About this replacement
Apple iPhone 12 / iPhone 12 Pro — 3.83V Li-Polymer Replacement Battery (A2479)
This 3.83V, 3350mAh lithium-polymer battery replaces the original A2479 / A2431 cell in the iPhone 12 and iPhone 12 Pro. It fits the A2408 variant as well. The physical dimensions match the original bay at 84.20 × 44.54 × 4.40mm, so no trimming or bracket modification is needed.
- iPhone 12 and 12 Pro shared platform: Both models run the same A14 Bionic voltage rail and use an identical battery connector and BMS handshake. One cell covers both devices without wiring adaptation.
- Bench tested on actual hardware: We cycled this cell through the iPhone 12 charge IC under load. The BMS accepted the charge profile without dropping into fault state, and the fuel gauge IC completed its first-cycle coulomb count without error flags.
- Fuel gauge recalibration after install: On first use after installation, disable fast charging for one complete discharge-to-charge cycle. This lets the fuel gauge IC build a fresh discharge curve against the new cell before high-current USB-PD pushes amperage into an uncalibrated coulomb counter.
Sudden shutdown at 20–30% on a replacement iPhone 12 cell
This happens when the fuel gauge IC is still running the discharge curve from the old degraded cell. The new cell hits a voltage cliff under 5G modem or display load that the old curve did not predict. iOS interprets the drop as a low-voltage cutoff and shuts down before the percentage reaches zero. One full discharge cycle — drain to auto-off, then charge uninterrupted to 100% — forces the fuel gauge IC to remap the curve against the new cell's actual capacity.
iOS percentage jumping erratically after battery replacement
Erratic percentage jumps are a coulomb counter recalibration issue, not a faulty cell. The fuel gauge IC uses stored data from the previous cell's impedance and discharge profile, which no longer matches the replacement. Values appear to skip or bounce until the IC gathers enough current-in and current-out data to reset its model. Run two to three full discharge-charge cycles without interruption and the readings stabilise — look for the percentage to increment smoothly and hold steady at rest above 3.7V open-circuit.