Nokia BL-5K N85 Replacement Battery 3.7V 1000mAh

Nokia N85 / N86 / C7 Series — 3.7V Li-ion Replacement Battery (BL-5K)

This is a 3.7V, 1000mAh Li-ion cell built to the BL-5K specification. It fits the Nokia N85, N86, C7, and C7-00, along with four additional compatible models that share the same battery bay and connector. Physical dimensions are 46.89 × 44.00 × 5.37mm — match these before ordering if you're unsure which variant you have.

• N85, N86, C7 platform compatibility: These models share the same BL-5K footprint, contact layout, and 3.7V nominal voltage rail. The BMS handshake is consistent across the group, so the fuel gauge IC on each device reads the cell correctly once calibration completes.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a BL-5K compatible test rig. The BMS engaged cutoff correctly at the low-voltage threshold and accepted charge current without error flags at the charge IC.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge mode and run one complete discharge-to-charge cycle. The fuel gauge IC in these Nokia devices calibrates its coulomb counter against the new cell's discharge curve — skipping this step causes erratic percentage readings in early use.

Why the N85 and C7 report wrong battery percentage after a cell swap

Nokia's fuel gauge IC stores a discharge curve profile built up over hundreds of cycles on the original cell. When a new cell goes in, that stored curve no longer matches the actual cell chemistry. The IC then reports state-of-charge based on stale data, so the percentage shown can be off by 15–25% until recalibration. Run one full discharge down to automatic shutdown, then charge uninterrupted to 100% with the screen on to force the coulomb counter to rewrite the curve against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — typically when the modem transmits or the display brightness spikes — and crosses the BMS cutoff threshold before the fuel gauge registers the drop. The OS reads 25% remaining but the actual cell voltage has already hit 3.2V under peak draw. It's more common in the first few cycles before the fuel gauge IC has recalibrated. Complete two full discharge-charge cycles and the shutdown floor typically rises back to the correct 3.0V cutoff region.