BenQ-Siemens P51 Compatible Battery 3.7V 1400mAh

BenQ-Siemens P51 — 3.7V Li-Polymer Replacement Battery (2C.2G3.D0.101)

This 3.7V 1400mAh Li-Polymer cell replaces the original battery in the BenQ-Siemens P51 smartphone. It matches the OEM part number 2C.2G3.D0.101 and fits the P51's battery compartment at 68.30 × 45.27 × 7.88mm. Capacity is rated at 5.18Wh — identical to the original specification.

• P51 platform fit: The P51 uses a single-cell Li-Polymer pack with a low-profile pouch form factor. The connector pinout and BMS communication lines match the OEM configuration, so the fuel gauge IC on the P51's motherboard reads charge state correctly from first boot.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge passes on a P51 unit, monitoring BMS cutoff thresholds at both ends. The cell held stable voltage across load pulses from the modem and display backlight without tripping low-voltage protection prematurely.
• Fuel gauge recalibration on first use: After installing this cell, disable any fast-charge mode and run one full discharge-to-charge cycle before normal use. This gives the P51's fuel gauge IC time to map its coulomb counter against the new cell's actual discharge curve before high-current charging begins on an uncalibrated cell.

Why the P51 reports wrong battery percentage after a cell swap

The P51's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual chemistry, so the reported percentage drifts — often reading full when the cell is not, or dropping sharply near the end. The fix is a single complete discharge down to auto-shutdown, followed by a full charge to 100% without interruption. After that cycle, the coulomb counter resets its baseline against the new cell and percentage reporting stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC is still calibrated to the old cell's voltage curve and the new cell hits its voltage cliff earlier than the OS expects. Under modem transmission load or full-brightness screen use, the cell voltage drops faster than the predicted curve, and the BMS trips protection before the OS has issued a low-battery warning. Run one full unconstrained discharge cycle — let the phone shut itself off at its hardware cutoff — then charge fully to 4.2V. That re-anchors the gauge and eliminates the early-shutdown symptom.