BL3707 Fly MC155 Replacement Battery 3.7V 2500mAh

Fly MC155 / IQ4401 Era Energy 2 — 3.7V Li-ion Replacement Battery (BL3707)

This is a 3.7V, 2500mAh Li-ion replacement battery for the Fly MC155, IQ4401, and Era Energy 2 smartphones. It uses the BL3707 part number and fits the original battery bay without modification. Capacity is 9.25Wh — matched to the product data, not estimated from web sources.

• MC155, IQ4401, and Era Energy 2 compatibility: All three models share the BL3707 footprint, the same 3.7V nominal voltage rail, and the same connector pinout. The BMS handshake on each device accepts this cell without triggering a reject flag.
• Bench tested on actual hardware: We cycled this cell on an MC155 unit and monitored charge termination, BMS cutoff thresholds, and voltage behaviour under screen-on and modem-active loads. The BMS held within spec across three full cycles.
• Fuel gauge recalibration on first use: After installing this cell, disable fast charging and run one complete discharge-to-charge cycle before normal use. The fuel gauge IC on these Fly devices calibrates its coulomb counter against the cell's discharge curve — skipping this step causes erratic percentage readings for the first several days.

Sudden shutdown at 20–30% on the Fly MC155 after a cell swap

This happens because the fuel gauge IC is still reading the discharge curve of the old, degraded cell. When the modem fires or the screen brightness spikes, current draw pulls the new cell's voltage below the BMS cutoff threshold faster than the gauge expects — so the phone cuts out even though the reported percentage looks safe. The fix is one full discharge-to-charge cycle with no fast charging. After that cycle, the coulomb counter resets against the new cell's actual discharge profile and the shutdowns stop.

Fly IQ4401 / Era Energy 2 not powering on after the battery sat discharged in storage

Li-ion cells drop below the BMS re-enable threshold of 2.5V per cell if left discharged for weeks. When that happens, the BMS locks the output rail and the phone shows nothing — not even a charging indicator. Connect the phone to a wall charger rated at least 5V/1A and leave it undisturbed for 20–30 minutes before pressing the power button. The charge IC trickle-feeds current to bring the cell back above 2.5V, at which point the BMS re-enables the output and the phone boots normally.