If you clicked on this article, then you already know that some battery degradation is normal. The range you see on a brand new EV will not be the same range you can get when the battery is five or ten years old. This is natural and there is no way to stop it. There are, however, ways to make degradation happen faster than normal.

The main culprits for accelerated battery degradation are heat, high voltage, and depth of discharge. Only one of these is easy to control, and it can have a big effect on the life of your battery. 

The easy trick to increase battery life is…charge more! That’s right – use less of your battery before recharging. 

Depth of Discharge Matters for Battery Health

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. It could mean starting at 100% state of charge (SoC) and using the battery down to 20%, or starting at 85% SoC and ending at 5%. In both cases, the depth of discharge is 80%.

Similarly, if you start driving when your battery is 70% full and stop to recharge at 20%, that is a 50% depth of discharge. Same as if you start at 92% and recharger at 42%.

To calculate depth of discharge, just subtract the ending state of charge (when you plug in) from the beginning state of charge!

The tl;dr is that if you recharge more often - when your depth of discharge is lower - your battery should last longer over its lifetime. Read on for the science.

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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 mean 5 - 10% of the physical battery remains. 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. 

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The Science - and Why it Matters

It is believed that depth of discharge contributes to battery health is because batteries are more chemically stable near the middle of their state of charge. They are less likely to experience physical or chemical stresses near 50% than when they are charged all the way to 95% or discharged down to 12%.

It has been fairly well studied in laboratory experiements that using lower depths of discharge can help preserve battery health, including lifetime, capacity, and power. How exactly this laboratory work translates to batteries configured with a battery management system in a car remains to be seen. EV batteries have a lot of software and hardware built to protect the health of individual battery cells, so the life extending effects seem in the lab may not be as pronounced in real EVs.

The Research

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 really 100% or 0%. See the note above on usable capacity vs. total capacity.

Guena and Leblanc 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 capacity as the batteries age.  LFP batteries show the least sensitivity to depth of discharge. 

Battery University is a free resource that collects and shares cutting edge battery science. They provided the data for the table below, which shows how many cycles it takes for different batteries to lose 20% of their original capacity, based on the depth of discharge. A battery that uses 40% of its depth of discharge lasts 2.5 times longer than one that uses 80%. This table has data for one popular EV battery chemistry.

Below is a charts from Battery University that show data on how depth of discharge affects battery life. There are three experiments: one when the depth of discharge is 50%, one when it’s 60%, and one when it’s 75%. The larger depth of discharge, the faster the capacity drops over time. In terms of an EV, this means the faster your range will decrease with the car's age.

By 2000 cycles (when the battery has been used 2000 times), the battery with the 75% depth of discharge has around 85% of its capacity remaining, while the one with the 50% depth of discharge still has over 90% capacity. By 4000 cycles, the battery with the 75% depth of discharge has lost another 5-7% capacity, while the one with the 50% depth of discharge has just shy of 90% capacity.

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Depth of Discharge Lessons for EV Drivers

The takeaway is that less is more when it comes to depth of discharge, but make sure that any change to your charging behavior makes sense with your driving needs and lifestyle. Since EVs have such sophisticated systems to protect their batteries, large depth of discharge will likely affect your car battery less than lab experiments suggest.

Still, it may matter. Here's how you can limit your depth of discharge in real life. 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%.

All this being said, 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.