One of the most common statements I get on my EV and Tesla videos is around battery longevity. Statements like, “you’ll have to replace your battery in 5 years.” Or, “it’s going to cost you $10,000 to replace in a few years. There goes your ‘savings’ from driving an EV.” And from EV owners I hear questions like, “I’ve lost 8 miles of rated range on my car. Should I be worried?” Well, I’ve partnered with the developer of the fantastic iOS app, Stats, to take a look at some real world data to see if we can address those concerns when it comes to the Model 3.
So first things first, let’s address the statement you often hear, which is “the battery will cost you thousands to replace in a few years.” This statement feels like it would be true. We’ve all had first-hand experiences with something like a smartphone that doesn’t hold a charge after a couple of years. Or a laptop that you can’t use for more than what feels like a few minutes before it has to be plugged in again to charge up.
A phone has a lithium ion battery. An EV has a lithium ion battery. So it must be true that an EV’s battery will be pretty much worthless after a few years, just like in your smartphone. The problem is that correlation doesn’t imply causation. Humans are natural pattern recognition machines, so we’re prone to find perceived connections between things when that connection actually isn’t there … or in this case … is just misunderstood.
I’ve put out several videos on battery technology and research that’s currently underway. I’ll include some links in the description if you’re interested in seeing more, but there’s some pretty big differences between the battery in your phone and the battery pack in an EV.
The first major difference is the number of batteries we’re talking about. You have one single lithium ion battery in your smartphone. An EV is made up of lots of tiny batteries strung together in a single battery pack. A standard range Tesla Model 3 battery back has 2,976 individual cells in the battery pack. A long range battery pack is made up of 4,416 individual cells1 that are lined up in rows with a sophisticated battery management system, which controls charging, discharging, and temperatures. The system is managing the individual cells to maximize their efficiency and longevity, so the degradation that happens over time is carefully controlled. It’s very different from a single cell in a smart phone.
The second major difference is charging behavior. How many of you plug your phone in at night when you go to bed and then unplug in the morning when you wake up and head off to work? And then sometimes plug your phone in at work or on your commute in the car? Lithium ion batteries are happiest when you use them between 30% – 70% state of charge. It’s harder on a battery when you drain it to zero and charge it to 100%. So constantly pushing your phones battery to 100% again and again, as well as leaving it plugged in at 100% for extended periods, will actually accelerate the degradation of the battery. Manufacturers have been implementing battery management systems to help combat this, like in Apple’s iOS 13 update. The system learns over time when you typically get up in the morning. It will charge your iPhone to 80% overnight and wait to trickle charge to 100% shortly before you normally get up.2 Systems like this can help reduce the strain on the battery and improve its lifespan.
EVs, like the Tesla Model 3, also recommend a daily charging amount. When you plug in your car the Tesla’s UI will show you the recommended range for daily charging, and also make it clear that going above 90% should only be done for longer trips. As soon as you hit 100%, you should start your trip immediately. Keeping your daily charging routine below 90% helps maintain good battery health.
Here are some results from a study that showed how charging behavior impacts lithium ion battery longevity.3
The Y axis represents the total charge capacity available and the X axis represents the number of charging cycles. The batteries that charged to 100% and drained down to various states of charge fared worse than the batteries that charged to 75% or 85% before draining them down. To make it very clear, let’s compare two data points from the chart. Both of these used 50% of the battery’s capacity for each cycle, but one charged to 100% and the other to 75%. After 1,000 cycles the 100% battery was down to just above 90% of original capacity. Extend that out and the gap widens after thousands of charging cycles.
And finally, an EV’s battery pack usually has a buffer built in to the top end and low end of the battery. That means the car’s display of 100% is probably not the actual 100% of the battery. Same for draining it to 0%. There’s wiggle room in the battery to maintain the cars electrical systems if the battery is run too low. This buffer gives the car’s battery management system some leeway in estimating the total range of the car. In some cases you can eat into that buffer slowly over time so the drivable range of the car appears unchanged.4 It’s not clear if Tesla is doing this or by how much, but there may be some wiggle room there.
What all this proves is that you can’t equate your phone’s battery lifespan to what you can expect out of your car’s battery lifespan.
Battery degradation – by the numbers
So let’s look at some data that Ramin, who’s the developer of Stats, compiled to see how Tesla Model 3 batteries are holding up. Looking at about 15,000 data points of Stats battery range data, you can see some pretty clear trends taking shape. If we look at the median range reported for the long range rear wheel drive Model 3, you can see a quick decline in range over the first 15,000 miles and then it starts to level off a bit. This fits in with what we’ve seen with the Model S, which also slows down degradation as you accumulate more miles. It’s a non-linear form of degradation.5
If we say that the average driver puts on about 12,000 miles a year, you might go from 324 miles to 313 miles, or a 3.4% drop in range in the first year. Year two goes from about 313 to 308, which is only a 1.6% drop for the following year. The long range AWD Model 3 shows similar drops; going from 310 to 301 in the first 12,000 miles, which is about a 2.9% drop. And just like the RWD Model 3, the AWD’s following 12,000 miles decreases 1.6.%.
Tesla warranties their long range batteries for 8 years, or 120,000 miles, at 70% of original charge.6 That means a 310 mile, AWD, long range battery pack could potentially be around 217 miles of range after 8 years in the worst case scenario. Break that down for a year by year rate of degradation and you might be looking at a loss of 11.6 miles per year. But as you can see from the data, Tesla, like any other company’s warranty, is based on a worst case scenario … and how much you drive. A 12,000-mile-a-year driver could see an 11 mile decrease on the long range RWD Model 3 in year 1, but only a 5 mile drop the following year. The long term degradation should, if it follows what we’ve been seeing on the Model S and X, come in well above that 70% of original charge in 10 years for an average, 12,000 mile/year driver.
We can also take a look at the data in a broader view to see the full spread of reported ranges for each 1,000 miles driven. If you’ve never seen a violin graph before, this helps to illustrate the spread and concentrations within each bucket. Each curve is essentially a tiny bell curve on it’s side. The majority of people will fall in the center of the curve where it’s highest, which is also where the median falls. The lowest points of the curve are the outliers, which are less common, but still might be experienced by a few of you out there. I’m actually one of those outliers myself in this data.
There’s a lot to take in looking at this type of view, but there was something that jumped out me from the distribution. There’s a lot of rated range fluctuations in the spread from grouping to grouping, but there’s a big wrinkle to remember with rated range: it’s an estimate.7 And that estimate, which is generated using the battery management system, can lose calibration with the battery packs true maximum capacity. If you’re charging to 80% everyday, there may be a little drift in that estimate, which can be rebalanced by charging to 90% or full for a long trip on occasion. Software updates that get pushed out to your car may reset some of those estimates, which can also contribute to why you see them fluctuate up or down at times. I’ve seen this myself. After the v10 release, my rated range started to climb back up. Charging and driving habits, like not charging every day or driving everyday, can play a role in those degradation numbers looking high even though their battery health is just fine.
Ramin recently released a new app, Battery Compare, where you can plug in your rated range, current charge percentage, and odometer to see where your car falls within the data that I’ve shared in the video. Looking at my car, I’m falling outside the norm, on the low end, for cars that match my criteria. I’m keeping an eye on it, but I’m not worried about it though.
As EV owners, we need to look past the small fluctuations we may see in our rated range and keep our eye on the longer term picture. Follow charging and usage best practices like charging to 80% or 90% each day. Only charge to 100% for longer trips and start driving as close to reaching 100% as you can. Just as I showed in the data, when you look long term, you should be seeing something minor like a 4-5% drop over the first 24,000 miles.
EVs are still relatively new to a lot of people, so there’s a lot of fear, uncertainty and doubt out there. It’s easy to jump to conclusions based on limited experiences and believe some of the misinformation that’s out there. But even EV owners, like myself, can get caught up in the range anxiety and be hyper vigilant in tracking our battery’s rated range estimates. Any small drop can make us worry and get concerned about how long our car is going to hold out. In the end, my takeaway after seeing these numbers and doing a little digging: just relax and enjoy the car.
Jack Rickard Tesla Model 3 Battery Breakdown: http://www.youtube.com/watch?v=PvCOcBynlq0
Like Tesla Kim charging recommendations: http://www.youtube.com/watch?time_continue=474&v=G5t1YOHQhlc
Stats graphs: http://www.maadotaa.com/firmwares