Many people will be familiar with how a smartphone’s battery can degrade over time. It can seem like that after barely a year of ownership, we’re all scanning for spare plug sockets as we walk into pubs, cafes and waiting lounges since our phones won’t even last a day without recharging.
Is the same thing happening in your electric car? Received wisdom is that it isn’t and early Tesla models, Nissan Leafs and battery-electric Renaults are not showing the rapid fall off in battery performance that some had predicted. Initial concerns that the original Nissan Leaf in particular, which doesn’t have active thermal management of its cells, would suffer when repeatedly stressed with high-current charging have been assuaged by what seems like software tweaks to prevent serial fast charging when the cells are hot.
THE AVERAGE BATTERY LIFE OF A NISSAN LEAF
Nissan recently indicated that monitoring its 400,000-strong fleet of Leafs has shown that the average useful battery life is between 10 and 12 years, with an all-out life of up to 22 years before the cells are completely dead.
Yet on the other hand, car companies (including Nissan) are working hard on second-life options for their automotive batteries, and it doesn’t take all that long scouring the internet to find posts from owners struggling with diminishing performance from their ageing EVs.
So what’s going on? Why do batteries lose effectiveness in the first place and is there anything you can do to extend their lives?
THE SCIENCE OF AN ELECTRIC CAR BATTERY
Lithium-ion (Li-ion) batteries comprise five main components: cathode; anode; separator; current collectors; and electrolyte. The anode and cathode store the lithium, and the electrolyte is the medium through which the positively-charged lithium ions travel. So, when you are charging the battery, the positive electrode (or cathode) gives up some of its lithium ions, which travel through the electrolyte and the separator to the negative electrode (or anode). When you discharge the battery, these lithium ions move from the anode to the cathode, generating a flow of electrons through the device that the battery is being used to power – in this case, the car’s electric motor.
In a Li-ion cell this process is reversible over many cycles and years, and furthermore Li-ion batteries carry more charge for less weight than alternatives such as lead-acid or nickel-metal-hydride cells, hence their use in electric cars.
WHY CHARGING AND DISCHARGING MATTERS
The charging/discharging process detailed above is crucial to battery health and longevity. “What’s happening during charge and discharge is a highly complex process and still not completely understood,” says Serena Corr, Chair in Functional Materials and Professor in Chemical and Biological Engineering at the University of Sheffield.
“The materials that make up your electrodes are typically layered structures, for example the cathode material can be lithium nickel manganese cobalt oxide (or NMC for short). One can think of this cathode like sheets of stacked paper, in between which the lithium ions can slot,” says Corr. “As this material undergoes multiple charge and discharge cycles, there are mechanical and chemical processes that can ‘weather’ the cathode and lead to its degradation. These can take the form of rearrangement of atoms, volume changes and even particle cracking.”
There is also a process of ‘coating’, where reactions of the electrolyte can lead to a permanent covering on parts of the electrode, which can reduce its capacity to accommodate ions and therefore its ability to hold a charge.
OTHER CAUSES OF BATTERY DEGREDATION
How you treat an EV’s battery can also affect how quickly it degrades. “There are a number of issues that can affect degradation, for example responses to external stimuli like temperature, charging and pressure,” says Corr. “Each set of battery chemistry brings its own set of challenges.”
Corr’s work is part of a UK-wide collaboration funded through the Faraday Institution, trying to understand the degradation in EV batteries and avoiding detrimental pathways and practises.
“Car batteries are moving to cathodes with high-nickel content, reducing the cobalt content” she says, “partly because of the cost of cobalt which has risen steeply in the last few years, and also the ethical concerns of extracting cobalt.”
Nissan Leaf e+ Review
Why 2019 is the Year of the Electric Car
It’s those concerns around mining cobalt that are behind the consumer-electric and battery-vehicle industries desire to seek alternative sources and alternative materials.
“Part of the work we are doing is to develop coatings that can prevent degradation and to look at the effect of nanostructuring on degradation mechanisms,” says Corr, “Our team of researchers around the UK has the expertise to assess these degradation mechanisms from the atomic to the battery pack level and provide powerful insights into how degradation might be avoided.”
TUNE YOUR CHARGING HABITS
One of these insights is that as an EV owner you should try to avoid charging the battery to 100% of its capacity. Doing so, says Corr, stresses the cathode: “It’s asking a lot, putting pressure on the battery; they don’t like extremes much.”
Corr recommends charging from 50 to 80% only, even in cars which have software limiting the proportion of the battery capacity it uses in the interest of longevity.
“If you can do your daily commute with less than 100% of charge, then I would avoid fully charging the battery and stay within a lower boundary, say 50-80% charge, to try to extend your battery lifetime. I would also try to avoid discharging all the way to zero per cent as again this can cause increased degradation to your battery,” she says.
Allan Paterson, head of programme management at the Faraday Institution, goes further by explaining that even though the “operational window moves around, in an automotive battery-electric vehicle it is good practise not to have the pack fully charged or discharged.”
This isn’t quite as straightforward as it sounds, because the software for the battery looks not only at state of charge, but also voltage, current and temperature and will adjust the pack’s operating window according to combinations of all four. A plug-in hybrid vehicle, for example, might boost to 100% by voltage when regenerating electricity travelling downhill for a long period, but the system should be able to cope with this and still protect for an overly high state of charge. Early hybrid cars couldn’t and simply threw excess current into an air-cooled resistor under the car.
For this reason, explains Patterson, battery management systems also provide control at low states of charge and voltage to protect longevity and to maintain compatibility with the electric motor’s inverter, which changes direct current into alternating current.
There are also different issues with batteries used in pure battery cars and those in plug-in hybrids with high power ratings; both experience different duty cycles. “If we try to understand the chemistry better, we can improve the models, which help develop better control systems and therefore better-performing batteries,” says Paterson.
OTHER FACTORS THAT CAN AFFECT BATTERY LIFE
Temperature extremes are also potential problems, accelerating battery deterioration, although most modern automotive batteries have sophisticated thermal management systems to maintain optimum values when it’s icy or when fast charging on the hottest day of the year.
There’s also an issue with how you store the vehicle when you leave it for long periods. “A lot of effort goes into manufacturers’ life analysis,” says Paterson. “There’s degradation caused by usage and then there’s degradation caused by calendar life and how the car is stored. Additionally, if you leave the battery at a high state of charge, there’s proportionately a higher rate of self-discharge than if you leave the battery at say at 50 per cent charge, but this is only really apparent over very long periods of inactivity, over several months.”
THE KEY TAKEAWAY
In the end we all want our vehicles to last as long as possible, both for the enhanced residual values that implies, and because keeping a vehicle in use for longer also reduces its environmental impact. For that reason, it could even be worth choosing a vehicle with a greater range than you strictly need so that you aren’t forced to recharge to the max every night. As Corr points out, “extra capacity is no bad thing.”
In the market for a used car?
CarGurus makes it easy to find great deals from top-rated dealers. CarGurus compares price, detailed vehicle data and dealer reviews to give each used car a deal rating from great to overpriced, and sorts the best deals first. Find out more and begin your used car search at CarGurus.