MeiZu MX4 BT40 Replacement Battery 3.8V 3100mAh Li-Polymer

MeiZu MX4 / M460 / M461 — 3.8V Li-Polymer Replacement Battery (BT40)

The BT40 is a 3.8V, 3100mAh lithium-polymer cell that fits the MeiZu MX4, M460, and M461 smartphones. It slots into the same battery bay as the original and connects to the same charge IC and fuel gauge circuit. Capacity is rated at 11.78Wh.

• MX4, M460, and M461 compatibility: All three models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The BT40 footprint — 70.00 × 54.92 × 5.17mm — fits each variant without modification to the chassis or connector.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the MX4 platform. The BMS accepted the new cell without error flags, balanced correctly across the protection circuit, and the charge IC reached termination voltage at 4.35V as expected.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated cell — preventing erratic percentage readings from the first hour.

Sudden shutdown at 20–30% on the MX4 after a cell swap

This is a voltage cliff issue, not a capacity fault. Under peak modem or display load, a fresh cell with an uncalibrated fuel gauge IC can reach the BMS undervoltage cutoff — typically 3.0V per cell — while the OS still shows 20–30% remaining. The fuel gauge was calibrated to the old cell's internal resistance profile, so its state-of-charge estimate is wrong. One full discharge-charge cycle at standard current corrects the coulomb counter and moves the reported percentage back in line with actual cell voltage.

MX4 not powering on after the replacement battery sat in storage

Li-polymer cells self-discharge in storage, and if the BT40 dropped below approximately 2.5V before installation, the BMS will have entered lockout to prevent cell damage. The phone will show no response — no charging indicator, no boot screen. Connect the device to a wall adapter, not a PC USB port, and leave it for 15–20 minutes without pressing the power button. The charge IC needs to trickle current into the cell until it clears the BMS lockout threshold, which sits around 2.9V, before normal charging resumes.