Battery Management Systems

Cell Balancing

Cell balancing is required due to the inherent differences in the manufacturing of each battery cell. As cells are charged, some might charge sooner than others, reaching their maximum allowable voltage. When this happens, charging must stop to prevent the cell from being damaged. Unfortunately, this means that cells that aren't at their maximum voltage will be left undercharged. Later, when the pack is discharged, the cells that have less energy in them will discharge to their minimum voltage sooner at which point the battery must be shut off to prevent any cell damage. This again leaves many cells that aren't at their minimum voltage with energy still stored within them. This is wasteful and allows poorly performing cells to degrade more rapidly due to their increased charge cycles. All of this can be mitigated using cell balancing which attempts to ensure equal distribution of energy amongst all cells during the charging and discharging processes.

Active Cell Balancing

Here is a good explanation from Analog Devices: Active Battery Cell Balancing

Passive Cell Balancing

Passive cell balancing is where a battery cell is discharged through a resistor in order to bring all the cell voltages in the battery pack down to the voltage of the lowest cell. This method is usually only used for low currents below 1A as the discharged energy is dissipated as heat. If a significant current is required then active balancing should be considered.

Here is a good explanation from Analog Devices: Passive Battery Cell Balancing

Safety and Monitoring

As lithium and some other battery chemistries can be quote volatile when outside of their operating range, it is important of a battery management system to be able to sufficiently measure properties like temperature and voltage of individual cells. This data can also be used to feed algorithms that provide insight like state-of-charge estimation.

Typical Cell Safe Operating Ranges

Chemistry	Min Voltage	Max Voltage	Discharge Temperature	Charge Temperature
Li-Po	3.0V	4.2V	10°C ~55°C	5°C ~ 45°C
LiFePO4	3.0V	3.6V	-20°C ~ 60°C	0 ~ 55°C

Temperature Measurement

As per FS rules and industry good practice, a significant number of cells should have their temperature measured at the negative terminal frequently to ensure safety and preventing events like thermal runaway from occurring.

Typically this is done by placing a thermistor on the negative terminal of the cell with a thermally conductive, but electrically insulating material in-between for purposes of electrical isolation.

Voltage Measurement

To ensure cells are not being over charged or discharged, accurate voltage measurement of each cell is needed. The battery management system only needs to measure each series connected cell as parallel connected cells will always have the same voltage potential.

Note

Poly eRacing(Polytechnique Montreal) developed an open hardware BMS, named BMSafe. You can reach out to them in order to get theschematic, code, and layout for free.