OK Big Button 3.7V Replacement Battery 900mAh Li-ion

OK Big Button / OMP80 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 900mAh (3.33Wh) Li-ion cell for the OK Big Button and OMP80 smartphones. It slots into the same compartment as the original and connects to the same charge IC and fuel gauge circuit. If your original cell no longer holds a charge or the phone shuts down unexpectedly, this cell replaces it directly.

• Big Button and OMP80 platform: Both models run the same 3.7V single-cell architecture with identical connector pinout and BMS handshake logic, so one cell covers both variants without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Big Button platform. The BMS accepted the cell on first connection, charge termination triggered correctly at 4.2V, and protection cutoff activated normally at the low-voltage threshold.
• Fuel gauge recalibration on first use: For the first complete cycle after installation, disable any fast-charge mode if the device exposes that setting. A slow first discharge-charge cycle lets the fuel gauge IC map the new cell's discharge curve before high-current charging is applied to an uncalibrated cell.

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

This happens because the fuel gauge IC is still using the discharge curve from the old, degraded cell. A new cell has a steeper voltage cliff at low state-of-charge than a worn cell does. The phone's modem or screen draws enough current to drop cell voltage below the protection threshold before the gauge registers empty. One full discharge-charge cycle recalibrates the coulomb counter and the shutdowns stop. Let the phone run down until it powers off on its own, then charge uninterrupted to 100%.

Phone won't power on after the replacement cell sat in storage

Li-ion cells self-discharge during storage. If the cell voltage dropped below approximately 2.5V, the BMS enters lockout mode and blocks current flow to protect the cell from damage. The phone won't respond to a short press of the power button and may show no charging indicator for the first several minutes on the charger. Leave it connected to a working charger for 15–20 minutes — the charge IC applies a low-current trickle to bring the cell above the BMS re-enable threshold, typically around 3.0V, before normal charging resumes.