CAT S31 Smartphone Replacement Battery APP00240 3.85V 3900mAh

CAT S31 — 3.85V Li-Polymer Replacement Battery (APP00240)

This 3.85V, 3900mAh Li-Polymer cell replaces the original APP00240 battery in the CAT S31 rugged smartphone. It fits the S31 directly — same connector, same BMS handshake, same physical footprint at 95.60 × 54.22 × 4.40mm. Capacity figure is taken from the product specification, not estimated.

• CAT S31 compatibility: The S31 uses a sealed chassis with a fixed connector pinout tied to a specific BMS communication protocol. This cell matches that pinout and negotiates correctly with the S31's charge IC — no adapter, no firmware workaround needed.
• Bench tested on actual hardware: We ran the APP00240 replacement through full charge cycles on the S31 platform. The BMS accepted the cell without fault flags, charge current ramped normally through CC/CV phases, and the fuel gauge IC registered capacity without throwing calibration errors on the first full cycle.
• Fuel gauge recalibration after installation: On first use, disable fast charging 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 CAT S31 after a cell swap

The S31's fuel gauge IC retains the discharge curve from the old, degraded cell in its coulomb counter. When a new cell is installed, the gauge reports percentage against that old curve — which means the displayed percentage no longer matches the actual cell voltage. Under modem transmit load or screen-on draw, the new cell hits a voltage cliff the gauge didn't predict, and the phone cuts out. One full discharge-to-shutdown followed by an uninterrupted charge to 100% forces a recalibration cycle. After that cycle, the percentage readings align with the new cell's actual state of charge.

USB charging not initiating after deep discharge storage

If the replacement cell sat in storage at low voltage, the BMS protection circuit can lock out below approximately 2.5V per cell — the USB port will show no charge activity at all when connected. The fix is a trickle pre-charge: use the original CAT wall adapter rather than a PC USB port, which delivers enough idle current to nudge the cell above the BMS recovery threshold. Hold the cable connected for 10–15 minutes before attempting a power-on. Once cell voltage clears 3.0V, the BMS releases the lockout and normal CC/CV charging resumes.