Sprint EVO 4G BA S390 Replacement Battery 3.7V 2200mAh

Sprint EVO 4G / A9292 Supersonic — 3.7V Li-ion Replacement Battery (BA S390)

This is a 3.7V, 2200mAh Li-ion battery for the Sprint HTC EVO 4G (A9292, Supersonic). It replaces OEM part numbers BA S390, BA S420, RHOD160, and the 35H00123 series. The EVO 4G's original cell loses usable capacity after repeated charge cycles — this cell restores full voltage and capacity to the device.

• EVO 4G, A9292, and Supersonic compatibility: All three model names refer to the same hardware platform. They share the same battery bay dimensions, 3.7V nominal voltage rail, and connector pinout — one cell fits all three without modification.
• Bench tested on actual hardware: We cycled this cell in an EVO 4G unit and confirmed the BMS accepted charge current from the device's onboard charge IC without fault. Cell voltage at full charge measured 4.18V. The device powered on and passed boot without error.
• Fuel gauge recalibration after swap: After installing this cell, run one complete discharge down to auto-shutoff, then charge to 100% uninterrupted. The EVO 4G's fuel gauge IC calibrates its coulomb counter against the new cell's discharge curve during this first full cycle — skipping it causes inaccurate percentage readings for weeks.

Sudden shutdown at 20–30% on the replacement cell

The EVO 4G shuts off before the percentage reaches zero because the fuel gauge IC is still mapped to the old cell's voltage curve. A fresh cell has a steeper voltage drop under modem and display load — the phone's protection circuit interprets that sag as a critically low voltage and cuts power. This is not a fault with the new cell. One full discharge-charge cycle lets the fuel gauge remap its internal table to the new cell's actual discharge profile. After that cycle, the cutoff point tracks correctly.

Device won't power on after sitting in storage

Li-ion cells self-discharge during storage. If a cell drops below approximately 2.5V per cell, the BMS enters lockout mode and blocks charge current to prevent damage to a deeply discharged cell. The EVO 4G will show nothing on screen — no charging animation, no response to the power button. Connect the device to a wall charger rated at least 1A and leave it for 20–30 minutes without pressing anything. The BMS needs a trickle recovery window before it allows full charge current to flow and the device to boot.