Thomson AKU400 Replacement Battery 6V 4200mAh Ni-MH

Thomson AKU400 Series — 6V Ni-MH 4200mAh Replacement Battery

This is a 6V 4200mAh Ni-MH replacement battery for Thomson cameras including the AKU400, BP33, BP40, and BP42 series. It powers the camera's imaging, flash, and onboard processing functions. Capacity is 4200mAh (25.2Wh), matching the original cell specification.

• AKU400 and BP-series fit: These Thomson models share the same 6V Ni-MH cell format, connector pinout, and BMS handshake protocol. Swapping between these models does not require a different battery — the voltage rail and physical housing are identical across the group.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on compatible Thomson camera bodies. The BMS accepted the cell cleanly, flash recharge current held steady across the discharge curve, and the protection circuit triggered correctly at the low-voltage cutoff threshold.
• First-install charge protocol: Insert the battery and run one complete charge cycle inside the camera body or OEM charger before shooting. Some Thomson BMS systems require this to calibrate the battery-remaining display against the new cell's discharge curve — skipping it can cause the indicator to read inaccurately from the first shot.

Flash output dropping mid-shoot on a fresh Thomson battery

Flash recharge draws a sharp current spike from the cell each time the capacitor refills. Ni-MH cells show measurable internal resistance rise as discharge progresses, which extends capacitor recharge time even when the battery indicator still shows charge remaining. On the AKU400, this appears as slower recycle times and softer flash output in the back half of a shoot. The fix is to treat flash-heavy sessions as high-drain use — carry a second charged cell and swap before the first cell drops below 5.5V under load.

Battery percentage jumping erratically on the AKU400 display

The AKU400's fuel gauge maps voltage thresholds to percentage steps calibrated to the original cell's discharge curve. A new or recently stored Ni-MH cell has a slightly different resting voltage profile, which can cause the indicator to skip between readings — jumping from 80% to 40% without warning. This is a display calibration issue, not a cell fault. Run two full charge-discharge cycles through the camera body and the gauge will re-align to the replacement cell's actual curve.