Fly DS120 Replacement Battery BL6203 3.7V 1300mAh

Fly DS120 — 3.7V Li-ion Replacement Battery (BL6203)

This is a 3.7V, 1300mAh Li-ion replacement battery for the Fly DS120 smartphone. It carries OEM part number BL6203 and slots directly into the DS120's battery bay. Capacity figure is 4.81Wh.

• DS120 platform fit: The DS120 uses a compact 60.00 × 43.80 × 4.50mm cell running at 3.7V nominal. This replacement matches that voltage rail and physical footprint, so the phone's power management IC sees the same parameters as the original BL6203.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a DS120 unit. The BMS accepted the charge handshake without fault flags, and the protection circuit tripped correctly at the low-voltage cutoff threshold — no thermal events recorded.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging pushes load into an uncalibrated state.

Sudden shutdown at 20–30% on the DS120 after a cell swap

The DS120's fuel gauge IC holds a discharge curve learned from the original cell. When a new cell goes in, the stored curve no longer matches actual cell behaviour, and the phone miscalculates remaining capacity. Under modem or screen load — both high-current draws — the new cell's voltage drops faster than the IC expects, and the phone hits its emergency cutoff while still reporting 20–30% remaining. One full discharge-charge cycle without fast charging resets the coulomb counter and lets the IC relearn the curve against the new cell.

Battery percentage jumping erratically on the DS120

Erratic percentage readings on the DS120 — jumping from 45% to 60% or dropping 15% in seconds — point to the fuel gauge IC recalibrating against a new cell it has not yet fully characterised. This is not a fault with the replacement cell itself. Drain the battery until the phone shuts off automatically, then charge uninterrupted to 100% with the screen on as little as possible. After one complete cycle, readings stabilise at the correct voltage-to-percentage mapping, typically within ±3%.