Motorola MB525 Droid 2 Replacement Battery BF5X 3.7V 2400mAh

Motorola MB525 / Defy — 3.7V Li-ion Replacement Battery (BF5X / SNN5877A)

This 3.7V 2400mAh Li-ion cell replaces the BF5X / SNN5877A battery in the Motorola MB525, MB520, and Defy. All three models share the same battery bay dimensions and connector pinout, so one cell covers the full group. Capacity is 2400mAh (8.88Wh) — sourced to match the original cell specification.

• MB525, MB520, and Defy compatibility: These three handsets use the same physical bay (60.50 × 42.00 × 10.50mm), identical 3.7V nominal voltage rail, and the same three-pin connector with shared BMS communication lines — one cell fits all three without modification.
• Bench tested on actual hardware: We cycled this cell through charge, load, and BMS handshake verification. The protection circuit tripped correctly at the low-voltage cutoff, and charge acceptance across the full capacity range was consistent with spec.
• Fuel gauge recalibration on first use: After fitting this cell, run one full uninterrupted discharge down to automatic shutdown, then charge to 100% without interruption. The Defy's fuel gauge IC reads coulomb count against the old cell's curve until it completes this reset cycle — skipping it causes erratic percentage readings from the first hour.

Why the Defy reports wrong battery percentage after a cell swap

The MB525 and Defy use a coulomb-counting fuel gauge IC that builds its discharge model from accumulated charge and load history. When you swap the physical cell, the IC retains the previous cell's calibration data — it does not auto-detect a new cell. This mismatch causes the reported percentage to lag or jump because the IC is mapping current draw against a curve that no longer reflects the installed cell. One full discharge-to-shutdown followed by a complete uninterrupted charge clears the accumulated error and resets the baseline to the new cell's actual capacity.

Sudden shutdown at 20–30% remaining on the Defy

This happens when the cell voltage drops sharply under load — typically during a cellular data burst, screen-on, or call — before the fuel gauge reaches its low-battery threshold. The phone's protection circuit sees the voltage cliff and cuts power to prevent cell damage, even though the reported percentage still shows charge remaining. It is not a fault in the replacement cell — it is the fuel gauge IC working from an uncalibrated curve after the swap. Complete the first full discharge-charge cycle described above, and the IC will recalibrate its cutoff prediction to match the new cell's actual voltage sag profile at load.