Brother BA-400 Superpower Note PN4400 Replacement Battery 6V 1500mAh

Brother Superpower Note PN4400 Series — 6V Ni-MH Replacement Battery (BA-400)

This is a 6V, 1500mAh Ni-MH replacement battery for the Brother Superpower Note portable electronic notebook series. It carries OEM part number BA-400 and fits the PN4400, PN5700DS, PN8500MDS, and PN8500MDSE, along with four additional models in the same line. Capacity is 9Wh, matching the original cell specification.

• PN4400 series compatibility: These models share the same 6V battery bay, connector pinout, and charge-control circuit. A single BA-400 cell covers the full range without any wiring or adapter changes.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Brother Superpower Note unit. The charge circuit accepted the Ni-MH chemistry correctly, the protection circuit tripped at the expected voltage floor, and the device powered on cleanly at full charge.
• First-cycle conditioning for the PN series: After fitting this cell, run the device until it shuts off from low battery, then charge it uninterrupted to full. The PN series charge controller recalibrates its fuel gauge against the actual cell capacity during that first complete cycle — skip it and the battery indicator will read inaccurately for weeks.

Why the PN8500MDS battery indicator stays stuck after a cell swap

The Brother Superpower Note line uses a simple fuel gauge IC that tracks cumulative charge and discharge against a stored reference value. When you swap to a new cell, that reference value is still calibrated to the old, degraded battery. The gauge reads the new cell as though it were the worn-out one, so the display appears stuck or jumps erratically. One complete discharge-to-cutoff followed by a full uninterrupted charge resets the reference and brings the indicator back into accurate range.

Device shuts down abruptly while still showing charge remaining

Ni-MH cells have a steep voltage cliff near end-of-discharge — voltage drops sharply in the final capacity window. If the battery indicator is not recalibrated after a swap, the device cuts power before the gauge reaches zero because the voltage collapses faster than the uncalibrated gauge predicts. This is not a fault with the replacement cell. Run one full conditioning cycle; the charge controller will map the new cell's actual voltage curve and the shutdowns will stop. Target a resting voltage of around 7.2V at full charge across the six cells.