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In the last article, we discussed battery fires and the hazards it poses. In an EV, there are several degrees of fire safety. There are safety measures at cell level and battery pack level to prevent short circuits and other failures that may result in overheating and thermal runaway in an ideal condition. 

Few protection measurement in place at cell level are: 

Separator:

It facilitates movements of ions across electrolyte and prevents physical contact between anode and cathode. They greatly impact the cell’s life and performance. Li-ion batteries majorly have polyolefins (polyethylene and propylene) as separator due to its high chemical stability and low cost. 

Gas vent mechanism:

In cases of thermal runaway or cell puncture there tends to be buildup of gas, if not released might lead to cell bulging to alarming levels and even rupture cell and lead to high pressure gas release. To ensure safe release of this gas buildup the cell has vent space at the positive terminal to ensure room for gas expansion and passage for escape. 

Cell casing:

The movement of ions between anode and cathode in Li-ion batteries takes place in electrolyte, which is sensitive to contaminants. The entire cell consisting of anode, cathode, electrolyte and separator also needs protection from mechanical rupture. The cell casing helps address this issue and ensures the cell is highly resistant to environmental factors.

Current Interrupt Device:

It’s a fuse-type device that disconnects the electrical circuit in case of high pressure, high voltage or over-temperature. Apart from these cell level protection, we also have several battery pack level protection which can be categorized into following categories:

Electrical protection:

Fuse: 

It prevents the battery pack circuit from reaching an over-current or short circuit condition. It protects the circuit by melting in case current higher than its rated value passes through.

Pre-charge circuit:

It is used to prevent high inrush current flow through the battery during turn-on.   The high inrush current occurs due to capacitor present at load side.  This capacitor, initially at 0V, draws high current from the battery pack. This can damage the fuse, contactors or cells itself. The pre-charge circuit is essentially a parallel path across the battery – load main power line, with resistor in place having high resistance.

Kapton tape: 

These tapes have properties like thermal insulation and electrical insulation, it’s used as a protection for routing the cables neatly and keeping the battery pack and Battery Management System and related components intact.

Thermal protection:

Modules:

The entire battery pack is divided into several modules, which makes the battery pack modular and easy  to service. Also, the modules provide more spacing compared to cell spacing, preventing thermal runaway.

Cell spacing:

Cells in a battery pack are kept at some distance from each other for effective heat dissipation and optimum performance of battery. The cell spacing can have air in-between or coolant, depending on the manufacturer for cooling.

Coolant: 

It is important to cool the battery as it is subjected to continuous charge and discharge which can generate a lot of heat. Considering the case of big battery packs, air spacing does not suffice in effective cooling. Coolants like Ethylene glycol or water not only prevent thermal protection but improve battery pack performance.

Mechanical protection:

Battery pack casing

The casing protects the battery from external shocks under normal circumstances and in cases of crashes. Batteries being the energy source contain large amounts of energy packed in small space, being vulnerable to punctures and fires they are extremely dangerous if not handled properly, The metal casings help in that case. Also, it’s extremely important to route the connectors properly along with firmly strapping the battery packs, battery management system and other components in place inside the battery pack casing. 

Conclusion:

Cells are grouped together to form modules which are placed adjacent to each other with enough spacing to minimize chances of a thermal runaway. The modules are placed together to form a battery pack kept in a housing made of cushion and metal which helps in dissipation of heat. Few battery pack housings are equipped with fire-prevention and extinguishment systems, ( Audi invents EV battery with fire extinguisher built in – ArenaEV news)  that detects the possibility of a fire and intervenes to prevent it or, if one has already started, put it out before it causes significant damage to the vehicle, or the passengers.

The battery management system (BMS) is capable of dealing with any risks to the battery, both internal and external, as well as interacting effectively with the vehicle’s other safety systems as needed. It provides several levels of cut-off based on different fault conditions like over-current, under-voltage, over-temperature etc. It also performs other functions like continuous monitoring of batteries and balances the cells in case of disbalance. BMS can effectively be called the brain of an Electric Vehicle.  Furthermore, the vehicle’s design should preferably contain safety features that safeguard the battery pack(s) in the event of an accident.