About this replacement
Samsung SGH-P850 / SGH-P858 — 3.7V Li-ion Replacement Battery (BST307ASE)
This is a 3.7V 850mAh Li-ion replacement battery for the Samsung SGH-P850 and SGH-P858 mobile phones. It carries OEM part number BST307ASE and slots directly into both models without modification. If your original cell no longer holds a usable charge, this restores the phone to working condition.
- SGH-P850 and SGH-P858 compatibility: Both handsets share the same battery bay geometry, connector pinout, and 3.7V single-cell architecture. The BST307ASE spec covers both — no adapters or wiring changes needed.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the SGH-P850 platform. The BMS accepted the charge IC handshake cleanly, hit full cutoff at 4.2V, and held the voltage curve through a complete draw-down without triggering an early protection trip.
- Fuel gauge recalibration on first use: On first install, disable any fast-charge accessory and run one full discharge-to-charge cycle at standard rate. This lets the phone's fuel gauge IC map the new cell's discharge curve before it starts reporting accurate percentages. Skipping this step is the most common reason users see erratic readings after a swap.
Why the SGH-P850 reports wrong battery percentage after a cell swap
The SGH-P850 uses a fuel gauge IC that builds its capacity model from past charge and discharge cycles. When you replace the cell, the IC still holds the old degraded cell's curve in memory. Until it relearns against the new cell, the percentage reading is pulled from that stale map. One full discharge below 3.4V followed by a complete charge to 4.2V forces the IC to flush the old model and write a new one. After that cycle, percentage accuracy normalises.
Sudden shutdown at 20–30% on the replacement cell
This happens because the fuel gauge IC hasn't yet mapped the new cell's true voltage cliff. Under load — screen on, active call, or radio scanning — the cell voltage dips sharply below what the IC predicted, and the protection circuit cuts power before the displayed percentage reaches zero. It looks like a dead battery at 25%. The fix is the same recalibration cycle: drain the phone to automatic shutdown, then charge uninterrupted to 100% at standard rate. After one complete cycle, the IC anchors its low-voltage threshold to the correct point on the new cell's curve.