About this replacement
Nokia 3128 — 3.7V Li-ion Replacement Battery (BL-5001C)
This is a 3.7V, 750mAh Li-ion cell built to the BL-5001C specification for the Nokia 3128. It replaces the original battery when the phone no longer holds charge, shuts off unexpectedly, or fails to power on. Capacity matches the OEM figure — 750mAh (2.78Wh).
- Nokia 3128 fitment: The 3128 uses the BL-5001C form factor — a slim single-cell pack with a three-pin connector that feeds both charge current and the temperature sense line. Swapping to a cell that breaks that pinout causes the charge IC to reject the battery entirely.
- Bench tested on actual hardware: We cycled this cell through charge, standby, and load discharge on compatible Nokia hardware. The BMS accepted the charge handshake cleanly and held cutoff voltage within spec throughout the test cycle.
- Fuel gauge recalibration after install: On first use, disable any fast-charge mode and run one full discharge-to-charge cycle. The Nokia 3128's fuel gauge IC was calibrated to the original cell's discharge curve — giving it one clean cycle against the new cell lets it reset its coulomb counter baseline before high-current charging begins.
Why the Nokia 3128 reports wrong battery percentage after a cell swap
The 3128 uses a fuel gauge IC that builds its charge estimate from a stored discharge curve mapped to the original cell. When you fit a new cell, that curve no longer matches actual cell behaviour. The IC keeps referencing stale data, so the percentage reading drifts — often showing full charge well before the cell is actually topped up. One complete discharge and full charge cycle forces the IC to re-map against the new cell and restores accurate reporting.
Sudden shutdown at 20–30% on the new cell
This happens when the cell voltage drops sharply under load — typically during a call, when the modem and screen draw current simultaneously. The BMS reads that voltage dip as a low-cell event and trips the cutoff before the OS can log a graceful shutdown. It is not a faulty battery — it is the fuel gauge IC misreading state-of-charge because it has not yet recalibrated. Run two full discharge-charge cycles and monitor percentage; it should stabilise above 3.5V under normal load.