Infocus UP130039 M521 Replacement Battery 3.8V 2400mAh

Infocus M521 — 3.8V Li-Polymer Replacement Battery (UP130039)

This 3.8V, 2400mAh Li-Polymer cell replaces the original battery in the Infocus M521 smartphone. It uses OEM part number UP130039 and fits the M521 directly. Voltage and capacity match the original spec: 3.8V nominal, 9.12Wh total energy.

• Infocus M521 fitment: The M521 uses a fixed connector pinout tied to this specific cell footprint — 73.00 × 54.26 × 4.63 mm. The BMS on the replacement communicates over the same two-wire bus the phone expects, so the charge IC accepts the cell without rejecting it as unknown hardware.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the M521 platform. The BMS held cutoff at the correct low-voltage threshold and the charge IC reached full termination without false-positive completion signals.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. The M521's fuel gauge IC is calibrated to the old cell's discharge curve — one full uninterrupted cycle lets the coulomb counter reset its endpoint references against the new cell before any high-current charge session runs.

Why the M521 reports wrong battery percentage after a cell swap

The M521 uses a coulomb-counting fuel gauge IC that tracks charge by integrating current over time against a stored discharge curve. That curve was built around the original cell's internal resistance and capacity. When a new cell goes in, the stored curve no longer matches the actual discharge behaviour, so the percentage reading drifts. One full discharge from 100% to auto-shutdown, followed by an uninterrupted charge to 100%, forces the IC to rewrite its endpoint calibration against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC still holds the old cell's voltage-cliff map. The new cell may hit a sharper voltage drop under modem or screen load at a different state of charge than the original. The phone's protection circuit reads that voltage sag as a hard cutoff, even though reported percentage still shows charge remaining. Run the recalibration cycle described above — after one full cycle, the IC re-maps the cliff point to match the new cell's actual low-voltage behaviour.