About this replacement
OnePlus 9 — 7.74V Li-Polymer Replacement Battery (BLP829)
This is a 7.74V, 2250mAh Li-Polymer replacement battery for the OnePlus 9 smartphone. It carries OEM part number BLP829 and fits the OnePlus 9 directly. Install it when the original cell no longer holds charge or the phone shuts down unexpectedly under load.
- OnePlus 9 fit: The BLP829 cell matches the OnePlus 9's connector pinout, physical envelope (85.40 × 62.40 × 5.20mm), and the voltage rails the PMIC expects. The BMS handshake on this cell is tuned to the charge IC inside the OnePlus 9 motherboard.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a OnePlus 9 unit. The BMS accepted the charge protocol without fault flags, and the fuel gauge IC tracked cell state across the full voltage window without a mid-cycle cutout.
- First-cycle calibration on the OnePlus 9: After installation, disable Warp Charge for the first complete discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state of charge register.
Why the OnePlus 9 reports wrong battery percentage after a cell swap
The OnePlus 9 uses a coulomb counter that was calibrated against the original cell's impedance and discharge curve. A new cell has different internal resistance, so the counter's stored model no longer matches actual cell voltage. The OS reads a stale curve and displays incorrect percentages — often showing 30% and then jumping to 5% without warning. One full discharge-charge cycle at standard current resets the calibration baseline.
Sudden shutdown at 20–30% on the replacement cell
This happens when the modem or display pulls a current spike the cell cannot sustain without its voltage dropping below the PMIC's cutoff threshold. The OS still shows 20–30% because the fuel gauge hasn't recalibrated yet, but the cell voltage collapses under load. It is not a faulty cell — it is an uncalibrated gauge paired with a high-draw moment. Run one full cycle with Warp Charge off, then check that the cutoff voltage holds above 3.0V per cell under screen-on load.