High Voltage Battery is rightly one of the largest concerns for people buying electric vehicles. In this episode I will review the recent Tesla Impact Report, where they talk about battery capacity retention and — out of curiosity — will also borrow the Tesla Model X with 30K miles on it from my friend Iliya to check the battery degradation. Stay tuned!
I am Michael Bakunin. Electric vehicles are my full time job and passion. I recently joined the Wisk’s Sustainables group session to discuss the electrification of transportation. Wisk is developing all-electric self-flying air taxis and one of the questions which came up during that discussion was high voltage battery durability and longevity. My answer was straight to the point — automotive batteries will outlast vehicle bodies. I know — this answer doesn’t normally sit well with everyone. Tons of doubts! But when I opened the Tesla’s Impact Report 2020 I was glad to discover the exact same wording: ‘Tesla’s battery packs are designed to outlast the vehicle. We estimate that a vehicle gets scrapped after approximately 200,000 miles of usage in the U.S. and roughly 150,000 miles in Europe.’
This time they’ve even published a chart to prove that. On x-axis you have thousands of miles driven and on “y” battery capacity or Battery State of Health. As you can see — battery capacity reduction happens a little bit faster in the beginning during the first 5 to 10 thousand miles, but then the curve stabilizes and at 200K miles on average a Tesla battery would retain 90% of its capacity. We don’t really know which models and battery technologies were included in this sample, but at least standard deviation in blue here helps us understand that the data is pretty consistent. Which made me think that I could just borrow a 3 years old Tesla from my friend for a few days and check whether or not the battery pack sits in the standard deviation range.
The first thing I did — I went to a supercharger in my neighborhood and charged it to 100%. What I saw on the screen one hour later was quite disappointing — 269 miles driving range. When new — 100D was supposed to go 305 miles on one charge and 30 thousand later I am only seeing 269 miles, which translates intp 12% capacity loss. This is kind of shocking. Aren’t we supposed to retain 90% after 200K miles? I don’t have Tesla’s data sets, but I tried to visually find battery retention value after 30K miles — and according to Tesla it needs to be 95% plus minus 2%.
The truth is that all Tesla forums and Facebook groups are full of similar discussions. New Tesla owners are panicking. They are paying for a certain driving range and suddenly after a few months the miles are melting! What if my battery is also melting? Not literally, but you get my point. So new owners would take screenshots and post them all over the place asking the same question again and again — what’s happening with my battery? Do I need to reach out to Tesla and complain? Is this covered by warranty?
Let me explain what is exactly happening here. First — your new battery capacity loss is a bit steep and noticeable in the beginning as you can see on the chart published by Tesla, but degradation is slowing down after the first 5–10K miles. Second — what you actually see on the screen is not only your real battery capacity loss. To make it simple — it’s a combination of both capacity loss and most likely lack of calibration of your Battery Management System. Battery Management System or BMS would monitor key battery parameters such as voltage, current, temperature; control cooling and also estimate your driving range. So the driving range estimate you see on the screen is actually coming from BMS, which is correlating battery’s voltage with its capacity.
The way we charge and use our Teslas doesn’t actually help BMS provide accurate estimates. Sentry Mode for instance would constantly bug the battery and will not allow BMS to do proper readings. Another issue is State of Charge. For normal daily use Tesla would recommend you to charge up to 90% and plug in your EV every day. And what it does — it doesn’t allow BMS to do all the necessary readings at 100%, at 40%, at 20% and so on and so forth.
12% battery capacity loss on my friend’s Model X after 30K miles looks outrageous. I’ve turned off sentry mode and decided to go for a couple of full charging and discharging cycles. Charing as high as 100% and going as low as 10% to calibrate the BMS. Luckily that weekend we had some family plans to visit San Francisco, which allowed me to drain the battery with a good purpose.
After the second charging cycle the driving range slightly improved from 269 to 272. I didn’t have much time for additional tests, but I am pretty sure that, if my friend continues providing additional data points to the BMS, the driving range will improve, but it still may not be in the Tesla’s average retention range. And the reason for that is frequent supercharging which my friend Ilya used. You may remember that early Model S and X owners were getting a very generous bonus from Tesla — free unlimited Supercharging. While it saves a lot of money and adds convenience — high power charging in general is one of the key reasons for battery degradation. Tesla has a couple of mitigation factors such as battery liquid cooling and reduced charging speed in cities versus freeways — that’s actually why it took me 1 hour to fully charge — but overall batteries are overheating during supercharging, which is not good for battery chemistry and obviously they are losing capacity.
What can be done in this case? Tesla’s battery warranty coverage is pretty good — 8 years or 120K miles for Model 3 and Y and 150K miles for Model S and X, but with minimum retention of 70%. Looking again at Tesla Impact Report — on average Tesla owners should never be in a position to request high voltage battery replacement. This obviously excludes outliers such as very frequent supercharging such as what we see on Ilya’s Tesla, but even for Iliya — I am not sure he would cross the 70% threshold.
A good lesson for those of you who are planning to buy a used Tesla which comes with a free supercharging option. The first thing you need to check is the retained battery capacity when the battery is fully charged. High chances that the previous owner was using supercharging quite a bit and the battery was losing capacity faster than what would normally be expected.
What truly excites me in the Impact report is the fact that Tesla is again talking about creating a battery which could last 1 million miles. They are rightly saying this technology could be a good fit for vans, trucks and robotaxis, but what I read between the lines — there is no intention yet to introduce those 1 million miles batteries in the B2C segment. I think it’s a bit of a conflict of interest. If we all start having electric vehicles which can last ten or twenty years, we won’t need that many car makers, so basically shooting in the foot. Anyway — exciting times!
Does your EV have a noticeable battery degradation? Do let me know in the comments section here below and see you next week!