Modern lithium¿ion packs are unforgiving. Push voltage too high and you plate lithium. Charge below freezing and you destroy the cell. Trust a noisy current sensor and your SoC drifts into fantasy. A BMS exists to prevent all of that - but building one that is accurate, safe, and robust is far from trivial. This book closes the gap between academic electrochemistry and real¿world engineering. It is written for the engineer who must take a stack of cells, a microcontroller, and a deadline - and deliver a system that does not catch fire and does not lie about how much energy is left. What you will learnHow lithium¿ion cells really behave: OCV curves, internal resistance, temperature effects, aging How to design the core BMS algorithms: SoC, SoH, power estimation, balancing How to architect a complete system: AFEs, current sensing, thermal design, safety paths How to implement firmware that is robust, testable, and fault¿tolerant How to validate a pack: test plans, standards, certification, field diagnostics How to avoid real failures seen in e¿bikes, telecom systems, ESS, and EV prototypes Why this book is differentWritten by an engineer, not a theorist Focus on practical trade¿offs, not idealized models Real field failures, real numbers, real constraints Clear math without unnecessary derivations Includes reference equations, BOM templates, code snippets, and troubleshooting playbooks If you design, review, or debug lithium¿ion battery systems - this book will make your work faster, safer, and more confident.