Sony Ericsson 788 Replacement Battery 4.8V 750mAh

Sony Ericsson 788 / T18 Series — 4.8V Ni-MH Replacement Battery (BKB1931022R2A)

This is a 4.8V, 750mAh Ni-MH replacement battery for the Sony Ericsson 788 and related handsets. It matches the original BKB1931022R2A and BKB1931030R1A part numbers. Compatible fits include the 788, 768, T18, and T10 series.

• 788, 768, T18, T10 platform fit: These models share a common 4.8V battery bay, connector pinout, and charge management circuit. The BMS handshake uses the same termination threshold across the range, so one cell fits the full group without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a T18 and 788 unit. The charge IC accepted the cell on first insertion, delta-V cutoff triggered correctly at full charge, and the protection circuit interrupted current cleanly at the low-voltage floor.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge or rapid-charge mode if available, then run one full discharge-to-charge cycle before normal use. This gives the fuel gauge IC time to map the new cell's discharge curve before reporting capacity to the OS.

Why the 788 reports wrong battery percentage after a cell swap

The fuel gauge IC on these Sony Ericsson handsets builds its percentage model from the previous cell's discharge curve stored in memory. A new Ni-MH cell has a different internal resistance and voltage profile, so the IC's stored reference no longer matches reality. The phone reads voltage at a point it associates with 60%, but the new cell is actually at 80% — or vice versa. One complete discharge cycle down to auto-off, followed by a full charge, forces the IC to rewrite its curve against the actual cell behaviour.

Sudden shutdown at 20–30% on the replacement cell

Ni-MH cells have a steep voltage drop near the end of their discharge curve. If the fuel gauge IC is still calibrated to the old cell, it underestimates how close the new cell is to its cutoff voltage. Under load — a call, backlight, or keypad activity — the cell voltage drops fast enough to cross the protection circuit's cutoff before the percentage reading catches up. The phone shuts down hard with the display still showing 20–30%. Run one full discharge cycle to auto-off and recharge fully; this recalibrates the IC to the correct voltage-to-capacity mapping on the new cell.