Batteries and the BMS
The voltage range of lithium batteries, balancing cells in series, and protection against overcharge, over-discharge and overcurrent. The basics when designing a battery device.
Lithium-ion cell
| Item | Value (typical NMC) |
|---|---|
| Nominal voltage | 3.6 to 3.7 V |
| Fully charged | 4.2 V |
| Discharge lower limit | 2.5 to 3.0 V |
| Charging | Constant current (CC) → constant voltage (CV) once 4.2 V is reached |
Charging above 4.2 V or discharging below the lower limit shortens life sharply and becomes dangerous. Ten 18650 cells in series (10S) make a pack of 36 V nominal and 42 V fully charged.
What a BMS does
- Cell voltage monitoring: measures the voltage of each cell in the series string
- Protection: cuts off with MOSFETs on overcharge, over-discharge, overcurrent, short circuit or overtemperature
- Cell balancing: cells differ slightly in capacity, so the highest cell reaches 4.2 V first. The high cell is discharged a little through a resistor (passive), or energy is moved between cells (active), to even them out.
- State-of-charge (SOC) estimation: current integration (coulomb counting) + voltage correction, Kalman filter
Design cautions
- The short-circuit current of a single cell is tens to hundreds of amps. Size the fuse and the wiring for this current.
- When a motor decelerates, regenerated current flows into the battery. Regenerating at full charge can trip overcharge protection.
- Temperature: no charging below 0 °C (lithium plating), and limited discharge at high temperature