Fly IQ451 Compatible Battery BL4257 3.7V 2000mAh

Fly IQ451 Vista — 3.7V Li-ion Replacement Battery (BL4257)

This is a 3.7V, 2000mAh Li-ion replacement battery for the Fly IQ451 Vista smartphone. It uses OEM part number BL4257 and slots directly into the IQ451's battery bay. Dimensions are 70.00 × 59.90 × 4.40 mm — verify these against your compartment before ordering.

• IQ451 Vista compatibility: The IQ451 and Vista share the same physical battery bay dimensions, connector pinout, and 3.7V nominal voltage rail. Both models use the BL4257 part number, so one cell covers both variants without any hardware modification.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the IQ451 platform. The BMS accepted charge handshake without fault, held voltage above 3.5V under screen-on and modem load, and tripped protection correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: On first installation, disable any fast-charge mode and run one complete discharge-to-charge cycle at standard current. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before high-current charging begins on an uncalibrated cell.

Sudden shutdown at 20–30% on the IQ451 after a cell swap

This is a voltage cliff failure, not a capacity problem. Under peak modem transmit or screen backlight load, a new cell with an uncalibrated fuel gauge can drop below the BMS cutoff voltage momentarily even when the reported percentage looks healthy. The phone's protection circuit sees the voltage dip and shuts down to prevent cell damage. Run one full discharge cycle — letting the phone reach automatic shutdown from 100% — then charge uninterrupted to full. After that cycle the coulomb counter tracks the new cell curve accurately and the premature shutdowns stop.

IQ451 not powering on after the replacement battery sat in storage

Li-ion cells self-discharge in storage. If the BL4257 cell dropped below approximately 2.5V per cell, the BMS enters lockout to prevent damage from charging a critically depleted cell at full current. The phone will show nothing — no boot screen, no charge indicator — even with a cable connected. Connect to a wall charger and leave it undisturbed for 20–30 minutes. Most BMS circuits include a trickle pre-charge path that slowly recovers cell voltage up to the safe threshold before allowing normal charge current; once it crosses roughly 3.0V the charge LED activates and normal charging resumes.