Sharp AD-N55BT Replacement Battery 1.2V 1200mAh Ni-MH

Sharp MD-MT66 Series — 1.2V Ni-MH Replacement Battery (AD-N55BT)

This is a 1.2V 1200mAh Ni-MH replacement battery for the Sharp MD-MT66 portable MiniDisc recorder/player. It uses the OEM part number AD-N55BT and also fits the MD-MT77, MD-MT770, and MD-MT877, along with 29 additional Sharp MiniDisc models. Capacity figures are from the product data — 1200mAh at 1.44Wh.

• MD-MT66 series compatibility: These Sharp MiniDisc models share the same single-cell Ni-MH bay, voltage rail, and connector pinout. The AD-N55BT slides into each without modification — no adapter, no rewiring.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the MD-MT66 platform and confirmed the onboard charge circuit accepted it without fault codes. The BMS tracked charge termination correctly at full cell voltage.
• First charge after a long gap: MiniDisc players from this era can enter a deep-discharge protection state after months on the shelf. If the device won't respond at all, connect the charger and leave it for 30 minutes before attempting to power on — the circuit needs a slow trickle to exit protection before it accepts normal charge current.

Playback cutting out before the battery indicator shows empty

The MD-MT66 audio amplifier draws a brief current spike during playback — especially when the laser seeks a new track. As the cell approaches end of discharge, its internal resistance rises and voltage sags under that spike. The device interprets the sag as a power fault and cuts out, even though the indicator still shows charge remaining. A fresh cell with low internal resistance eliminates this sag and restores clean playback through the full discharge cycle.

Battery percentage jumping erratically after swapping the cell

The MD-MT66 estimates charge level using voltage thresholds calibrated to the original cell's discharge curve. A new Ni-MH cell has a flatter discharge curve and slightly different resting voltage at each state of charge. This causes the indicator to jump or read incorrectly for the first few cycles. Run two or three full charge-to-discharge cycles and the voltage-threshold indicator will recalibrate to the new cell's actual curve.