Microsoft Surface P21GK3 Replacement Battery 7.4V 4250mAh

Microsoft Surface / Surface RT / Surface Pro 2 — 7.4V Li-Polymer Replacement Battery (P21GK3)

This 7.4V, 4250mAh Li-Polymer battery replaces the original P21GK3 cell in the Microsoft Surface, Surface RT, and Surface Pro 2 tablets. It restores power to the display, processor, and internal components when the factory battery no longer holds a charge or fails to power the device at all. Voltage and capacity match OEM spec: 7.4V, 4250mAh (31.45Wh).

• Surface / Surface RT / Surface Pro 2 compatibility: These three models share the same battery form factor, connector pinout, and BMS handshake protocol — the P21GK3 cell fits all three without modification to the battery bay or flex cable.
• Bench tested on actual hardware: We cycled this battery through charge and discharge on a Surface Pro 2 unit, confirmed BMS communication with the charge IC, and verified the cell holds voltage under combined display and Wi-Fi load without triggering a low-voltage cutoff prematurely.
• Fuel gauge recalibration after install: After fitting this battery, run one full discharge to automatic shutoff, then charge uninterrupted to 100%. The fuel gauge IC calibrates against the new cell during that first full cycle — skipping it causes the percentage readout to drift or display inaccurate values from the first use.

Surface Pro 2 shutting down at 15–25% remaining

The Surface Pro 2 draws a combined load from the display backlight and Wi-Fi radio that creates a voltage dip the fuel gauge IC interprets as a near-empty cell. On an aged or freshly swapped battery that hasn't been calibrated, the BMS triggers a protective shutoff before the actual cell capacity is exhausted. The display alone can pull enough current to push the cell voltage below the BMS cutoff threshold under certain brightness settings. Run the calibration cycle first — one full discharge to shutoff, then a full uninterrupted charge — before drawing conclusions about the battery's actual capacity.

Fast charging unavailable after battery swap on Surface RT

The Surface RT charge IC negotiates charging rate on the first accepted charge cycle after a battery replacement. Until that cycle completes, the device may default to a lower charge rate as a precaution. This is a firmware-level handshake between the charge controller and the new cell's BMS — not a fault with the battery or the charger. Complete one full charge from below 10% to 100% without interruption, and the charge IC will recognise the new cell and restore the normal charge rate.