About this replacement
I-Mate Ultimate 8502 — 3.7V Li-Polymer Replacement Battery (303POL0000A)
This 3.7V, 1600mAh Li-Polymer cell is a direct swap for the battery in the I-Mate Ultimate 8502 Windows Mobile smartphone. It matches the original part number 303POL0000A and the alternate reference 745WS00685. At 5.92Wh and 66.89 x 44.37 x 4.71mm, it fits the original battery bay without modification.
- Ultimate 8502 fitment: The 8502 uses a dedicated Li-Polymer cell with a fixed connector pinout tied to the device's charge IC and fuel gauge circuit. This cell matches that pinout, voltage rail, and physical footprint — so the BMS handshake completes correctly on first boot.
- Bench tested on actual hardware: We cycled this cell through a full charge and discharge sequence, monitoring BMS cutoff behaviour at both the high-voltage ceiling and the low-voltage floor. The protection circuit tripped correctly at each threshold with no false cutoffs under steady load.
- Fuel gauge recalibration on first use: After installation, run one complete discharge-charge cycle with fast charging disabled. This gives the fuel gauge IC a clean reference curve against the new cell before high-current charging begins on an uncalibrated counter.
Why the Ultimate 8502 reports wrong battery percentage after a cell swap
The 8502 uses a fuel gauge IC that tracks charge state by measuring cumulative current flow — a coulomb-counting method. When you swap the cell, the IC still holds the discharge curve it built around the old, degraded cell. The new cell has a different impedance profile, so the gauge drifts from the actual state of charge immediately. One full discharge-to-cutoff and charge-to-100% cycle resets the reference and brings the percentage readout back in line with real capacity.
Sudden shutdown at 20–30% on the replacement cell
This happens when the fuel gauge IC has not yet recalibrated to the new cell's discharge curve. At around 3.6–3.5V, the modem radio or screen backlight draws a current spike the gauge wasn't predicting at that state of charge, and the BMS cuts power to protect the cell. It looks like a random crash but it is a voltage cliff the old curve did not map correctly. Complete two full discharge-charge cycles without fast charging — after that, the coulomb counter tracks the new curve accurately and the shutdowns stop.