About this replacement
Lenovo Z5 Pro (L78031 / L78032) — 3.85V Li-Polymer Replacement Battery (JR40)
This is a 3.85V, 3100mAh Li-Polymer replacement battery for the Lenovo Z5 Pro smartphone (model numbers L78031 and L78032). It carries OEM part number JR40 and slots into the same physical bay as the original cell. Voltage, capacity, and connector position match the factory specification.
- L78031 and L78032 compatibility: Both Z5 Pro variants share the same battery bay geometry, connector pinout, and BMS handshake protocol — one JR40 cell covers both boards without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the Z5 Pro board. The BMS accepted the cell, charge IC ramped normally, and protection cutoffs triggered at expected voltage thresholds on both ends of the curve.
- First-cycle fast charge hold-off: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC build an accurate discharge curve for the new cell before high-current charging pushes into an uncalibrated coulomb counter.
Sudden shutdown at 20–30% on the Z5 Pro after a cell swap
The Z5 Pro's fuel gauge IC retains the discharge curve of the old, degraded cell in its coulomb counter. When a fresh cell is installed, the IC's internal map no longer matches the new cell's actual voltage curve. Under high load — modem transmit bursts or screen-on peaks — the new cell's voltage briefly sags below the threshold the IC expects at that reported percentage, triggering an emergency shutdown. One full discharge-charge cycle at standard current resets the calibration and eliminates the early cutoff.
Phone won't power on after the JR40 replacement sat in storage
Li-Polymer cells self-discharge during storage. If the replacement cell drops below approximately 2.5V before installation, the BMS enters lockout mode to prevent damage and will refuse to pass current to the board. The phone shows no response — no charging indicator, no boot attempt. Connect a charger and leave it for 15–20 minutes without pressing the power button; the charge IC will trickle current into the cell until voltage climbs above the BMS re-enable threshold, typically around 2.8–3.0V, after which normal charging resumes.