CAT S41 Replacement Battery APP00223 3.85V 4400mAh

CAT S41 — 3.85V Li-Polymer Replacement Battery (APP00223)

This is a 3.85V, 4400mAh Li-Polymer battery for the CAT S41 rugged smartphone. It replaces the original APP00223 cell when the existing battery no longer holds a usable charge. Voltage and capacity match the S41's charge IC and fuel gauge IC specifications exactly.

• CAT S41 cell fitment: The S41 uses a sealed rear housing with a specific cell footprint — 66.40 × 58.22 × 7.65mm. This cell matches that footprint and the 3.85V nominal rail the S41's power management IC expects. A voltage mismatch here triggers an immediate BMS lockout.
• Bench tested on actual hardware: We cycled this cell on the S41 platform and confirmed the BMS handshake completes correctly on first boot. The fuel gauge IC accepted the new cell without reporting an error state, and charge termination triggered at the correct upper cutoff voltage.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before high-current charging is applied to an uncalibrated cell.

Why the S41 reports wrong battery percentage after a cell swap

The S41 uses a coulomb-counting fuel gauge IC that builds its percentage model from the previous cell's discharge history. When a new cell goes in, that stored model no longer matches the actual charge curve of the replacement cell. The IC keeps referencing stale calibration data until it completes at least one full discharge-charge cycle with the new cell. Until recalibration completes, reported percentage can read 10–15% higher or lower than the true state of charge.

Sudden shutdown at 20–30% on the S41 after replacement

This is a voltage cliff issue, not a capacity fault. Under modem transmission load or peak screen brightness, the cell voltage can drop sharply below the S41's minimum operating threshold — even when the fuel gauge still shows charge remaining. The phone's PMIC interprets that voltage drop as a critical low-power state and shuts down immediately to protect the system. Run one full recalibration cycle first; if shutdowns persist, check that the connector is fully seated and the BMS protection circuit reads at least 3.5V at rest.