Thomson AKU400 Replacement Battery 6V 2100mAh Ni-MH

Thomson AKU400 / BP40 Series — 6V Ni-MH Replacement Battery

This is a 6V 2100mAh Ni-MH replacement battery for Thomson vintage video cameras and camcorders, including the AKU400, BP33, BP40, BP42, and over 27 additional models from the same platform. It slots into the same battery bay as the original cell and delivers the same voltage rail the camera's recording circuits expect. Capacity is 2100mAh (12.6Wh), matching the original Thomson specification.

• AKU400 and BP-series platform: These Thomson models share a common 6V power rail, identical battery bay geometry, and the same connector orientation — which is why one cell covers the full range. The BMS on these cameras reads voltage directly rather than using a digital authentication handshake, so a correctly rated Ni-MH cell is accepted without firmware negotiation.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on Thomson camcorder-class hardware. The BMS accepted the cell on first install, held charge through full recording sessions, and showed no mid-cycle cutoff under combined sensor and viewfinder draw.
• First-use charge cycle on these bodies: Thomson camcorders of this era map their battery-remaining indicator to a fixed voltage-drop curve. Run one complete charge cycle through the camera body or OEM charger before shooting — this lets the indicator calibrate correctly against the new cell's discharge profile. Skipping this step often causes the display to show depletion earlier than actual cutoff.

Why the AKU400 battery indicator drops to zero before the cell is actually flat

Thomson camcorders from this generation use a voltage-threshold method to estimate remaining charge — they compare live cell voltage against a fixed lookup table burned into firmware. A new Ni-MH cell has a slightly different discharge curve shape than an aged original, so the camera reads the voltage crossover point earlier than the actual depletion point. The result is the indicator hitting zero while the cell still holds usable charge. This corrects itself after one or two full charge-discharge cycles, as the cell's voltage curve stabilises around 1.2V per cell at mid-discharge. No firmware change is needed — cycle the battery fully two to three times and the indicator tracks correctly.

Camera shuts down mid-recording despite showing battery remaining

This happens when the cell voltage sags briefly under the combined draw of the image sensor, recording circuits, and LCD — even if resting voltage looks healthy. Thomson camcorders of this age have a low-voltage cutoff set conservatively around 5.4V; a momentary sag under peak load trips the shutdown before the indicator has updated. It is more common in cold conditions, where Ni-MH internal resistance rises sharply. Warm the camera body to room temperature before recording and ensure the replacement cell has completed at least one full charge cycle before a long session.