About this replacement
Apple iPhone 15 Plus — 3.85V Li-Polymer Replacement Battery (A3039)
This is a 3.85V, 4650mAh (17.9Wh) Li-Polymer battery for the Apple iPhone 15 Plus. It replaces part number A3039 — the original cell fitted to this model at manufacture. Fit this when the original cell has lost capacity through normal charge cycling and can no longer sustain the phone under load.
- iPhone 15 Plus fit: The A3039 cell uses a specific flex connector and BMS handshake tied to the iPhone 15 Plus power management IC. The physical dimensions — 98.80 × 50.00 × 4.80mm — match the battery bay exactly. No adapter or modification is needed.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on an iPhone 15 Plus unit. The BMS accepted the full charge cycle without cutoff errors, and the power management IC reported capacity correctly after the fuel gauge completed its first calibration pass.
- Fuel gauge recalibration on first install: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes additional load into an uncalibrated cell.
Why the iPhone 15 Plus reports wrong battery percentage after a cell swap
The fuel gauge IC on the iPhone 15 Plus uses a coulomb counter calibrated to the old cell's discharge curve. After fitting a new cell, the stored curve no longer matches actual cell voltage behaviour. The percentage reading drifts — often showing 100% then dropping fast, or stalling at mid-range. One full discharge down to automatic shutdown followed by a complete charge to 100% forces the coulomb counter to remap against the new cell. After that cycle, percentage readings stabilise.
Sudden shutdown at 20–30% on the replacement cell
This happens when the fuel gauge IC hasn't yet aligned its voltage threshold map to the new cell. The new cell may hit a voltage cliff — dropping below the modem or display's minimum rail voltage — before the reported percentage reaches zero. The phone shuts down as a protective measure, not because the cell is faulty. Run one full discharge-to-shutdown and full recharge cycle; the fuel gauge recalibrates its low-voltage cutoff point against the actual cell, and the shutdowns stop.