About this replacement
LG Optimus Me / P350 / C550 — 3.7V Li-ion Replacement Battery (BL-42FN)
The BL-42FN is a 3.7V, 1250mAh Li-ion cell for the LG P350, Optimus Me, and C550 smartphones. It replaces the factory battery when the original cell no longer holds a charge or causes unexpected shutdowns. Voltage and connector match the original spec exactly.
- P350 / Optimus Me / C550 platform fit: All three models share the same battery bay dimensions, 3.7V single-cell architecture, and BL-42FN connector pinout. Swapping between these devices does not require any adapter or modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the P350 mainboard. The BMS accepted charge current without fault flags, and the protection circuit tripped correctly at the low-voltage cutoff threshold during deep-discharge testing.
- Fuel gauge recalibration on first install: After fitting this cell, run one full discharge to automatic shutdown, then charge uninterrupted to 100% before use. This gives the phone's fuel gauge IC a clean reference curve for the new cell — skip it and the percentage readout will drift from the first charge.
Sudden shutdown at 20–30% on the BL-42FN replacement
This happens because the P350's fuel gauge IC was calibrated against the old cell's discharge curve. The new cell hits a voltage cliff — the point where cell voltage drops sharply under modem or screen load — at a different state of charge than the IC expects. The phone reads 25% but the actual cell voltage has already dipped below the shutdown threshold under load. One full discharge-charge cycle resets the coulomb counter and eliminates the early cutoff.
Phone warm near the battery during the first charge after replacement
A new cell starts with higher internal impedance than a conditioned one. The charge IC on the P350 pushes the same current profile it used on the broken cell, but the higher impedance converts more energy to heat during the initial cycles. This is normal and resolves after two or three full cycles as impedance drops. If the back cover becomes hot to the touch — not just warm — remove the phone from charge and let it cool before continuing.