About this replacement
Sony Ericsson P800 Series — 3.7V Li-Polymer Replacement Battery (BST-15)
This is a 3.7V, 1400mAh Li-Polymer cell replacing the BST-15 in the Sony Ericsson P800, P800C, P802, and P900 series smartphones. These were early touchscreen PDA-phones from the early 2000s, and original BST-15 cells are long past their cycle life. This replacement restores full voltage to the handset's calling, PDA, and display functions.
- P800, P800C, P802, and P900 fitment: All four models share the same BST-15 footprint, connector orientation, and 3.7V nominal voltage rail. The physical cell dimensions and contact placement are identical across the series, so one cell covers the full group.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on a P900 handset. The BMS accepted the new cell without fault flags, charge termination triggered correctly at 4.2V, and cutoff activated cleanly at the low-voltage threshold without hard lockout.
- Fuel gauge recalibration on first use: After installing this cell, run one complete discharge to shutdown before recharging. The P800 series uses a basic coulomb counter that retains calibration data from the old degraded cell — a full cycle forces it to reset against the new cell's actual discharge curve and report percentage accurately.
Why the P800 reports wrong battery percentage after a cell swap
The P800's fuel gauge IC tracks charge by counting current in and out of the cell over time. When the original degraded cell is replaced, the IC still holds the old cell's capacity baseline — often far below 1400mAh. Until recalibrated, the percentage readout is effectively guesswork against the wrong reference. One complete discharge-to-shutdown followed by a full uninterrupted charge resets the baseline. After that single cycle, percentage reporting stabilises against the new cell's actual 1400mAh capacity.
Sudden shutdown at 20–30% on the replacement cell
This happens when the fuel gauge IC has not yet recalibrated and the displayed percentage is ahead of the cell's actual state of charge. The P800's screen and GSM modem together draw enough current to pull cell voltage below the BMS cutoff threshold before the percentage reaches zero. The shutdown is not a faulty cell — it is the gauge reading incorrectly. Run one full discharge cycle as described above, and the cutoff will shift back to the correct low-voltage point near 3.0V per cell.