How Do Electric Car Batteries Work?

Rowan Harris 8 minutes Published: 15/02/2022

Electric cars, we’re told, are much simpler than petrol or diesel cars. This should make them cheaper to run, repair, and (eventually) buy than conventional combustion engine cars. 

But EV batteries are a chemically complex bit of kit! In this article, we take a look at the different types of EV batteries, how they work, how they are recharged, and how they are rated. 

Types of electric car batteries

All battery electric vehicles (BEVs) run purely off electricity, which is stored in a large battery pack, typically located on the underside of the car. This powers one (or several) electric motor(s), which drive the car forward.

Almost all electric cars use lithium-ion batteries. These are similar to the ones found in laptops and mobile phones, though EVs use more robust thermal management solutions to extend their life (phew!).

One of the most common types of lithium-ion battery is Lithium-Nickel-Manganese-Cobalt-Oxide, abbreviated as NMC

What electric car batteries are made of can have big implications for things like their electric range and performance in extreme temperatures. Increasingly, manufacturers like Tesla are turning to Lithium-Iron-Phosphate batteries (LFP), which are significantly cheaper, though less energy dense. 

One of the other main electric car battery types is Nickel-Metal-Hydride (NMH). Toyota has used this in its Plug-in Hybrid Vehicles (PHEVs), though its newly-announced BZ4X EV uses a lithium-ion battery. By 2025, Toyota also hopes to be among the first to produce a mass-market solid state battery after promising results in early prototypes. 

Most electric cars still use a separate, 12V lead-acid battery to power things like the lights, windows and locking system. 

Tesla has already started moving to a 12V Li-on battery to address some of the issues with their lead acid batteries dying faster than usual, and it plans to eventually replace 12V batteries with a 48V configuration to allow for thinner, cheaper and lighter wires throughout the car. 

How do electric car batteries work?

A lithium-ion battery consists of an anode and the cathode (electrodes), a separator between the two electrodes, and an electrolyte that fills the remaining space of the battery. The anode and cathode are capable of storing lithium ions. Energy is stored and released as lithium ions travel between these electrodes through the electrolyte.

These batteries can be charged and discharged hundreds of times, but their performance will gradually start to diminish. This is a consequence of the fundamental chemistry of the battery, which produces unavoidable ‘parasitic’ chemical reactions. 

As the battery starts to degrade, you will start to notice a difference in how often you need to charge your electric car as your maximum range begins to drop. 

Future electric car batteries may see less battery degradation, with solid electrolyte batteries (solid state batteries) poised to offer up to 30 years of near-optimum battery State of Health (SOH)!

All lithium-ion batteries gradually self-discharge even if not connected and delivering current. Typically, the rate of self-discharge in a lithium-ion battery is around 2% a month, though this can be affected by external variables such as temperature, and State of Charge (SOC) when it is no longer being used. In EVs, the self-discharge rate may be slightly higher as the battery is often used to power auxiliary functions in a low power state.

How does charging work?

Unlike hybrid cars, all electric cars must be plugged in at some point to charge. But don’t worry if you don’t know how to charge an electric car, it’s pretty simple:

Home Charging

Electric car charging at home can either be carried out with a three pin plug and a ‘granny cable or a professionally installed smart home wallbox. 

While a granny cable is usually provided with your EV and can help you to get started with home charging without the additional expense of a wallbox, it’s much slower, and doesn’t allow you to pre-programme your charging if you are on a variable rate energy tariff. 

The smart wallbox will use something called a ‘mode 3’ charging cable to allow the car and charger to communicate with each other. This means you won’t have to unplug it when it’s full and you don’t get charged extra for leaving it plugged in.

Electric car charging at home with no driveway requires an on-street residential charging station. This will offer similar charging speeds to a home wallbox. 

Public Charging

Electric cars can also be recharged at public charging stations. 

There are a variety of EV charger types, and public charging stations are rated for different charging speeds (e.g. slow, fast, rapid, ultra-rapid), depending on whether they supply AC or DC power. How long it will take to charge your electric car depends on your car’s own charging capability, which is rated in kilowatts (kW).

Charging points can either be ‘tethered’ or ‘untethered’. Most slow charging points are ‘untethered’, which means you will need your own cable to connect the EV to the charging station. A Tesla supercharger, by contrast, will be ‘tethered’. 

Trials for wireless electric car charging are currently underway. Inductive charging pads may be used to charge the vehicle when stationary, or when moving, as we’ve seen in Sweden’s Smart Road Gotland project.

Regenerative Braking

Finally, electric car batteries can also be recharged using something called regenerative braking, or ‘Regen’. Put simply (sort of), this is the process of transforming kinetic energy into electrical energy through the act of braking. 

All electric cars have friction brakes, just like conventional cars. The problem with friction brakes is that a lot of the energy from braking is lost as heat, which also produces toxic ‘brake dust’ as a byproduct. 

With regenerative braking, when the motor stops, it disengages and begins to run backwards. This produces an electrical current as the coils pass by the magnets in reverse, which can be used to charge the battery and extend the electrical range of the vehicle. This is one of the reasons why EVs are much better suited to start-stop urban driving than petrol or diesel cars. 

How are electric car batteries and chargers rated? 

Moving from a petrol or diesel to an electric car can feel a little daunting at first. There’s a lot of new ‘jargon’ to learn. There may be no spark plugs or cambelts in an electric car, but things like ‘kilowatts’ and ‘kilowatt-hours’ are worth understanding, because they’ll tell you a lot about the kind of range you can expect from your car, the charging speed of the battery, and how efficient it is. 

Battery Capacity

Put simply, ‘watts’ are a unit of power. It’s a measure of amps and voltage combined, or the potential difference in charge between two points in an electrical field (Voltage) and the volume of electrons (Amperage). 

Watts are used to define how much power runs through a given power supply. A kilowatt (kW) is simply a thousand watts. A kilowatt-hour (kWh) is the amount of energy consumed in a given period. 

Electric car battery capacity is measured in kilowatt-hours. Think of it as the equivalent of the fuel tank size in a petrol or diesel car. 

Charging Speed

This unit of measurement is useful for calculating charging times, as chargers are also rated in terms of their power, measured in kilowatts (kW). With a 7kW wallbox charger installed outside your house, it will take one hour to deliver 7kWh of energy to your car. 

As a general rule of thumb, you can divide a car’s battery capacity (kWh) by the power of the charger (kW) to work out the amount of time it would take to charge your car. 

The Tesla Model S Plaid has a 95kWh battery capacity. If you decided to plug it into a 7kW wallbox at home, it would take around 13.5 hours to reach a full charge. Plug it into a 250kW rapid charging station and it should charge in 0.38 hours, or 22.8 minutes, according to this formula. 

This is where the formula becomes slightly less accurate. All EV batteries have what’s called a ‘charging curve’. While DC rapid and ultra-rapid charging stations are able to charge the batteries much faster than the AC wallboxes in our driveways, they can’t deliver full power to the vehicle from 0-100%. The car and/or the charging station will reduce the amount of power supplied as the car nears full charge to protect the battery. 

Efficiency

Whether you’re looking for the best family car or the best company car, ‘miles per gallon’ will no doubt be an important factor in your decision making process. Higher MPG figures mean the car is more economical to drive as you’ll burn through less fuel, and may also bring financial incentives (Lower BiK rates) as your car will produce less CO2 emissions. 

The efficiency of an electric car is probably not the first thing on your mind. EVs generally produce less lifetime CO2 emissions than petrol or diesel cars anyway, and if you drive an EV there’s no incentive for driving a more economical small electric car over a less economical electric SUV - even though the differences can be stark. 

A Volkswagen e-Up! has an efficiency rating of 5.3 miles per kWh, compared to the Mercedes EQC, which manages about 2.7 miles per kWh - almost half that of the e-Up!

With the cost of electricity already so much cheaper than petrol or diesel (with certain EV tariffs you could be paying just 5p/kWh of electricity overnight), choosing a more efficient EV may not be your top priority. 

However, as the above example demonstrates, you could easily cut your charging costs in half if you’re willing to go without a higher seating position and a more commanding road presence. 

If you do want to calculate the efficiency of your EV, simply divide the electric range of the car (miles/km) by the battery capacity (kWh)

Conclusion

If you’re new to EVs, it’s important that you understand the basics of electric car batteries and how to charge them. If this article had you scratching your head at points - don’t worry - that’s about as complex as it gets!

In fact, it’s because EVs are so much simpler that we can expect them to be cheaper to produce, repair and insure than petrol and diesel cars in the future. 

Check out the rest of our EV guide for a full run-down of the pros and cons of electric cars. Or, if you’re set on an electric car as your next ride, check out our electric car lease deals now!