Motorola BF5X Defy MB525 Replacement Battery 3.7V 1500mAh

Motorola Defy / MB525 Series — 3.7V Li-ion Replacement Battery (BF5X)

This is a 3.7V, 1500mAh Li-ion cell that replaces the original BF5X battery in the Motorola Defy, MB525, MB520, and ME525. It fits the same compartment as the OEM unit and connects via the standard three-contact interface. Voltage and capacity match the original spec exactly — no modifications needed.

• MB525 / MB520 / ME525 platform fit: These models share the same BF5X battery footprint, three-pin connector layout, and 3.7V nominal voltage rail. The BMS on each handset communicates over that third contact, so cell identification and charge termination work the same across the platform.
• Bench tested on actual hardware: We ran this cell on an MB525 through a full discharge-charge cycle and monitored BMS handshake, charge termination voltage, and current ramp. The BMS accepted the cell on first insertion and terminated charge cleanly at 4.2V.
• Fuel gauge recalibration on first use: After installing this cell, disable fast charging if your device supports it and run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. This gives the fuel gauge IC a clean reference curve against the new cell before normal use resumes.

Why the Motorola Defy reports the wrong battery percentage after a cell swap

The Defy uses a coulomb-counting fuel gauge IC that builds its model of cell capacity over charge cycles. When you swap in a new cell, the IC still holds the discharge curve it learned from the old, degraded battery. Until it re-learns the new cell's curve, percentage readings will be inaccurate — often showing full charge earlier than true capacity warrants. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the counter. After that single cycle, percentage reporting aligns with the actual cell capacity.

Sudden shutdown at 20–30% on the Defy after a replacement cell

This happens when the fuel gauge IC's old discharge model predicts remaining voltage incorrectly, and the cell voltage drops below the protection threshold under load — particularly during screen-on or radio activity — before the displayed percentage reaches zero. The BMS cuts power to protect the cell even though the percentage still reads 20–30%. Running one full calibration cycle as above corrects the fuel gauge model. If shutdowns continue past that point, check resting voltage with a meter — the cell should sit at or above 3.7V after a full charge.