Battery Management Systems

A Battery Management System (BMS for short) is essential on electric cars due to the inherent danger of the cells. The primary responsibility of the BMS is to prevent the accumulator from entering conditions where it would be unsafe to use. It can also perform cell balancing to preserve the life of the battery cells. BMSes also serve as an interface for the rest of the car to gain information on the batteries (so that a datalogger can log the accumulator's state of charge, for example).

Safety and Monitoring

A 3Ah lithium ion battery has a similar amount of stored energy to an entire 20 round AK-47 magazine. Because lithium and some other battery chemistries can be quite volatile when outside of their operating range, it is important that the BMS can 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 frequently to ensure safety and preventing events like thermal runaway from occurring. Over-temperature conditions are most important, as they can lead to battery fires, but under-temperature conditions are also important if you live in a colder climate, since they can lead to premature degradation of the cells.

Typically temperature 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. The Enepaq (formerly Energus) battery segments use a temperature sensing diode. This is useful as diodes can be strung together in parallel to naturally get the highest temperature without any fancy circuitry.

Both FSAE and FS rules require that temperature sensing is done at the negative terminal. This is probably because more heat is produced in the cell's cathode than the cell's anode. However, the negative terminal might see more cooling than the positive terminal (depending on what cells you're using), so it could be worth having thermistors near both.

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.

Cell Balancing

Cell balancing is needed 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.

Balancing does not need to be "on" all the time, and is not required by the rules. Balancing only during charging or even every few weeks could be reasonable, depending on the health of your cells. If your cell voltage measuring is working, you can wait until you pass a voltage imbalance threshold before you turn on 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.

This can be done extremely simply by manually putting a resistor across each cell for a calculated amount of time. If you don't have enough freshmen to do this, it can be automated, either with custom circuits or off the shelf components.

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

Active Cell Balancing

Active cell balancing is where the energy released from the higher voltage cells gets sent to the lower voltage cells. Often times this is implemented with a system where any one cell can "charge" the entire pack, thus redistributing the energy. Very little energy is lost during active balancing, so the system can be designed with higher balancing currents in mind. Because of the inherent complexity of this system, it's usually only used if the extra efficiency is necessary (which it often isn't in FSAE applications).

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

This video outlines an interesting method of active balancing, where capacitors are switched back and forth across cells to equalize them. It would take a while to balance, but it would be simple to implement. Battery balancing by switched-capacitors : Theoretical consideration

Off–the–Shelf Battery Management Systems

Off–the–shelf BMSes are attractive due to the relatively little work involved in implementing them. Careful consideration should be taken that the system passes the rulebook, as well as any electrical and packaging constraints of your vehicle.

Orion BMS 2

The Orion BMS 2 is massive but has a good feature set, and can support up to 168S battery packs. One thing to watch out for is the galvanic isolation regulations. According to the FSAE rules, teams must have galvanic isolation between the segments. While the voltage sense connectors have 2.5kV isolation between them, the cell groups within the connectors only have 100V isolation between them.

Elithion Lithiumate Lithium-ion BMS

The Elithion Lithumate system is a distributed system, with one board per parallel cell block, and a separate control box that connects to the rest of the car. I am not positive but the temperature sensors on the distributed cell boards may not count towards the FSAE temperature sensing requirement due to their placement; review the rules to make sure.

Open–Source Battery Management Systems

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