T-Mobile Comet HB4J1 Replacement Battery 3.7V 1100mAh

T-Mobile Comet U8150-B / Rapport — 3.7V Li-ion Replacement Battery (HB4J1)

This is a 3.7V, 1100mAh Li-ion replacement cell for the T-Mobile Comet, Comet U8150-B, and Rapport smartphones. It fits devices using OEM part numbers HB4J1 or HB4J1H. Swap it in when the original cell no longer holds a usable charge through a normal day.

• Comet and Rapport platform fit: The Comet U8150-B and Rapport share the same battery bay dimensions, connector pinout, and BMS handshake protocol — all three models accept this 54 x 43 x 5mm cell without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Comet U8150-B. The BMS accepted the charge curve correctly, protection circuits triggered at the expected voltage floor, and the fuel gauge IC tracked state of charge without error after the first full cycle.
• Fuel gauge recalibration on first use: After installation, run one complete discharge-charge cycle with fast charging disabled. The Comet's fuel gauge IC is calibrated to the old cell's discharge curve — one full cycle against the new cell resets the coulomb counter and prevents erratic percentage readings from the first week of use.

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

The Comet's fuel gauge IC stores a learned discharge curve from the original cell. A new cell has different internal resistance, so the voltage drops faster under modem or screen load than the IC expects. At around 3.5V under load, the phone's protection circuit cuts power — even though the reported percentage still shows 20–30%. One full discharge-charge cycle lets the coulomb counter remap to the new cell's actual voltage-to-capacity curve. After that cycle, shutdowns at false-high percentages stop.

Phone warm near the battery compartment on the first few charges

A new Li-ion cell has higher internal impedance than a broken-in cell. The Comet's charge IC pushes current into that higher resistance, generating more heat per charging cycle until the cell's impedance drops after a few cycles. This is normal and not a sign of a faulty unit. If the device stays warm beyond the third full charge, check that the back cover is fully seated — a lifted cover traps heat against the cell and can cause the charge IC to throttle current unpredictably. Confirmed normal temperature is below 40°C at the back cover surface.