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

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

This 3.7V, 2200mAh Li-ion cell replaces the original BA S390 in the HTC EVO 4G (A9292, Supersonic). It fits the original battery bay without modification and connects to the same charge circuit. Voltage and capacity match the OEM spec your phone's fuel gauge IC was calibrated to read.

• EVO 4G / A9292 / Supersonic fit: All three model names cover the same hardware platform. The battery connector, BMS handshake, and 3.7V nominal rail are identical across the EVO 4G, A9292, and Supersonic variants — one cell fits all three.
• Bench tested on actual hardware: We ran this cell through charge and discharge on an EVO 4G mainboard. The BMS accepted the charge IC handshake on the first cycle, and the protection circuit tripped correctly at the low-voltage cutoff threshold without false triggers under modem load.
• Fuel gauge recalibration on first use: After installing this cell, disable fast charging for one full discharge-to-charge cycle. The EVO 4G's fuel gauge IC carries the discharge curve from your old, degraded cell. One full cycle at standard current lets the coulomb counter map the new cell's actual capacity before you return to normal charging.

Why the EVO 4G reports wrong battery percentage after a cell swap

The EVO 4G uses a coulomb-counter-based fuel gauge IC that builds its capacity model from repeated charge and discharge data. When the original cell degrades, the IC adjusts its curve to match a lower actual capacity. A new 2200mAh cell presents a steeper, healthier discharge curve the IC has never seen. Until one full cycle completes, the gauge reads from the old curve — so it may show 100% charge draining faster than expected, or the percentage may jump erratically. Run one full discharge down to automatic shutdown, then charge uninterrupted to 100% without fast charge active. After that single cycle, the fuel gauge IC resets its reference and percentage accuracy stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops below the modem's or display's minimum operating threshold before the fuel gauge reaches 0%. A fresh cell with full capacity delivers voltage cleanly at low state-of-charge, but if the phone shuts down abruptly at 20–30% on the first few cycles, the fuel gauge IC is still using the old degraded cell curve as its reference. Under combined screen and radio load, the phone sees the voltage as acceptable but the actual cell voltage sags and hits the protection cutoff. Complete two full calibration cycles — full discharge to auto-shutdown, full charge to 100% — and the shutdowns will stop as the IC learns the new cell's voltage-to-capacity relationship.