Electric cars are young enough in 2021 that we are still learning about how their batteries age over the years. One thing that scientists understand well, after 20 years of studying lithium batteries, is that battery health is sensitive to something called depth of discharge.
Depth of discharge refers to how much battery you use in between charges. For instance, if you have a 100 kWh battery, a 80 kWh depth of discharge is 80% of the battery's total capacity.
Although depth of discharge is often measured as the kWh that represents a certain percent of total capacity, it can also be helpful to think about it as the difference between your starting and ending state of charge when you use your EV. For instance, if you start a trip at 80% charge and end when you’re at 30%, you can think of this as a 50% depth of discharge - this is the definition we’ll use for depth of discharge in this article.
Nerdy side note: There is a nuance that we do not discuss in this article which is the difference between the physical and virtual battery, or total vs. usable capacity. For electric cars, many vehicle manufacturers prevent the battery from reaching fully charged or discharged states to mitigate rapid degradation. As such, 100% charge on the dashboard may be 90 - 95% of the physical battery. Similarly, 0% on the dashboard may be 5 - 10% of the physical battery. For the purposes of this article, we are referring to the usable battery capacity seen on your dash, which the research is talking about the physical battery capacity.
Depth of Discharge Battery Research
It has been fairly well studied that using lower depths of discharge can help preserve battery health, including lifetime, capacity, and power.
Research by Guena and Leblanc shows a “four-fold improvement is expected between 100% DOD and 50% DOD" meaning that a battery that is only cycled between 80% and 30% will hold its capacity four times as long as the expected life of a battery cycled from 100% to 0% -- although, in the real world, lithium ion batteries in cars never reach 100% or 0%.
They test the expected lifetime, measured in “expected number of cycles” and “expected number of equivalent full cycles,” based on various depths of discharge of some kind of battery. Here are some of their results:
Another study by Rechkemmer et al. also shows that battery health is improved by limiting depth of discharge and state of charge. They saw that holding the state of charge constant, a depth of discharge between 40-60% and 25-70% saw only a 12% capacity reduction after the equivalent of 700 full cycles. With a depth of discharge from 100% to 5%, though, the cells saw a 20% reduction in capacity with the same number of equivalent cycles.
These graphs from Preger 2020 show that as the depth of discharge increases (blue is the smallest; red is the largest), most battery chemistries see faster degradation and shorter lifetimes.The x axis measures how many “equivalent full cycles” (a measure of lifetime) the battery can be expected to have, while the y axis shows the percent of as the batteries age. LFP batteries (LiFePO4) show the least sensitivity to depth of discharge.
Depth of Discharge Lessons for EV Drivers
The takeaway is that less is more as long as it matches your driving needs and lifestyle.
Let’s say you need to use 50% of your battery over a weekend. Here are two ways you might use your car.
In other words, rather than using 50% of your battery before recharging, you might use 20% of the battery, recharge, and then use another 30%. In the example above, instead of going from 80% to 30%, you could go from 60% to 40%, then recharge back to 60% and discharge until 30%.
Rules are meant to be broken. Be sure to drive your EV in a way that supports your lifestyle. If you need 120 miles out of your 2019 Leaf - charge it up and use it! However, if you are only driving 20 miles a day, consider charging your battery to a lower state of charge than full and recharging more often.