Casio BTR811B Commando 2 Replacement Battery 3.7V 1800mAh

Casio Commando 2 / Commando 4G LTE — 3.7V Li-ion Replacement Battery (BTR811B)

This is a 3.7V, 1800mAh Li-ion cell that fits the Casio Commando 2, Commando 4G LTE, C811, and CA201. It replaces OEM parts BTR811B and CA-201L1. Capacity is 6.66Wh — matching the original spec so the fuel gauge IC has a clean reference curve to work from.

• Commando 2 and 4G LTE compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The 65.53 × 45.10 × 5.40mm cell fits either chassis without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Commando 2 platform. The BMS accepted the handshake on first contact, and the charge IC hit a full 4.2V termination voltage without flagging a fault.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated reference.

Sudden shutdown at 20–30% on the Commando 2 after a cell swap

The Commando 2 runs a modem that pulls sharp current spikes during signal acquisition. A new cell with a slightly higher internal impedance than the aged original can cause a momentary voltage sag under that load. The fuel gauge IC reads that sag as a low-voltage cutoff event and tells the OS the battery is empty — even though the coulomb counter still shows 25%. One full discharge-charge calibration cycle brings the reported state of charge back in line with actual cell voltage.

Phone reports wrong battery percentage after installing a new cell

The fuel gauge IC on the Commando 2 builds its percentage model against the discharge curve of the previous cell. Drop in a new cell and that learned curve is stale — the IC will over- or under-report by 10–20% until it relearns. Drain the battery until the phone shuts itself off, then charge uninterrupted to 100% without disconnecting. After one complete cycle, the coulomb counter resets its reference and percentage accuracy returns to normal.