AT&T GoPhone U2800A HBU83S Replacement Battery 3.7V 700mAh

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

This 3.7V, 700mAh Li-ion cell replaces the original battery in the AT&T GoPhone U2800A. It matches OEM part numbers HBU83S and HB4A1H. When the original cell loses capacity through normal charge cycles, this replacement restores full functionality without replacing the handset.

• U2800A platform fit: The U2800A uses a fixed 3.7V single-cell architecture with a spring-contact connector. This cell matches that voltage rail and physical footprint — 53.78 × 34.05 × 4.80mm — so the door closes flush and the contacts seat correctly.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the U2800A platform. The onboard BMS accepted charge current without triggering overvoltage cutoff, and the protection circuit responded correctly at low-voltage threshold during deep discharge simulation.
• Fuel gauge recalibration on first install: After fitting this cell, run one full uninterrupted discharge down to automatic shutdown, then charge to 100% without interruption. The U2800A fuel gauge IC reads coulomb count against a stored cell curve — a new cell needs that first full cycle to reset its baseline before percentage readings are trustworthy.

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

The U2800A's fuel gauge IC carries a discharge curve calibrated to the old cell. A new cell with a different internal resistance profile hits a voltage cliff at a different state of charge than the stored curve predicts. When the modem fires a high-current burst — during a call or data sync — the cell voltage sags below the BMS cutoff threshold before the gauge reaches 0%. One full discharge-recharge cycle lets the IC relearn the actual curve. Until that cycle completes, expect early shutdowns under load.

Phone warm near the battery compartment during first charges

A new Li-ion cell starts with higher internal impedance than a broken-in cell. During initial charges, the charge IC pushes current into a higher-resistance load, and that resistance converts more energy to heat than usual. This is normal for the first two to three cycles as the cell's electrolyte settles. If the case stays warm to the touch beyond the third full charge, check that the charge port is clear and no debris is partially blocking the contact — the U2800A charge IC will sustain higher current longer if contact resistance at the port is elevated.