Google Pixel 4a BH39100 Replacement Battery 3.7V 1400mAh

Google Pixel 4a G20 — 3.7V Li-ion Replacement Battery (BH39100)

This is a 3.7V 1400mAh Li-ion battery for the Google Pixel 4a, identified by model code G20 and OEM part number BH39100. It replaces a degraded or failed original cell in the Pixel 4a chassis. Capacity is 5.18Wh, matching the stock specification.

• Pixel 4a G20 fitment: The G20 shares its battery bay dimensions and connector pinout with a narrow range of Pixel 4a production runs. The BMS handshake on this cell matches the charge IC expectations of that platform — voltage thresholds, temperature reporting, and the four-pin flex connector orientation are confirmed against the G20 board revision.
• Bench tested on actual hardware: We cycled this cell on a Pixel 4a G20 unit under mixed screen-on and standby load. The BMS accepted charge current without fault flags, held the voltage rail stable through high-draw events, and passed temperature reporting checks on the host board.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate discharge curve map against the new cell before fast charge current is applied to an uncalibrated coulomb counter.

Why the Pixel 4a reports wrong battery percentage after a cell swap

The Pixel 4a uses a coulomb counter in the fuel gauge IC that stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The OS reads percentage from this stale map, not from raw cell voltage, so the number on screen drifts from reality. One full discharge down to automatic cutoff followed by an uninterrupted charge to 100% forces the fuel gauge IC to rewrite its reference curve. After that cycle, percentage reporting stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops below what the modem or display load can sustain, even though the fuel gauge still shows charge remaining. The Pixel 4a's Snapdragon modem draws sharp current spikes that can pull the cell voltage below the BMS cutoff threshold — around 3.2V under load — before the gauge registers empty. It is more common in the first few cycles on a new cell before the fuel gauge IC has recalibrated. Run one full discharge-charge cycle without fast charging, then check whether shutdowns persist at that percentage.