About this replacement
Microsoft Surface Pro — 7.4V Li-Polymer Replacement Battery (P21GU9)
This is a 7.4V, 5670mAh Li-Polymer battery for the Microsoft Surface Pro and Surface Pro 64GB, including the Surface Pro 2. It matches the original cell dimensions at 244.80 × 96.20 × 5.16mm and connects to the same charging circuit. When the original cell degrades and the device loses portability, this unit restores it.
- Surface Pro and Surface Pro 2 fitment: Both generations use the same 7.4V two-cell Li-Polymer pack with the same BMS connector and voltage rail. The P21GU9 part number covers both. No adapter or modification needed.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Surface Pro unit. The BMS handshake completed on first connection, charge IC accepted current without fault codes, and the pack held voltage within spec through a full cycle.
- Fuel gauge recalibration after swap: After fitting this battery, run the Surface Pro down to automatic shutoff, then charge uninterrupted to 100% before unplugging. This single full cycle recalibrates the fuel gauge IC against the new cell and corrects the inaccurate percentage readings that appear after most battery replacements.
Surface Pro shutting down at 15–25% remaining
This is a voltage cliff issue, not a defective cell. When the display backlight and Wi-Fi radio draw current simultaneously, the combined load pulls cell voltage below the BMS cutoff threshold before the fuel gauge reaches zero. The gauge was calibrated against the old cell's discharge curve, so it reports 20% while the new cell's actual voltage is already at cutoff. One full discharge-to-shutoff cycle followed by an uninterrupted charge to 100% resets the curve and stops the early shutdown.
Surface Pro percentage dropping faster than expected from 100%
Fast percentage drop from a full charge is fuel gauge drift — the IC is still using the old cell's capacity map. The new 5670mAh cell has a different discharge curve, and until the IC recalibrates, it misreads early voltage drop as large capacity loss. This is not a capacity defect. Discharge the device fully to automatic shutoff, then charge to 100% in one uninterrupted session. After that cycle, the percentage display should track accurately.