B2PQ9100 HTC One A9 Replacement Battery 3.85V 2100mAh

HTC One A9 — 3.85V Li-Polymer Replacement Battery (B2PQ9100)

This 3.85V, 2100mAh lithium-polymer cell replaces the original battery in the HTC One A9, One A9 LTE, and Hima Aero smartphones. It matches OEM part numbers B2PQ9100 and 35H00252-00M. Capacity figure is sourced from the product specification — 8.09Wh total energy.

• One A9 and Hima Aero platform fit: These models share the same 3.85V battery rail, connector pinout, and BMS handshake protocol, which is why one cell covers the full variant range including the 2PQ9120 chassis.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the One A9 platform. The BMS accepted the cell without error flags, and the charge IC completed a full constant-current to constant-voltage transition at the correct cutoff threshold.
• Fuel gauge recalibration on first use: 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 high-current charging pushes current into an uncalibrated cell — preventing erratic percentage readings early on.

Sudden shutdown at 20–30% on a replacement cell

This is a voltage cliff failure. The fuel gauge IC still holds calibration data from the old, degraded cell, so it reads the new cell's state-of-charge incorrectly. When modem transmit bursts or screen brightness spikes draw peak current, the cell voltage briefly dips below the BMS cutoff threshold — even though the gauge shows charge remaining. One full discharge-charge cycle forces the coulomb counter to re-anchor to the new cell's actual chemistry. After that cycle, shutdowns at false-high percentages typically stop.

Device not powering on after the battery sat in storage

Lithium-polymer cells self-discharge in storage. If the cell drops below approximately 2.5V, the BMS enters lockout to prevent damage — and the phone will not respond to a normal charger connection. Connect the device to a wall adapter, not a PC USB port, and leave it for 20–30 minutes without attempting to power it on. The charge IC on the One A9 runs a trickle pre-charge sequence that slowly brings the cell back above the BMS reactivation threshold, typically around 2.9V, before switching to normal charge current.