About this replacement
Bea-fon SL651 — 3.7V Li-Polymer Replacement Battery (SL651a)
This 3.7V, 900mAh Li-Polymer cell replaces the original SL651a battery in the Bea-fon SL651 mobile phone. It restores power to calling, messaging, and standby functions on this device. Dimensions are 51.30 × 42.60 × 4.50mm — confirm physical fit before installation.
- SL651 and SL651a compatibility: Both models use the same OEM part number SL651a, share the same connector pinout, and accept the same BMS handshake at 3.7V nominal. One cell covers both variants.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the SL651 platform. The BMS accepted the cell without fault codes, and charge termination triggered correctly at full capacity.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging begins on an uncalibrated cell.
Sudden shutdown at 20–30% on the replacement cell
A new Li-Polymer cell has a slightly different voltage-versus-capacity curve than the original. The SL651's fuel gauge IC was calibrated to the old cell, so it misreads the remaining charge under load. When the modem or screen draws peak current, the cell voltage drops below the BMS cutoff threshold before the percentage readout reaches zero. Run two full discharge-to-charge cycles without interruption. After the second cycle, the coulomb counter recalibrates and the shutdowns typically stop.
OS reporting wrong battery percentage after a cell swap
The SL651 uses a fuel gauge IC that tracks charge by counting coulombs against a stored cell profile. Swapping the cell resets the physical reference but not the stored profile, so the percentage display drifts — often reading 100% when the cell is partially charged, or dropping suddenly. Let the phone discharge fully until it powers off, then charge uninterrupted to 100%. One full cycle is usually enough to anchor the gauge to the new cell's actual capacity at 3.7V nominal.