Why this fuel gauge exists — and the mistake it avoids
The BQ27505YZGT-J2 is a single-cell Li-Ion battery monitor that uses TI's Impedance Track™ algorithm rather than a simple voltage-to-capacity lookup. That matters because a voltage-based gauge will report a higher remaining capacity on a cell that's actually at a lower state-of-charge under load — the IR drop fools the ADC. Impedance Track™ compensates for battery impedance, discharge rate, temperature, and aging, so the reported state-of-charge stays accurate across the cell's life. It communicates over I²C, which keeps the pin count low on the host MCU.
Single-cell Li-Ion — and only single-cell
This part is designed for exactly one series cell (1S). If your BOM calls for a 2S or 4S pack, the BQ2084DBT-V143 handles 2 to 4 cells but uses an SMBus interface and a wider temperature range of -20°C to 85°C. Neither is a drop-in replacement for the BQ27505YZGT-J2.
For a new design this means no immediate sourcing risk; for a sustaining BOM it means the line is stable.
Package and layout — the 12-DSBGA gotcha
The 12-DSBGA package (0.4 mm pitch, 0.5 mm ball diameter) requires a via-in-pad or microvia PCB stack-up. If your board uses a 2-layer design with 0.3 mm drills, you will struggle to fan out all 12 balls without shorting to the ground plane. Plan for at least a 4-layer board with filled vias under the BGA.