HBU83S AT&T GoPhone U2800A Compatible Battery 3.7V 950mAh

AT&T GoPhone U2800A — 3.7V Li-ion Replacement Battery (HBU83S)

This is a 3.7V Li-ion replacement battery rated at 950mAh (3.52Wh) for the AT&T GoPhone U2800A. It replaces OEM part numbers HBU83S and HB4A1H. The U2800A is a basic GSM handset used for calls and messaging.

• U2800A compatibility: Both listed OEM part numbers — HBU83S and HB4A1H — share the same 3.7V single-cell configuration and connector pinout on the U2800A. The BMS charge termination voltage and protection thresholds match what the phone's charge IC expects.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a U2800A-class platform. The BMS tripped correctly at the low-voltage cutoff and accepted a full charge termination without fault flags.
• Fuel gauge recalibration on first cycle: After fitting this battery, run one complete discharge-charge cycle before enabling any power-saving shortcuts. The U2800A's fuel gauge IC was calibrated to the old cell's discharge curve — one full cycle resets that baseline so percentage readings track accurately.

Sudden shutdown at 20–30% on the U2800A after a cell swap

The U2800A's fuel gauge IC still holds the discharge curve from the original cell after a swap. When the new cell's actual voltage drops faster than the old curve predicts, the phone sees a valid percentage reading right up to the point the cell voltage collapses under GSM transmit load. That transmit burst draws a short, sharp current spike the new cell's internal resistance can't sustain without a voltage dip below the BMS cutoff threshold. One full discharge-to-cutoff and charge-to-full cycle forces the coulomb counter to rebuild the curve against the new cell.

Phone feels warm near the battery compartment on the first charge

A fresh Li-ion cell has higher internal impedance than a broken-in one. On the first charge cycle, the U2800A's charge IC pushes current into that higher-impedance cell, which converts more energy to heat than it would after a few cycles. This is a one-time effect — internal impedance drops as the cell forms. If the phone stays warm past the second full charge cycle, check that the battery contacts are seated flat and the back cover isn't trapping heat against the cell.