AT&T Radiant Max PT40H426487W Replacement Battery 3750mAh

AT&T Radiant Max U705AA — 3.85V Li-Polymer Replacement Battery (PT40H426487W)

This is a 3750mAh (14.44Wh) lithium-polymer cell for the AT&T Radiant Max (U705AA). It slots directly into the phone and restores power to the processor, display, and wireless radios. Voltage is 3.85V nominal, matching the OEM spec from part PT40H426487W.

• Radiant Max U705AA fit: The U705AA shares a single battery platform with the Radiant Max branding. Both names reference the same handset, the same connector, and the same BMS handshake — one cell covers both.
• Bench tested on actual hardware: We cycled this cell on the U705AA mainboard. The BMS accepted the charge handshake without error flags, and the fuel gauge IC tracked state-of-charge across a full discharge cycle with no dropout events.
• First-cycle fuel gauge recalibration: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate its coulomb counter against the new cell's actual discharge curve before high-current charging pushes load into an uncalibrated register.

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

This is a voltage cliff issue, not a capacity issue. A new cell has a steeper discharge curve in the lower state-of-charge range than the degraded cell the fuel gauge IC was calibrated against. When the modem fires a high-draw burst — LTE handoff, screen-on event — the cell voltage drops faster than the IC predicts and the phone shuts down before reaching 0%. One full discharge-to-shutdown, followed by an uninterrupted charge to 100%, resets the coulomb counter and flattens the reporting curve. After that cycle, shutdowns at 20–30% stop.

AT&T Radiant Max not powering on after sitting in storage

If the battery self-discharged below 2.5V per cell during storage, the BMS trips a deep-discharge lockout and the phone shows nothing on screen — no charge indicator, no boot loop. Plug into a wall adapter rated at 5V/1A or higher and leave it for 20–30 minutes without pressing any buttons. The BMS needs trickle current at that voltage to release the lockout before the charge IC can begin a normal charge cycle. Once the screen shows a charge indicator, the cell is above the lockout threshold and will charge normally.