AT&T Z431 Li-ion Replacement Battery 3.7V 900mAh

AT&T Z431 — 3.7V Li-ion Replacement Battery (Li3710T42P3h483757)

This 3.7V, 900mAh Li-ion battery replaces the OEM cell in the AT&T Z431 smartphone. It matches the original voltage rail and physical footprint — 47.67 x 37.27 x 5.80mm — so it seats correctly in the battery bay. Swap it in when the original cell no longer holds voltage under screen or modem load.

• Z431 platform fit: The Z431 uses a compact, low-capacity cell on a 3.7V rail with a recessed contact strip. This replacement matches that contact geometry and BMS handshake parameters so the phone's charge IC recognises the cell and proceeds through normal charge stages.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS held voltage within spec across the full discharge curve and did not trigger early cutoff under simulated screen-on and modem-active load.
• Fuel gauge recalibration on first cycle: After installation, run one full discharge to around 5% and then charge uninterrupted to 100% before resuming normal use. The Z431's fuel gauge IC is calibrated to the old cell's discharge curve — one complete cycle lets it relearn the new cell's characteristics before reporting percentage accurately.

Sudden shutdown at 20–30% on the Z431 after a cell swap

A new Li-ion cell has a slightly different internal resistance profile than the aged cell it replaces. The Z431's fuel gauge IC still references the old discharge curve, so it misjudges how much usable capacity remains. When modem transmit or screen brightness spikes current draw, the cell voltage dips momentarily — and the phone's protection circuit reads that dip as a critically low-voltage event and shuts down. The fix is one full discharge-to-charge cycle: drain to roughly 5%, then charge to 100% without interruption. After that cycle, the coulomb counter resets against the new cell's actual curve and the false shutdowns stop.

Z431 won't power on after sitting in storage with a flat battery

Li-ion cells that discharge below approximately 2.5V during storage trigger a BMS lockout — the protection circuit disconnects the cell to prevent damage from over-discharge. The phone shows nothing on screen when you press power, even connected to a charger. To recover, connect the Z431 to a wall adapter — not a USB port — and leave it for 20 to 30 minutes without pressing any buttons. The charge IC trickle-charges the cell back above the BMS re-enable threshold, typically around 2.8–3.0V, at which point the BMS reconnects and normal charging resumes.