T-Mobile Sparq II Replacement Battery CAB3120000C1 3.7V

T-Mobile Sparq II — 3.7V Li-ion Replacement Battery (CAB3120000C1)

This 3.7V, 800mAh lithium-ion battery replaces the original cell in the T-Mobile Sparq II (also sold as Sparq 2, One Touch 875, and model 875). It slots into the same battery bay and connects to the same charging circuit as the factory cell. OEM part numbers CAB3120000C1 and BTR875B both cross-reference to this unit.

• Sparq II / One Touch 875 compatibility: These model names all refer to the same hardware platform — same voltage rail, same connector pinout, same BMS handshake. One cell fits all listed variants without modification.
• Bench tested on actual hardware: We cycled this cell through a full charge-discharge sequence and monitored BMS cutoff at both ends. The protection circuit tripped correctly at low-voltage threshold and charge termination voltage held at 4.2V as expected.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate its discharge curve against the new cell before high-current charging pushes into an uncalibrated state.

Why the Sparq II reports wrong battery percentage after a cell swap

The Sparq II uses a fuel gauge IC that tracks capacity by measuring charge moved in and out of the cell over time. When you swap the cell, the IC still holds the discharge curve it mapped to the old, degraded battery. Until it runs one full cycle on the new cell, the percentage reading is comparing fresh capacity against a worn baseline — so the numbers will be off. One full discharge to auto-shutoff followed by an uninterrupted charge to 100% resets the coulomb counter and brings the percentage back in line.

Sudden shutdown at 20–30% on a replacement cell

This happens when the phone's modem or display draws a current spike that the cell cannot sustain at that state of charge. Even a healthy new cell can hit a voltage cliff under load before the fuel gauge reads zero — the BMS sees voltage drop below the cutoff threshold and shuts down to protect the cell. It is not a defective battery. Let the phone cool for two minutes, then power it back on — it will often boot to 15–25% and continue normally. If the shutdowns repeat at the same percentage, run two full calibration cycles to let the fuel gauge IC anchor its low-voltage model to the new cell's actual discharge curve.