Motorola C300 SNN5725A Replacement Battery 3.7V 750mAh

Motorola C300 / C335 — 3.7V Li-ion Replacement Battery (SNN5725A)

The SNN5725A is a 3.7V, 750mAh Li-ion cell that fits the Motorola C300 and C335 compact mobile phones. These phones date from the early 2000s, and original batteries at this age have almost universally degraded past usable capacity. This replacement matches the OEM voltage, footprint, and connector so the phone powers on without modification.

• C300 and C335 compatibility: Both models run the same 3.7V single-cell architecture and share the SNN5725A form factor, connector pitch, and BMS communication lines — one cell covers both.
• Bench tested on actual hardware: We cycled the cell through full charge and discharge on a C300 chassis. The BMS accepted the cell without fault codes, and the charge IC stepped through its standard CC/CV stages cleanly.
• Fuel gauge recalibration on first use: After fitting this cell, run one full discharge down to automatic power-off, then charge uninterrupted to 100% before using the phone normally. The C300's fuel gauge IC calibrates its coulomb counter against that first complete cycle — skipping it causes percentage jumps and early shutdown warnings on the replacement cell.

Sudden shutdown at 20–30% on the C300 after a cell swap

The C300 fuel gauge IC holds a discharge curve built from hundreds of cycles on the original cell. A new cell has a different internal resistance profile, so the IC misjudges the voltage cliff — it reads 25% remaining while the actual cell voltage has already dropped below the modem's minimum draw threshold. The phone cuts power to protect the circuit before the gauge catches up. One full discharge-to-shutdown cycle followed by a full uninterrupted charge resets the coulomb counter against the new cell's actual curve and eliminates the early cutoff.

C300 not powering on after the replacement cell sat in storage

Li-ion cells shipped or stored without a partial charge can drop below 2.5V per cell — the threshold where the BMS latches into deep-discharge lockout and blocks normal charge current to prevent plating damage. The C300 will show nothing on screen and appear completely dead. Connect the phone to a charger and leave it for 15–20 minutes without pressing the power button; the charge IC will trickle current at a low rate to recover cell voltage before the BMS re-enables full charge. If the screen shows a charge indicator at any point, the cell is recovering — wait until it reaches at least 3.0V before attempting to power on.