Nokia C21 BL-29CI Replacement Battery 3.8V 2700mAh

Nokia C21 / C20 / C10 — 3.8V Li-ion Replacement Battery (BL-29CI)

This is a 3.8V, 2700mAh Li-ion replacement battery for the Nokia C21, C20, and C10 smartphones. It replaces OEM part numbers BL-29CI and BL-29CL. Fit this battery when the original cell no longer holds charge through a normal day of use.

• C10, C20, and C21 compatibility: All three models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. Nokia used the same power management IC across this entry-level series, so one cell covers all three variants without modification.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the C21. The BMS accepted the cell immediately, charge termination triggered correctly at full capacity, and the protection circuit responded to overcurrent conditions within spec.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell — preventing early percentage jumps.

Sudden shutdown at 20–30% on the Nokia C21 after a cell swap

This happens when the fuel gauge IC is still calibrated to the old, degraded cell's voltage curve. A fresh cell holds voltage higher for longer, then drops steeply near the bottom. The phone's power manager hits what it reads as a cutoff threshold — but the actual cell still has charge left. Run one full discharge to near-zero and a full charge without interruption. After that cycle, the coulomb counter re-anchors to the new cell's actual curve and the shutdowns stop.

Phone reports wrong battery percentage after fitting the BL-29CI

The Android fuel gauge on the C21 stores learned capacity data from the previous cell. When a new cell goes in, that stored data no longer matches reality — so percentage readings jump or stall at odd values. The fix is a full calibration cycle: drain the phone until it powers off automatically, then charge uninterrupted to 100% without using the phone. One complete cycle is usually enough to bring the reported percentage in line with actual cell state. If jumping persists after two cycles, check that the battery connector is fully seated — a loose pin causes intermittent voltage reads that confuse the fuel gauge IC.