Electric car battery life explained

Everything you need to know about your electric car's battery life

One of the key questions potential electric-vehicle buyers have is how far they can drive on electric power. Many people mention the term ‘range anxiety’ when talking about electric cars, saying they’re worried about having to find charging stations on a long journey.

While this was a valid concern in the past, electric vehicles have come a long way over the years. The latest technology on cars such as the 40kWh Nissan Leaf allows for a range of more than 160 miles in real-world use, while the Tesla Model S P100D’s 100kWh batteries can keep going past 360 miles.

However, remember that all cars are ‘consumables’. Petrol and diesel vehicles have many parts – such as clutches and cam belts – that wear and degrade over time, and electric cars are no different. The on-board batteries don't have an infinite lifespan – as with other components, their performance eventually degrades.

Electric car battery life explained

The most common type of battery in an electric car is a lithium-ion one. This is very different from the lead-acid 12V battery found in your car that works components such as the starter motor.

You’ll hopefully recognise the term lithium-ion (or li-ion), as this type of battery is already used in smartphones and computers. They're lighter than lead-acid batteries, and can also hold a charge for longer – both characteristics are important for electric vehicles.

The downside to lithium-ion batteries has traditionally been cost. Even though prices have fallen by 80% since 2010, according to a report by the McKinsey consultancy, lithium-ion batteries are still costly to manufacture, which is partly why electric vehicles are more expensive than normal cars.

As with all batteries, li-ion units have a finite lifespan. They lose capacity with every full charge and discharge – known as a ‘cycle’ – and the more cycles the battery undergoes, the more its capacity degrades. As a result, the car’s range eventually decreases.

However, this doesn't mean that after a year or so, your electric vehicle's performance and range will drop noticeably in performance and range. The drop is very gradual and slow, and if you do have any worries, carmakers often give extra cover – over and above the standard new-car warranty – to the components in the electric drive.

The Nissan Leaf 40kWh, for example, comes with a 100,000-mile, eight-year battery warranty. The Renault ZOE, meanwhile, has a 60,000-mile, five-year warranty. However, as with an internal-combustion engine, you’ll have to do plenty of miles before seeing a drop in performance. According to independent research from owners on the Dutch-Belgian Tesla Forum, the batteries in a Tesla still work at 90% capacity after 186,000 miles.

That said, the level of degradation you experience will depend on how you treat the car. There are a few factors that can adversely affect an electric vehicle’s battery life:

- Overheating
- Deep discharge
- Overcharging or high voltage
- High discharge or charge current

Batteries suffer if they’re exposed to very high outside temperatures, or if the battery’s own temperature rises. This can happen if the battery is plugged in for too long, although most come with safety mechanisms to prevent overheating. However, it’s always a good idea to unplug the car after a full charge.

Extreme cold will also negatively affect a car’s battery. This is because batteries work off chemical reactions. A cold battery means these reactions take longer and the battery isn’t able to give full charge capacity.

Overcharging is another concern. During charging, a battery’s voltage slowly rises. When a battery has been fully recharged, the voltage has reached a 'ceiling' – otherwise known as max voltage. This can't be breached, as doing so can damage or destroy the battery. To prevent this happening, batteries come a battery management system (BMS).

Deep discharge is using a battery’s maximum capacity repeatedly. Drivers who regularly drain the battery down to 10 or 5% wear it out faster, and scientists have found that battery life improves when deep discharge is avoided. This means it’s better to charge your car between 80 and 40% battery, for example, than between 80 and 5%.

Similarly, high discharge is using a large proportion of the battery’s energy over a short period of time. Tesla warns owners that frequent use of 'Ludicrous Mode' – a high-performance setting for the Model S P100D that allows for sub-three-second 0-62mph acceleration – will negatively affect the battery’s lifespan.

It’s not all bad news, though: you can recover lost battery capacity by using your car more sympathetically. Matt Cleevely from specialist independent garage Cleevely Electric Vehicles once took in a used Nissan Leaf with a State of Health (SOH) of 87%, but after a programme of regular use, better charging patterns and a different driving style, the SOH went up to 91% in nine months.

What happens to my car once the battery degrades?

First of all, we should emphasise it's extremely unlikely that a car’s battery will degrade to the point that it needs replacing. Nissan says it has only ever had to replace ‘a handful’ of Leaf batteries, despite there being thousands of models on the road.

In fact, unless you mistreat your battery, you’re likely to get 100,000 miles at the very least from it – and probably much more. By then, you’ll probably be looking for a new car anyway.

Should your batteries require replacing, though, what you need to do will depend on how you bought the car. If you have a battery hire or battery lease, you buy the car and lease the battery in a separate agreement. That way, you effectively own the car, but not the battery. So, if the battery’s performance degrades beyond a certain point, it'll be replaced or repaired free of charge.

On the other hand, if you bought the car and battery together, you may have to buy a new battery. How much that costs varies depending on the type and size of battery, but you should expect to pay several thousand pounds for one.

For example, Indra Renewable Technologies in Worcestershire quotes £6,500 to replace a degraded 24kWh battery on a Nissan Leaf – a 16-hour job. That may sound like a lot, but it’s comparable to the cost of replacing a petrol or diesel engine in a conventional car.

According to Matt Cleevely, though, you may not need to replace the whole battery. "The battery is made up of modules of cells, so batched sets. And even if the traction battery does fail, it can be due to a cell or two falling below parameters, and these can be individually recovered, or replaced individually."

What happens to the old battery once it’s been replaced?

Because many electric-car batteries are replaced once capacity drops below 70% of the original, there's still plenty of life in them. Increasingly, manufacturers are converting them for home purposes, where they can be used to store energy generated through solar panels.

Nissan, for example, has partnered with US power-management company Eaton to convert old electric-vehicle batteries into home electricity-storage units. Because the batteries come from older electric vehicles, they’re cheaper than new home energy-storage units, and can still hold plenty of charge.