AT&T U668AA Replacement Battery 3.85V 4000mAh

AT&T Motivate Max / U668AA / U626AA — 3.85V Li-Polymer Replacement Battery (386786)

This is a 3.85V, 4000mAh (15.4Wh) Li-Polymer replacement battery for the AT&T U668AA, Motivate Max, U626AA, and Maestro 3 smartphones. It slots in where the original cell sits and connects to the same battery management circuit on the mainboard. Capacity comes from the product specification — 4000mAh, matching the original cell rating.

• U668AA / Motivate Max / U626AA / Maestro 3 fit: These four AT&T handsets share the same battery bay geometry, connector pinout, and BMS handshake protocol — all run the same 3.85V nominal cell with identical thermistor wiring, so one part number covers the group.
• Bench tested on actual hardware: We cycled this cell on the Motivate Max and monitored BMS response through charge, load, and cutoff. The protection circuit tripped correctly at the low-voltage threshold and the charge IC accepted a full CC/CV cycle without fault flags.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before the charge controller pushes high current into an uncalibrated cell — skipping this step causes erratic percentage readings for the first several days.

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

This is a voltage cliff, not a capacity problem. Under modem transmit or screen backlight load, the new cell's terminal voltage drops faster than the fuel gauge predicts — because the gauge is still using the old cell's discharge curve as its reference. The phone's PMIC sees voltage fall below the safe operating floor and shuts down to protect the SoC, even though the coulomb counter says 25% remains. One full discharge-charge cycle forces the fuel gauge IC to relearn the actual voltage-to-capacity relationship for the new cell and eliminates the cliff.

USB-PD fast charge not triggering on first cycle after replacement

On the first charge after installation, the charge IC may default to standard 5V/0.9A rather than negotiating the fast-charge contract. This happens because the BMS on a new cell reports a high internal impedance, and the charge controller responds by holding back current until it builds confidence in the cell state. Plug in, let it charge fully at the slower rate without interrupting the session. On the second or third cycle, the fast-charge handshake resumes once the controller sees stable impedance data from the BMS.