UMI UMIDIGI Z2 Replacement Battery 3.85V 3650mAh

UMI UMIDIGI Z2 — 3.85V Li-Polymer Replacement Battery (Z2)

This is a 3650mAh Li-Polymer battery rated at 3.85V, built to replace the original cell in the UMIDIGI Z2 smartphone. It restores power to the processor, display, and wireless radios when the factory cell has degraded or failed. Dimensions are 84.00 × 62.35 × 4.50mm — confirm clearance inside the chassis before installation.

• UMIDIGI Z2 fitment: The Z2 uses a fixed connector pinout tied to a specific BMS communication protocol. This cell matches that pinout, so the charge IC and fuel gauge IC receive the handshake they expect without throwing a fault code.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the Z2 platform. The BMS accepted charge current normally, and the protection circuit tripped correctly at the low-voltage cutoff threshold — no runaway, no false cutoff mid-cycle.
• First-cycle calibration on the Z2: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle at standard rate. This lets the Z2's fuel gauge IC map the new cell's actual discharge curve before fast-charge current is applied to an uncalibrated cell.

Sudden shutdown at 20–30% on the UMIDIGI Z2 after a cell swap

A new Li-Polymer cell has a slightly different voltage-versus-capacity curve than the worn cell the fuel gauge IC was calibrated against. At 20–30% reported charge, the modem or display draws a brief high-current spike. If the fuel gauge IC hasn't re-mapped the new cell, it cannot predict the voltage drop accurately — the phone reads that spike as a collapse to cutoff voltage and shuts down. One full discharge cycle at standard charge rate recalibrates the coulomb counter and eliminates these false shutdowns.

USB fast charge refusing to engage on the first cycle after replacement

The Z2's charge IC negotiates fast-charge current only after it confirms the BMS state is within expected parameters. On a brand-new cell, internal impedance is slightly higher than a broken-in cell, and the charge IC may interpret that as an out-of-range condition and fall back to standard 5V charging. This typically clears after the first full charge cycle as cell impedance settles. If fast charge still does not engage after two full cycles, check the USB-C port for debris and confirm the charger output matches the Z2's rated input.