One of the biggest concerns for car owners as the world switches to electric vehicles (EVs) is what happens to batteries as they’re used.
Various studies have been conducted: for instance, during a taxi trial at Gatwick Airport, five Tesla Model S 90D cars completed 1.5 million miles, each driving around 300,000 over a three-year period. Afterwards, the batteries were at an impressive 82% state of health (SoH).
The Model S was initially offered with an unlimited-mileage, eight-year warranty. However, that was reduced to 150,000 miles, eight years and 70% SoH in 2017.
Tesla will replace the battery in your recent Model S or Model X if it drops below 252-mile range in less than 150,000 miles or eight years.
Mercedes-Benz, on the other hand, offers a 100,000-mile, eight-year warranty on its EV batteries, and it’s the same for the Jaguar I-Pace. It’s clear, then, that second-hand electric car owners will have queries around long-life battery ownership.
That EV batteries degrade faster in the first year or two is widely known. The first year dropped about 8% in 100,000 miles in the Gatwick trial, but after that, the degradation was almost a straight line of 5% per year (driving 100,000 miles per year). At this rate, the vehicles would have easily passed 500,000 miles before they reached 70% SoH. This is more than three times the warranted distance.
Guest editorial originally published in Autocar sister title Car Aftermarket Trader (CAT) by Alex Johns, sales development manager at Altelium
How do I stop my EV’s battery degrading?
First, a technical term: knee-point. This is when a battery undergoes a rapid degradation – something that every battery and vehicle manufacturer is obviously trying to avoid. The phenomenon is subject to a great deal of study, but what is clear is that, on the assumption that your battery is well manufactured (without faults), then knee points tend to occur if the battery is overstressed.
This occurs through too much supercharging (which raises the battery temperature too much too often) and charging and discharging to the maximum too often. Running a battery from 10% or more to less than 90% state of charge (SoC) most of the time and rarely supercharging it will help ensure it lasts longer. ‘Resting’ the battery once a week for a few hours at a fairly low SoC will also help.
Battery SoH is an absolute measure and will become the currency of battery trading in the future. Although EV drivers focus on SoC, because it tells them if they have enough power to reach their destination, the measure that really matters in economic terms is SoH.
What could happen to EV batteries after their SoH drops below 70%?
A battery at 70% SoH may no longer be suitable for use in an EV, but it will be very useful in a ‘second life’ battery energy storage system (BESS) for several years (at least five) until it reaches 50% SoH, at which point it’s no longer commercially useful. The industrial knee-point has been reached.