AP21D8M Acer Nitro 16 Replacement Battery 15.4V 3550mAh

Acer Nitro 16 AN16-51 Series — 15.4V Li-ion Replacement Battery (AP21D8M)

This is a 15.4V, 3550mAh (54.67Wh) Li-ion battery for the Acer Nitro 16 AN16-51 series, including the AN16-51-74BV, AN16-51-78GX, AN16-51-56VT, and AN16-51-76BS. It replaces OEM part numbers AP21D8M and KT.0040G.015. Fit this when the original cell no longer holds a charge or has failed entirely.

• Nitro 16 AN16-51 compatibility: Every AN16-51 variant listed here shares the same 15.4V four-cell architecture, identical connector pinout, and the same BMS handshake protocol — so one replacement cell covers the full sub-series without modification.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the AN16-51 platform. The BMS negotiated correctly with the EC firmware, charge current stepped down as expected at top-of-range, and the protection circuit tripped at the correct under-voltage threshold under sustained CPU and GPU load.
• Post-install calibration on Nitro 16: After fitting, run the laptop down to hibernate cutoff under normal load, then charge uninterrupted to 100% without using the machine. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on this platform.

BIOS reporting poor battery health immediately after fitting a new cell

The Nitro 16 EC stores learned capacity data from the previous cell in non-volatile memory. When a new cell goes in, that old data conflicts with the EEPROM values from the replacement, and the BIOS flags the battery as degraded before it has run a single cycle. This is not a fault with the new cell. Run one full discharge-to-hibernate cycle followed by a full uninterrupted charge — the learn cycle rewrites the stored data against the actual cell chemistry. After two to three cycles, the health reading normalises.

Laptop shuts down at 20–30% charge shown on screen

This happens when the fuel gauge IC has not yet calibrated against the new cell's actual voltage curve. The reported percentage drifts ahead of the real state of charge, so the system hits the low-voltage protection threshold while the display still shows 20–30%. Under full CPU and display load, the voltage cliff arrives faster than the gauge expects. Force a full discharge to hibernate cutoff — do not interrupt it — then charge to 100%. Repeat this two to three times until the gauge tracks correctly down to 5% or below before shutdown.