Xiaomi Mi Max 2 BM50 Replacement Battery 3.85V 5300mAh

Xiaomi Mi Max 2 — 3.85V Li-Polymer Replacement Battery (BM50)

This is a 3.85V, 5300mAh (20.41Wh) Li-Polymer replacement cell for the Xiaomi Mi Max 2 smartphone. It fits the MDE40 and MDT40 variants alongside the standard Max 2. When the original BM50 cell degrades, capacity drops sharply on a phone this large — swapping the cell brings the device back to full charge capacity.

• Max 2 / MDE40 / MDT40 fit: All three variants share the same battery bay geometry, connector pinout, and BMS handshake protocol. The BM50 footprint — 104.10 × 76.40 × 3.40mm — is consistent across this entire model group, so no adapter or modification is needed.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Mi Max 2 platform. The BMS accepted charge without tripping over-current protection, and the fuel gauge IC tracked state-of-charge accurately after a full calibration cycle.
• First-cycle fuel gauge reset: On first use after installation, disable fast charging and run one complete discharge down to automatic shutdown, then charge fully on a standard 5V/2A input. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current charging begins.

Why the Mi Max 2 reports wrong battery percentage after a cell swap

The Mi Max 2 uses a coulomb-counting fuel gauge IC that builds its state-of-charge model against the original cell's discharge curve over hundreds of cycles. When a new cell goes in, that learned curve no longer matches the cell's actual chemistry. The gauge reads a stale model, so it may show 40% and then cut off — or show 15% while the phone keeps running. One complete discharge-to-shutdown followed by a full charge resets the coulomb counter to the new cell's real endpoints. After that cycle, percentage readings stabilise.

Sudden shutdown at 20–30% on the replacement cell

This happens because the modem and the large 6.44-inch display on the Mi Max 2 pull current spikes that cause a voltage sag on a freshly installed cell. If the cell voltage dips below the BMS cutoff threshold under that combined load — even briefly — the BMS trips and shuts the phone down before the gauge reads zero. It is not a faulty cell; it is the fuel gauge misreading available capacity before calibration. Run the first-cycle calibration discharge at lower screen brightness and with mobile data active so the load mirrors real-world use. After calibration, the BMS cutoff and the gauge reading align, and the shutdowns stop.