In an old World War II training film, the United States military tried to teach marines and soldiers how to avoid getting killed when fighting Imperial Japan. Instead of telling them what to do, they use the device of a captured Japanese fighter to explain why people get killed. Obviously, nobody wants to learn how to cause themselves big problems, but knowing how bad things happen can help you avoid them.
So, it was cool to see Jason at the Engineering Explained YouTube channel take this approach with EV batteries: (article continues after embedded video)
In the video, Jason explains three very common mistakes that not only make EV ownership undeniably worse, but cause the vehicle’s serviceable life to go way down. This is a big deal, because having to prematurely replace an EVs battery can negate the cost advantages of owning an EV in the first place.
But, in usual Engineering Explained practice, he doesn’t just do what I do and tell you what things to do and what things not to do. He dives much deeper and explains, down to the microscopic level, why some things are terrible for NMC batteries (and not necessarily for LiFePO4 or “LFP” batteries, which work differently at the atomic level).
He starts with three main rules owners should follow:
- Don’t store the car’s battery at 100% charge, especially when it’s hot outside
- Don’t wait to recharge the battery until it’s low.
- Don’t regularly charge to 100%
After all, why not? Why shouldn’t we charge to 100%?
EVs already generally have limited range compared to gas cars, so it’s more convenient to charge to 100% isn’t it?
Storing at 100% is bad because every kind of problem that ages battery cells happens faster in the heat and at higher voltages. We have to keep in mind that chemical reactions (which are what happens in a rechargeable battery) happen faster with more energy. So, keeping the energy levels in the cells lower helps greatly.
A big driver of degradation is called Solid Electrolyte Interface Formation, or SEI/CEI formation. (the C stands for Cathode). In both cases, it’s a buildup that happens on the surface of the battery’s original particles. Right off the bat, about 10% of the theoretical energy storage is lost the first time the battery’s charged, but the battery factory and vehicle manufacturers know this is going to happen and advertise the energy storage capacity that the cells happen after the first charge, so you’re never aware of this issue.
But, the growth continues over time, even if not nearly as quickly as it happened on the first charge. This buildup gets in the way and also takes away useful chemicals and materials in the battery, slowly whittling down the battery’s storage and power ability over time. Studies and real EV driver experience shows us that in just over six months, storing the cells at 100% charge in 50C temperatures causes a 40% loss in energy storage.
For long term storage (weeks to months), it’s best to store an NMC car at 30% SOC. So, when you’re going to go on a long trip away from home, be sure to keep the charge low until you get back. But, in cold temperatures, you really don’t have to worry about this.
Why We Shouldn’t Run The Battery Down Before Charging
Older rechargeable battery chemistries would develop “memory” problems if you didn’t run them down low before recharging them. But, newer NMC designs aren’t like this. It’s actually a bad idea to run them down to almost dead, or even to run them down more than needed to get to a destination. It’s better to reduce this “depth of discharge” to only what’s needed, for example going from an 80% charge down to 60% for your daily commute and then charging back up to 80% again.
Why? Because research shows that a bunch of little charging sessions is better for the battery than a bunch of deep recharges. This is because the cathode’s particles are made of little crystallites that let the lithium move back and forth. The crystal structures shrink and expand in different directions as the lithium comes and goes. Just like a brittle piece of metal, the more expanding and contracting and stretching and bending that goes in in these crystalline structures, the more you’ll start to have cracking and breaking, which gives more surface area for buildup.
So, discharging less puts less stress and damage on the lithium crystals, captain! It’s best to plug in after every trip and only use a quarter of the battery at most.
Avoiding 100% Is Key
Automakers want to get away from this problem by using single crystal particles instead of many small crystals that can more easily crack up and give surface area for buildup. Another way to avoid problems is with more nickel in the mix. But, the expansion of crystals happens more at 100%, and oxygen can escape from the battery cells even with smaller crystal structures.
So, if you don’t have a LifePO4 (LFP) battery, you’re best off to avoid 100%. The best recommendation is 75% for a good balance of range and longevity, but this is tough if you don’t have the ability to plug in every night.
If you can avoid all of these three things, you’re likely to have a battery that outlasts the whole vehicle. But, do what you can do even if you can’t get it perfect. Even an abused battery (assuming it’s a decent vehicle with liquid cooling) will still likely last a long, long time.
What About Fast Charging?
Many people worry about the stress of DC fast charging on EVs. For the most part, experts don’t recommend worrying about it anymore. Vehicles tend to control the flow of energy and try to avoid things that cause degradation. This will become even better over time.
But, do keep in mind that charging gets a lot slower above 80%, so it’s best to avoid a full charge except when you actually need the range to get s0mewhere. Not only does this help the vehicle get better longevity, but it can also save you a lot of time on road trips.
Featured image by the U.S. Department of Defense (Public Domain).
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