Nokia 8820 BL-6U Replacement Battery 3.7V 750mAh

Nokia 8820 / Erdos — 3.7V Li-ion Replacement Battery (BL-6U)

The BL-6U is a 3.7V, 750mAh lithium-ion cell that fits the Nokia 8820 and Erdos mobile phones. It slots directly into the original battery bay using the same contact layout as the factory cell. Capacity figures here come from the product specification — 750mAh / 2.78Wh.

• 8820 and Erdos compatibility: Both handsets run off the same BL-6U footprint and 3.7V nominal rail. The contact spacing, connector polarity, and BMS communication lines match across both models, so the same cell fits without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the 8820 platform. The onboard BMS accepted charge current without tripping, and the protection circuit responded correctly to low-voltage cutoff at the expected threshold.
• Fuel gauge recalibration after swap: After fitting this cell, run one full discharge to automatic shutdown followed by a complete uninterrupted charge before enabling any battery-saving or fast-charge modes. This gives the phone's coulomb counter a clean reference curve against the new cell before it starts making load predictions.

Why the Nokia 8820 reports wrong battery percentage after a cell swap

The 8820 uses a fuel gauge IC that builds its percentage estimate from a stored discharge curve tied to the original cell's impedance profile. When a new cell goes in, that stored curve no longer matches the actual electrochemistry in the new pack. The IC keeps calculating against old data until it has a fresh full cycle to work from. One complete drain-to-shutdown followed by a full charge resets the reference point and brings the readout back into alignment.

Sudden shutdown at 20–30% remaining on the new cell

This happens when the cell voltage drops below the modem or display's minimum operating threshold under load, even though the fuel gauge still shows charge remaining. A fresh cell with an uncalibrated coulomb counter can misread state-of-charge by 15–25% in early cycles. The phone cuts out not because the battery is faulty, but because actual terminal voltage has hit the cutoff floor before the gauge catches up. After two full discharge-charge cycles, the gauge recalibrates and the shutdown point moves back to below 10%.