Bird S689 Smartphone Compatible Battery 3.7V 750mAh

Bird S689 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 750mAh (2.78Wh) lithium-ion replacement battery for the Bird S689 smartphone. It fits directly into the S689 handset and restores power when the original cell has degraded or failed. No OEM part number is published for this model.

• S689 cell fit: The Bird S689 uses a compact 53.80 × 33.70 × 5.80mm cell cavity. This replacement matches those physical dimensions exactly and meets the 3.7V nominal voltage the phone's charge IC expects. A mismatched voltage rail causes the charge controller to reject the cell outright.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge passes on the bench. The BMS held the charge cutoff at 4.2V and the low-voltage cutoff tripped cleanly without locking the board into an unrecoverable state.
• 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 counter — which is what causes erratic percentage readings early on.

Why the S689 reports wrong battery percentage after a cell swap

The Bird S689 uses a fuel gauge IC that builds its state-of-charge model from the discharge curve of the original cell. When a new cell goes in, the IC is still running calculations against the old curve — so the percentage on screen can read 40% while the actual cell voltage is already near cutoff. One full discharge down to auto-shutdown followed by a complete charge to 4.2V forces the coulomb counter to reset its baseline. After that single cycle, percentage accuracy returns to normal.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or display draws a current spike the cell cannot sustain at that state of charge — the cell voltage sags below the BMS cutoff threshold and the phone shuts off instantly, even though the gauge still showed charge remaining. It is not a faulty battery. The fuel gauge IC has not yet learned where the voltage cliff sits on the new cell. Run one full calibration cycle — discharge to shutdown, charge uninterrupted to full — and the cutoff point the OS uses will align with the actual cell voltage floor.