03.4 Electric vehicle batteries

What is an electric vehicle traction battery?

How an electric vehicle 
lithium ion traction battery pack is constructed

An EV traction battery is rechargeable energy storage that supplies power to the electric motor very quickly, giving EVs high performance & rapid acceleration.

To better understand a traction battery, & the challenges of extinguishing one should it ignite, let's look at how EV traction battery packs are constructed.

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Traction battery

Multiple individual lithium ion battery cells are connected to make a battery module. A group of connected battery modules is contained within an enclosed battery casing with underbody protection. This is known as a battery pack.

In a passenger electric vehicle, the battery pack is typically located along the floor pan of the vehicle, in a rectangle or 'T" shape.

Commercial & public transport electric vehicles may have multiple battery packs located at the front, rear, sides & even the roof of the vehicle.

1.

Battery cells

Cylindrical, prismatic & pouch lithium battery cells are most commonly used in electric vehicle traction battery packs.

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Cylindrical
(see below for detail)

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Pouch

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Prismatic

2.

Battery modules

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Battery cells contained within a module (these are prismatic)

3.

Battery pack 

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Battery modules connected in series create a pack

4.

Electric vehicle chassis (exploded view)

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Battery pack

How do lithium ion battery cells work?

5.

Electric vehicle chassis

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Battery pack contained in a battery housing with underbody protection & battery management system

Like all batteries, lithium ion battery cells contain two electrodes; the cathode (positive) & anode (negative). The anode is made from carbon, primarily graphite, & the cathode from metal oxides, including lithium.

Structure of a lithium ion battery EV.png

Positive terminal

Anode -
negative plate

Positive tab

Battery cell case

Porous separator

Cathode -
positive plate

Negative tab

Electrolyte (between anode & cathode)

Lithium ions move between the electrodes through an electrolyte, creating an electric current through the conversion of chemical energy to electrical energy. A very thin porous separator sits between the cathode & anode.

Later, we look at thermal runaway, which may happen due to a short circuit in a battery cell & is how electric vehicle lithium ion traction battery fires start.