Apple iPhone 6s Replacement Battery 616-00036 3.8V 1900mAh

Apple iPhone 6s — 3.8V Li-Polymer Replacement Battery (616-00036)

This 3.8V, 1900mAh Li-Polymer battery replaces the original 616-00036 cell in the iPhone 6s. It fits models A1691, A1633, and A1688. Voltage and capacity match the OEM specification so the charge IC and fuel gauge IC receive the correct input from the first cycle.

• A1691, A1633, A1688 compatibility: All three A-series variants of the iPhone 6s share the same battery bay geometry, connector pinout, and BMS handshake protocol. A single replacement cell covers the full production run without modification.
• Bench tested on actual hardware: We cycled this cell on an iPhone 6s running iOS with screen-on load and modem active. The BMS accepted the charge IC handshake without fault flags, and voltage held above 3.6V through the bulk of the discharge curve.
• Fuel gauge recalibration after installation: After fitting this cell, disable fast charging and run one full discharge down to automatic shutdown, then charge uninterrupted to 100%. This gives the coulomb counter a clean reference curve against the new cell before high-current cycles begin.

Sudden shutdown at 20–30% on a replacement iPhone 6s cell

The iPhone 6s fuel gauge IC builds its percentage estimate from a stored discharge curve, not a live voltage read alone. When a new cell goes in, the stored curve still reflects the old, degraded cell. Under combined modem and display load, the new cell hits a voltage point the IC maps to near-empty — even when physical capacity remains. The phone shuts down to protect against over-discharge based on a curve that no longer matches reality. One full uninterrupted discharge-to-shutdown followed by a full charge forces the coulomb counter to remap its curve to the actual cell behaviour.

Phone warm near the battery during the first few charges after swap

A fresh Li-Polymer cell has higher internal impedance than a broken-in one. The charge IC on the iPhone 6s pushes current into that impedance, and the resistance difference generates more heat in the first three to five cycles than you will see long-term. This is normal and temporary. If the case stays warm past the fifth full charge cycle, check that the battery connector is fully seated — a partially connected cell forces the IC to work harder to maintain current flow. Confirm the connector clicks flat and re-run a charge cycle to 4.2V.