03.4 Electric vehicle batteries
What is an electric vehicle traction battery?
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.
Traction battery
How an electric vehicle
lithium ion traction battery pack is constructed
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.
Cylindrical
(see below for detail)
Pouch
Prismatic
2.
Battery modules
Battery cells contained within a module (these are prismatic)
3.
Battery pack
Battery modules connected in series create a pack
4.
Electric vehicle chassis (exploded view)
Battery pack
5.
Electric vehicle chassis
Battery pack contained in a battery housing with underbody protection & battery management system
How do lithium ion battery cells work?
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.
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.