Caterpillar S31 Replacement Battery APP00240 3.85V 3900mAh

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

This 3.85V, 3900mAh Li-Polymer battery is a direct cell replacement for the CATERPILLAR S31 rugged smartphone. It matches the OEM part number APP00240 and fits the S31's battery bay without modification. Voltage, connector pinout, and BMS communication protocol match the original specification.

• S31 platform fit: The S31 uses a sealed battery compartment with a specific flex-ribbon connector. This cell matches that connector and the three-wire communication line the S31's charge IC uses to verify cell identity during power-on — a mismatch here stops the device from booting.
• Bench tested on actual hardware: We ran this cell on the S31 through three full charge-discharge cycles. The BMS handshake completed on each power-on, charge IC accepted the cell without fault codes, and cutoff triggered correctly at the low-voltage threshold — no premature shutdowns observed.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge down to automatic shutoff, then charge to 100% uninterrupted. This gives the S31's fuel gauge IC a clean reference curve against the new cell before the coulomb counter begins tracking incremental cycles.

Sudden shutdown at 20–30% on the CATERPILLAR S31

The S31's fuel gauge IC stores a discharge curve calibrated to the original cell. When a new cell is installed, that stored curve no longer matches the actual voltage profile of the replacement. Under high-current load — active LTE, GPS, or screen at full brightness — the cell voltage drops faster than the fuel gauge predicts, and the phone shuts off to protect the cell before the reported percentage reaches zero. One full discharge-charge cycle after installation resets the coulomb counter and rebuilds the curve. After that cycle, shutoffs at 20–30% typically stop.

USB fast charge not activating after cell swap on the S31

The S31's charge IC uses a BMS handshake on the first post-swap charge session to verify the cell is within safe parameters before enabling higher current. If the cell sat in storage and arrived at a low state of charge, the IC may default to trickle mode — around 5V at low current — until the cell reaches approximately 3.0V per cell. Plug in via the stock cable, leave the phone off, and allow at least 20 minutes at trickle before attempting to power on. Once the cell clears the recovery threshold, the charge IC will step up to normal charging current automatically.