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What happens when an EV battery reaches ‘end of life’?

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A profound question at the best of times, but in this case, thankfully we’re referring to the batteries used in Tesla electric vehicles. Alex Johns, sales development manager at Altelium questions whether or not some batteries may be getting retired prematurely, and wonders, what does the ‘afterlife’ look like for a Tesla battery?

Earlier this year, Tesla, the world’s leading producer of electric vehicles, made a number of high-profile announcements on its 2020 Battery Day, including an outline of its recycling scheme for end of life batteries. But, one thing I felt was missing was what Tesla would be doing with those retired EV batteries before they’ve reached the end of their useful life.

I have first-hand experience of the longevity and excellence of Tesla batteries and know they support the company’s goal to accelerate the world’s transition to sustainable energy.  

Five Tesla Model S 90D vehicles completed 1.5 million miles whilst stationed at Gatwick Airport when I was overseeing the trial of Tesla electric taxis there. They each drove approximately 300,000 miles during the three-year trial, yet still the batteries were at 82% State of Health (SoH).

The Model S was initially offered with an unlimited mileage, eight-year warranty. However, that was reduced to a 150,000 miles, eight-year warranty and 70% State of Health (SoH) in 2017. This means Tesla will replace the battery in the recent Tesla Model S or X if it drops below the  252-mile range in under 150,000 miles and eight years.

The fact the battery degrades faster in the first year or two is widely known. In the first year dropping about 8% in 100,000 miles during our trial, but after that, the degradation was almost a straight line of 5% per annum (100,000 miles per annum).

At this rate, the vehicles would have easily passed 500,000 miles before they reached 70% SoH. This impressive performance is more than three times the warranted distance.

What counts as end of life?

Tesla has announced a recycling scheme for its ‘end of life’ batteries but what counts as an ‘end of life’ battery? The consensus from other manufacturers looking at this question seems to be that a battery is still commercially useful if used carefully down to about 50% SoH.

One important feature which affects the viability of a battery for second life use is a ‘knee point’. A knee point is when a battery undergoes a rapid degradation, something every battery and vehicle manufacturer is trying to avoid. The phenomenon is subject to a great deal of study. On the assumption that your battery is well manufactured (i.e. without faults), then knee points tend to occur if the battery is over stressed.

This occurs through too much supercharging (which raises the battery temperature too much too often) and charging and discharging to the maximum too often. The probability of a knee point occurring gets too high for commercial reliability at below 50% SoH. That leaves a very wide band of life in the battery and begs the question, what is happening to the batteries (which have been removed from Tesla vehicles) between 70% and 50% SoH?

Battery SoH (when properly measured) is an absolute measure and will become the currency of battery trading in the future. Although drivers of pure electric cars focus on State of Charge, because it tells them if they have enough power to get to their destination – linked to ‘range anxiety’ – the measure that really matters in economic terms is SoH.

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 (usually at least five) until it reaches 50% SoH.

The battery afterlife

The BESS market is a retired EV battery’s afterlife, and it is set to take off dramatically.

The Australian Hornsdale Power Reserve, which boasts a power capacity of 150MW/194MWh, held the title of the world’s largest first life BESS for just three years before it was surpassed by the Californian Gateway Energy Storage in 2020 which has a storage capacity of 230MW/hour.

In itself this is less than half the capacity of the new Tesla plant planned at Moss Landing in Monterey County, California, at 730 MWh. All of them, however, will be dwarfed by the size of the 1,500 MW/6,000 MWh installation which was given approval in October 2020, also in Moss Landing California, for Vistra Energy.  

It is Lithium-ion BESS which is revolutionising the energy market, allowing energy from renewable sources of wind, sun and waves to be stored. Most BESS’ are made using first life batteries, but an increasing amount are being made from ex-EV second life batteries.

Tesla has told us how its batteries will last for one million miles or even longer (presumably to 70% SoH), but has made no mention yet of using second life batteries in BESS or any other second life use. So where are they going?

The EV battery sweet spot

The cost of second life batteries is so much lower than first life batteries that the Return on Investment (ROI) of second life batteries is higher in spite of the shorter lifespan of the second life BESS’.

A battery at 75% SoH will last half the length of the time of a new battery but cost far less than half the amount to buy. This is exactly the sweet spot where the ‘missing’ Tesla batteries will be: not yet ready for complete recycling, but also no longer in cars on the road.

Various companies (including Altelium) are already providing the information to enable vehicle manufacturers to pivot their batteries from EV to BESS applications. Advances in Artificial Intelligence (AI), machine learning and secure data sharing are making this possible. The more information we know about a battery, the higher its value. If we know what the battery has done all its life – its voltage parameters, temperature parameters and C rates – then we’re dealing with a known quantity.

With this information it is possible to buy the battery with confidence, in the full knowledge that some fuel cells have failed or will fail, and when, and especially if, a battery has hit or will be about to hit the knee point.

A new life

When an EV reaches between seven and nine years old, its battery will typically become available for second life uses. It will not be long before a whole generation of EV batteries, including Teslas, are ready to make the leap to their next stage in life. 

This article originally appeared in Electrical Review January/February 2021.

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