can't respond fast enough, and internal resistance causes a momentary voltage sag. The BMS reads this sag as a low-cell event and may throttle output or trip protection. A fresh pack with lower internal resistance handles these transient spikes without triggering the cutoff.
Speaker reads full charge but audio cuts out after extended play
This symptom points to fuel gauge drift, not a faulty new battery. The onboard gauge loses calibration when a degraded original pack never reached a true full discharge — the gauge's low-end reference becomes inaccurate over time. After fitting the replacement, charge to 100% uninterrupted, then discharge fully in a single session without topping off. This recalibrates the low-end reference and the gauge will read accurately again. Confirm the pack voltage sits between 8.2V (full) and 6.0V (cutoff) with a multimeter at the charge port if the behaviour persists.