About this replacement
Lenovo Zuk Z1 / Z1221 — 3.8V Li-Polymer Replacement Battery (BL255)
This is a 4000mAh Li-Polymer battery for the Lenovo Zuk Z1 (including Z1221 and Z1221 Dual SIM variants). It replaces OEM part BL255 and runs at 3.8V nominal, matching the original cell specification. If the stock battery swells, drains unusually fast, or fails to hold a charge, this is the direct replacement cell.
- Zuk Z1 / Z1221 platform compatibility: All three model variants — Z1, Z1221, and Z1221 Dual SIM — share the same battery bay dimensions, connector pinout, and BMS handshake protocol. One cell covers all three configurations without modification.
- Bench tested on actual hardware: We ran the BL255 replacement through full charge-discharge cycles on the Zuk Z1 platform. The BMS accepted the cell without error flags, and the charge IC stepped through trickle, CC, and CV phases correctly.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. The fuel gauge IC on the Zuk Z1 calibrates its coulomb counter against the discharge curve — running a full cycle on standard charge gives it accurate baseline data before fast charging pushes higher current into an uncalibrated cell.
Sudden shutdown at 20–30% on the Zuk Z1 after a cell replacement
The Zuk Z1 uses a software fuel gauge that was calibrated to the original cell's discharge curve. A new cell has a slightly different voltage-to-capacity curve, so the gauge misreads the remaining charge. Under screen or modem load, the cell voltage drops faster than the gauge expects, and the phone cuts power before the percentage reaches zero. This is not a defective battery — it is the fuel gauge IC chasing stale calibration data. Run two full discharge-charge cycles on standard charge (not fast charge) and the gauge recalibrates. After recalibration, shutdowns at 20–30% should stop.
Phone feels warm near the battery bay on the first charge after fitting BL255
A freshly installed Li-Polymer cell has higher internal impedance than one that has completed several cycles. During the constant-current phase, the charge IC has to work harder to push current through that higher impedance, which produces more heat at the cell surface. This is normal on the first one or two charges and not a sign of a faulty cell. If the phone stays warm past the second full charge, check that the charging cable and adapter output match the Zuk Z1 spec — input must not exceed 5V/2A on standard charge mode. Warmth that persists beyond early cycles points to a charger mismatch rather than the cell itself.