Motorola RAZR2 BX40 Replacement Battery 3.7V 720mAh

Motorola RAZR2 / V8 / V9 Series — 3.7V Li-ion Replacement Battery (BX40)

This is a 3.7V, 720mAh Li-ion replacement cell for the Motorola RAZR2, V8, V9, and V9m flip phones. It replaces OEM part numbers BX40, BX50, SNN5805, SNN5805A, FNN7012AA, SNN5807, and SNN5807A. If your original cell no longer holds a charge through a normal day of calls and standby, this is a direct swap.

• RAZR2 / V8 / V9 platform fit: These models share the same 3.7V single-cell architecture, connector pinout, and BMS handshake protocol — which is why one cell covers the full lineup. Swapping between models in this family does not require a different part.
• Bench tested on actual hardware: We cycled this cell through the RAZR2 charge IC and confirmed the BMS communication handshake completed without fault flags. Charge acceptance was clean from 3.0V up to the 4.2V cutoff with no mid-cycle interruptions.
• Fuel gauge recalibration on first cycle: On first use after installation, run one full discharge down to automatic shutdown, then charge uninterrupted to 100% without using fast-charge modes. This gives the fuel gauge IC a clean reference curve against the new cell before the OS begins reporting percentages.

Sudden shutdown at 20–30% on the RAZR2 after a cell swap

The RAZR2's fuel gauge IC was calibrated to the discharge curve of the original, degraded cell. When a new cell with a different internal impedance goes in, the IC misreads the voltage-to-capacity relationship at the low end of the curve. The phone sees what it thinks is 25% remaining, but when the modem fires up for a call, voltage sags below the cutoff threshold and the device shuts off. One full discharge-charge cycle resets the coulomb counter and eliminates this behaviour.

Phone warm near the battery slot during the first few charges

A new Li-ion cell has higher internal impedance than a mid-life cell. On the first one or two charge cycles, the RAZR2's charge IC pushes current into higher resistance, which generates slightly more heat than normal. This is not a fault — it resolves as the cell completes its first few cycles and impedance settles. If the back cover feels hot rather than warm, or if the phone is inside a case during charging, remove the case and let it charge in open air.