AT&T Z331 Replacement Battery 3.7V 800mAh Li-ion

AT&T Z331 / X930 Series — 3.7V Li-ion Replacement Battery (Li3709T42P3h504047-H)

This is a 3.7V, 800mAh Li-ion cell that replaces the original battery in the AT&T Z331, X930, UX990, and GX991 smartphones. It matches the OEM part number Li3709T42P3h504047-H and sits within the same 50 × 39 × 5mm footprint as the factory cell. Capacity and voltage figures are taken from the product data — 2.96Wh total energy.

• Z331 / X930 platform fit: These models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell covers the full group without modification or adapters.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Z331 platform. The BMS accepted the cell, protection thresholds engaged at expected voltages, and the charge IC completed a full cycle without error flags.
• Fuel gauge recalibration on first install: On first use after installation, disable fast charging and run one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before the charge controller pushes higher current into an uncalibrated cell.

Sudden shutdown at 20–30% on the Z331 after a cell swap

The Z331's fuel gauge IC stores a learned discharge curve from the old cell. When a new cell goes in, the IC's stored model no longer matches the actual voltage-versus-capacity curve of the fresh Li-ion cell. Under modem transmit or screen load, the phone hits a voltage cliff the IC didn't predict and cuts power to protect the SoC. This isn't a faulty battery — it's a calibration mismatch. Run one full discharge down to automatic power-off, then charge uninterrupted to 100%. After that single cycle, the fuel gauge re-anchors its endpoints and the shutdowns stop.

Phone shows 100% immediately after install but drops fast

A replacement cell at storage charge — typically 3.7–3.8V — can read as full on an uncalibrated fuel gauge because the IC maps voltage to percentage using the old cell's impedance model. The new cell has lower internal resistance, so the voltage sits higher than expected for its actual state of charge. The percentage then collapses quickly once current draw exposes the real charge level. Fix: discharge the phone fully until it shuts off on its own, then charge to 100% without interruption. After one complete cycle, the coulomb counter resets its reference points to match the new cell. Target 4.15–4.20V at end of charge as confirmation the cycle completed correctly.