Understanding electric car battery health and performance

Battery health is one of the great unknowns – and therefore one of the great concerns – when it comes to buying and owning an electric vehicle.

After all, we all know an engine can eventually wear out. We understand that clutches fail, automatic transmissions can need rebuilding and turbos don’t last forever.

But what about the substantial lithium-ion battery sitting underneath an electric car?

How long will it last? Should you charge it to 100 per cent? Is fast charging bad for it? Should you avoid running it flat? Does driving your EV hard damage the battery? And how can you tell whether a used EV has a good battery or a dud?

There’s plenty of misinformation surrounding EV batteries, but there are also some relatively simple things owners can do to improve their chances of getting a long and healthy life from one.

And it’s something more Australians will need to understand as EV sales grow and increasing numbers of used electric cars enter the market.

To separate fact from fiction, carsales spoke to Nathan Gore-Brown, founder of Australian EV battery diagnostics specialist Test EV.

Gore-Brown has spent around 30 years in the automotive industry, starting as an auto electrician before becoming an engineer and working in the UK for Jaguar and Aston Martin.

After returning to Australia, he worked in the heavy vehicle industry before joining Tesla around a decade ago, becoming the first sales manager/dealer principal of its Brisbane store in Fortitude Valley.

He subsequently moved into EV consulting and has spent the past decade working on the transition to electric vehicles with councils, businesses and vehicle manufacturers.

About two years ago, he established Test EV, which distributes and supports Austrian company Aviloo’s independent EV battery diagnostic equipment in Australia and New Zealand.

Aviloo equipment is used locally by dealers, auction houses, workshops and pre-purchase inspection services, including carsales-owned RedBook Inspect.

The test takes about three minutes, gathering data from the vehicle before Aviloo’s system analyses it against its battery database to produce an independent assessment of battery condition.

Test EV has now conducted thousands of battery tests in Australia and that has helped Gore-Brown develop a comprehensive picture of what’s going on.

Importantly, Gore-Brown says battery health and battery performance aren’t necessarily the same thing.

Think of state of health as the size of the battery’s fuel tank. A battery with 90 per cent state of health can essentially store about 90 per cent of the energy it could when new.

But performance also depends on things such as whether the individual cells within the battery remain properly balanced and whether the battery’s sensors are operating correctly.

Gore-Brown’s number one piece of advice is to follow the manufacturer’s instructions.

Different battery chemistries and different manufacturers have different charging recommendations, so there isn’t one magic percentage that applies to every EV.

Many EVs with nickel manganese cobalt (NMC) batteries recommend limiting routine charging to around 80 per cent, while some lithium iron phosphate (LFP) batteries are designed to be regularly charged to 100 per cent.

“Follow your manufacturer’s guidance on where to charge to and how often to charge,” Gore-Brown says.

That also allows the battery management system to perform tasks such as cell balancing.

Running your EV’s battery down occasionally isn’t necessarily a disaster, but running it very low and then abandoning the car for an extended period is a different matter.

Gore-Brown says Test EV has seen surprisingly poor battery health in some low-kilometre vehicles.

In one group of cars he showed carsales, some examples with only about 20,000km had lost roughly 10 percentage points more capacity than comparable higher-kilometre vehicles.

While he cannot definitively establish why, the histories available suggest underuse may have contributed.

Leaving an EV sitting for months while its battery slowly discharges towards empty can stress the battery. At the other extreme, leaving some batteries sitting continuously at a very high state of charge isn’t ideal either.

If an EV is going into storage, check the manufacturer’s recommendations and periodically monitor its state of charge rather than simply parking it and forgetting about it.

The oft-repeated advice that you should never charge an EV beyond 80 per cent is too simplistic.

It depends on the battery. LFP batteries, increasingly common in more affordable EVs, can generally be charged to 100 per cent in accordance with manufacturer recommendations.

NMC batteries have greater energy density – helping make them lighter for a given capacity – but manufacturers commonly recommend a lower everyday charging limit.

Gore-Brown says Test EV’s results suggest LFP batteries are holding up well compared with NMC batteries, although he cautions against drawing firm conclusions because many of the oldest EVs being tested use NMC chemistry and therefore don’t provide a like-for-like age comparison.

Regular use of DC rapid chargers has long been regarded as something EV owners should avoid if they want maximum battery life.

There is some logic behind that. The faster energy is pushed into a battery, the more heat is generated, and heat is one of the enemies of battery longevity.

But Gore-Brown says the effect in modern EVs appears considerably smaller than many people believe.

Modern battery thermal management systems can pre-condition a battery before charging and then maintain it within its preferred temperature window during the charging session.

“The perception is probably greater than the actual impact on most cars,” Gore-Brown says.

That doesn’t mean you should fast-charge simply for the sake of it. Home AC charging is gentle on the battery and generally the most convenient solution anyway.

Even charging from a conventional power point is easy on the battery, although Gore-Brown says it is less energy-efficient than using a dedicated home wallbox.

Where an EV spends its life can influence its battery health.

Test EV’s Australian data shows vehicles operating in warmer climates tend to suffer greater degradation than equivalent vehicles from cooler areas.

Gore-Brown says analysis of auction vehicles found greater degradation for a given age and distance travelled among cars from warmer locations such as Brisbane compared with those from Melbourne or Tasmania.

Again, heat is the issue. It also helps explain why the quality of a vehicle’s battery thermal management system is important, particularly if it is regularly fast-charged or driven hard.

One of an EV’s great attractions is the instant performance available when you flatten the accelerator.

Enjoy it, but doing it constantly isn’t necessarily great for the battery.

Large amounts of energy rapidly flowing out of the battery during hard acceleration – and back into it under heavy regenerative braking – generate heat and stress.

Gore-Brown says Aviloo’s data indicates vehicles that are repeatedly driven hard suffer greater degradation than those driven more moderately.

That doesn’t mean you need to drive your EV as though there are eggshells under the throttle.

But if maximum battery longevity is the objective, smooth acceleration and braking are preferable to continually exploiting all that instant electric torque.

Plug-in hybrids add another consideration because they generally use much smaller batteries than pure EVs while still asking them to provide substantial power.

That means considerably more cycling and potentially greater stress on the battery.

Gore-Brown says PHEVs are disproportionately represented among the approximately two per cent of Test EV results that flag some form of battery fault.

For anyone buying a used PHEV, the basic message is the same as for a BEV: don’t assume the battery is healthy simply because the car has relatively low kilometres.

Mileage alone won’t tell you how healthy an EV battery is.

Gore-Brown showed carsales examples of low-kilometre EVs with unexpectedly high degradation and higher-mileage vehicles with excellent batteries.

Test EV has even encountered cars where an individual cell has deteriorated sufficiently that the vehicle shuts down while its range display still indicates around 15 or 20 per cent charge remaining.

An independent battery test can identify degradation, cell imbalance and sensor problems that may not trigger a dashboard warning.

Aviloo tests are available in Australia through Test EV’s testing network, including RedBook Inspect, and typically cost about $200.

As a broad guide, Gore-Brown says 70 per cent state of health is an important threshold because it is commonly the point around which manufacturer battery warranties are structured, although buyers should always check the warranty conditions applying to the particular vehicle.

Of the thousands of vehicles tested through Test EV’s Australian network, Gore-Brown says only about two per cent have flagged a battery fault of some description, which can include degradation, cell imbalance or sensor problems.

So buying an EV doesn’t mean you’re purchasing a battery with a ticking clock attached to it.

As Gore-Brown puts it: “There’s nothing to be scared about as long as you’re informed.”

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