A complete battery management system for multi-cell Li-Ion packs assembled from 18650 cells, built around Texas Instruments' bq76930 — an AFE-BMS, the analog front end that sits between the cells and the controller and does the measuring, protecting, and balancing.
Why
Off-the-shelf BMS boards are cheap but opaque — you cannot see per-cell behaviour, tune protection thresholds, or log anything useful. I wanted a pack I could actually instrument.
What it does
- Per-cell voltage monitoring across the series stack, read over I²C from the AFE-BMS
- Temperature sensing with thermistors placed against the cells
- Protection for over-voltage, under-voltage, over-current, and short circuit, with thresholds configured in firmware rather than fixed by hardware
- Passive cell balancing to keep the stack matched across charge cycles
- Coulomb counting for a state-of-charge estimate that survives partial cycles
Hardware notes
The design puts the AFE-BMS and its protection FETs on a board sized to sit directly on top of the cell pack. The high-current path is kept short and wide; the sense lines are routed away from it. Cell taps use a connector that cannot be seated incorrectly — a lesson learned the expensive way.
Firmware
Bare-metal C driving the AFE-BMS over I²C, with a small state machine for charge / discharge / fault handling and a serial interface for reading live cell data during testing.
Status
Working prototype, running on the bench and inside a portable power pack.