Motorola BC50 C261 Replacement Battery 3.7V 750mAh

Motorola SLVR L7 / C261 / K1 Series — 3.7V Li-ion Replacement Battery (BC50)

This is a 3.7V Li-ion cell rated at 750mAh (2.78Wh), cross-referenced to OEM part numbers BC50, SNN5779, CFNN1043, and variants. It fits the Motorola C261, SLVR L7, SLVR L7i, K1, and over a dozen additional Motorola handsets that share the same BC50 battery platform. Voltage and connector pinout match the original spec.

• BC50 platform compatibility: The C261, SLVR L7, and K1 all draw from the same 3.7V single-cell architecture with an identical connector and BMS handshake protocol. Motorola standardised this across mid-2000s flip and slider models, so one cell covers the full lineup without adaptation.
• Bench tested on actual hardware: We ran this cell through charge cycles on an SLVR L7 and monitored the BMS charge termination. The protection circuit cut off correctly at full charge, and the cell accepted current without thermal event across multiple cycles.
• Fuel gauge recalibration after install: On first use, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. The coulomb counter in these Motorola handsets was calibrated to the original cell's discharge curve — one full cycle re-anchors it to the new cell.

Why the C261 and SLVR L7 report wrong battery percentage after a cell swap

These handsets use a simple coulomb-counting fuel gauge with no adaptive learning — it tracks charge in and out against a fixed discharge curve stored at the factory. When you install a replacement cell, that stored curve no longer matches the new cell's actual voltage-versus-capacity profile. The gauge reads the voltage correctly but maps it to the wrong percentage point on the old curve. One full discharge-to-shutdown followed by an uninterrupted charge to 100% forces the counter to reset its endpoints against the new cell.

Phone shuts down suddenly at 20–30% remaining on the replacement cell

This is a voltage cliff problem, not a capacity problem. At low state of charge, the cell's internal resistance rises — when the phone's transmit circuit pulls a brief high-current spike, the terminal voltage drops sharply below the BMS cutoff threshold even though the displayed percentage still looks safe. The fuel gauge IC hasn't recalibrated yet, so it's reporting a percentage that doesn't reflect actual usable voltage under load. Perform a full discharge-to-shutdown cycle, then charge to 100% without interruption — after one complete cycle the gauge re-anchors and the premature cutoffs typically stop.