Lenovo A5 BL291 Replacement Battery 3.85V 3900mAh

Lenovo A5 / L18021 — 3.85V Li-Polymer Replacement Battery (BL291)

This 3.85V, 3900mAh Li-Polymer cell replaces the original BL291 battery in the Lenovo A5 (model L18021) smartphone. It fits the stock battery bay without modification and connects via the original ribbon connector. Capacity figures come directly from our sourced cell spec — 3900mAh at 15.02Wh.

• A5 and L18021 compatibility: Both model numbers refer to the same physical handset sold across different regional markets. The battery bay dimensions, connector pinout, and BMS communication protocol are identical — one cell fits both.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on an A5 unit. The BMS accepted the charge IC handshake without error flags, and the protection circuit tripped correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard current. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before higher charge rates are introduced.

Why the Lenovo A5 reports wrong battery percentage after a cell swap

The A5 uses a fuel gauge IC that builds its battery model from accumulated charge and discharge data on the original cell. When a new cell goes in, the IC still holds the old cell's impedance and capacity curve in memory. This mismatch causes the reported percentage to drift from the actual state of charge — sometimes by 10–15 points. One full discharge down to automatic shutdown followed by an uninterrupted charge to 100% forces the coulomb counter to reset and relearn against the new cell. After that cycle, percentage reporting stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops below what the modem or display load demands, even though the reported percentage looks healthy. A fresh Li-Polymer cell has a steeper voltage cliff under high-current draw until it has been broken in through a few charge cycles. The phone's protection circuit interprets that voltage sag as a critically low cell and cuts power immediately. Run two to three full discharge-charge cycles — after that, the cell's internal resistance stabilises and the voltage cliff flattens. If shutdowns persist past cycle three, check that the battery connector ribbon is fully seated and making clean contact at all four pins.