lithium ion batteries
Picture shows Lithium Battery beneath the floor of a Nissan Leaf
Lithium ion batteries are used in many electronic devices, phones, laptop computers, power tools, and electric cars as they can store a lot of energy in a small space, are light, and can be recharged many times. An electric car battery will weigh 300-500Kg and consists of many small Lithium Ion 'cells' often looking similar to the 'Dry Cell' batteries you would put in a torch or a radio. These are assembled together into a strong casing, and controlled by a computer which manages their charging and discharging, called the 'Battery Management System' or BMS.
Lithium Ion Cells
A green coloured lithium Ion Cell is shown here alongside a conventional AA size dry cell battery as used in a radio torch etc, and available in the supermarket.
These Lithium Ion Cells contain an Anode, a Cathode and an electrolyte which between them can store electricity efficiently - that means nearly all the energy you put into the battery when you charge it, comes back out to power the car. Compare this to a petrol car where perhaps 25-35% of the energy in the petrol ends up powering the car.
The Tricky Part - The Battery Management System
Assembling many cells together is straightforward enough, however when powering the car, each cell performs slightly differently, and the Battery Management System must monitor the cells and adjust how the energy is taken from each, so that some cells do not age prematurely.
The battery Pack
This battery pack from a Nissan Leaf comprises 48 modules made up from individual cells, linked in series. Each module has a voltage of 7.5V, giving overall voltage for the car of 48X7.5V = 360 Volts.
At 360 Volts the battery pack is a High Voltage device and requires complex safety systems to protect users and mechanics. Any significant store of readily available energy brings hazards (the dangers of Petrol are well known) but with robust safety standards in place, lithium ion batteries are an excellent power source for light vehicles like bicycles and cars; even for low speed larger vehicles like city buses.
The 12 Volt battery in an electric car (extra little one)
For completeness I should mention the small extra 12V battery in an electric car which powers many systems in the car, and is charged from the main battery. This is needed because there are still power demands when the main battery is charging. This is usually a conventional Lead Acid battery , mounted under the bonnet/hood. This should last longer than a petrol or diesel car 12V battery, as it does not see the current draw those engines require while starting. For a Nissan Leaf this battery costs around £50 to replace should that be necessary, a possibility in older cars.