Apple iPhone 4G Replacement Battery 616-0520 3.7V 1420mAh

Apple iPhone 4G Series — 3.7V Li-Polymer Replacement Battery (616-0520)

This 3.7V, 1420mAh Li-Polymer cell replaces the original battery in the iPhone 4G, iPhone 4G 16GB, and iPhone 4G 32GB. It matches OEM part numbers 616-0520, 616-0521, and 616-0512, and fits the same connector and cavity without modification. Capacity is 5.25Wh — identical to the factory specification.

• iPhone 4G platform fit: All three 4G variants — base, 16GB, and 32GB — share the same battery bay dimensions, flex connector, and BMS communication protocol. One cell covers the full lineup because Apple did not change the power architecture between storage tiers.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on an iPhone 4G unit. The BMS accepted the cell without error flags, the charge IC ramped correctly to 4.20V, and the fuel gauge IC began tracking coulombs from the first cycle.
• Fuel gauge recalibration on first use: After installation, run one complete discharge to automatic shutdown followed by a full charge before re-enabling any background activity. This gives the fuel gauge IC a clean reference cycle against the new cell's actual discharge curve — skipping it causes erratic percentage readings for days.

Sudden shutdown at 20–30% after fitting a new cell

This is a voltage cliff problem, not a capacity problem. Under peak load — LTE modem transmission, screen at full brightness, GPS active simultaneously — the cell voltage can drop below the BMS cutoff threshold even when the fuel gauge still reads 25%. The fuel gauge IC was calibrated to the old, degraded cell's internal resistance curve, so it misreads how much usable charge remains in a fresh cell under high current draw. Run one full discharge cycle to forced shutdown and recharge to 100% without interrupting it. After that single cycle the coulomb counter resets its reference and the shutdowns stop.

Phone stays at 0% and won't power on after the battery sat in storage

Li-Polymer cells self-discharge in storage. If the cell dropped below approximately 2.5V, the BMS enters lockout mode and blocks all charge current to prevent thermal runaway in an over-discharged cell. The phone will show nothing — no Apple logo, no charging indicator. Connect to a wall adapter rated at 5V and leave it completely undisturbed for 20–30 minutes; the charge IC trickle-charges the cell at a safe low current until it climbs back above the BMS re-enable threshold of around 3.0V. Only after that will the phone boot. USB ports on computers often supply insufficient current to trigger recovery — use a wall adapter.