Understanding Depth of Discharge
Depth of discharge (DoD) is the amount of battery capacity you use between charges. For example, with a 100 kWh battery, an 80 kWh usage represents an 80% depth of discharge.
While DoD is often described as a specific percentage of the battery's total capacity in kWh, it’s also useful to think of it as the difference between your starting and ending state of charge (SoC). For instance, if you begin a trip with 80% charge and end with 30%, you’ve used 50% depth of discharge. This is the definition we’ll use in this article.
Side Note for the Enthusiasts: There’s a difference between the physical battery capacity and how much of the battery a driver can use. Since charging fully to 100% or discharging all the way down to 0% is pretty bad for a lithium ion battery, most EV manufacturers add a buffer to both ends of the battery that can’t be accessed by the driver. In other words, when you see a 100% charge displayed on your dashboard, you might actually be at 95% of the physical battery capacity. Similarly, 0% might really be 5%. For simplicity, we’ll be talking about the usable battery capacity as shown on your dashboard, although researchers often study the physical battery capacity.
Depth of Discharge and Battery Degradation
Lithium-ion batteries degrade with both age and use, but it isn’t anything to worry about. Most EV batteries have enough life to last as long as the body of the car and then some. In Recurrent’s community of EV drivers, very few batteries have needed to be replaced and even high-mileage cars with odometers over 175,000 mi. still maintain more than 86% of the original battery range.
However, it’s possible to affect the lifespan of your EV battery with certain behaviors. For instance, research indicates that using lower depths of discharge can help maintain battery health, including its lifespan, capacity, and power. While the science is well researched and conclusive, EV batteries have both software and hardware to protect the health of individual cells. Since EV batteries have these sophisticated protections, it’s hard to tell exactly how much of the laboratory science translates to EVs.
Key Findings in Depth of Discharge Research
A number of studies have been published that show the effect of different depths of discharge for batteries. Keep in mind that not every study looked at electric car batteries, which have sophisticated software and cooling systems to help maximize their useful lives.
- Research by Guena and Leblanc shows that a battery cycled between 80% and 30% can last four times longer than one cycled from 100% to 0%. However, EV batteries rarely reach these extremes in practice.
- Another study by Rechkemmer et al. found that maintaining a depth of discharge between 40-60% and 25-70% resulted in only a 12% capacity reduction after 700 equivalent full cycles. In contrast, a depth of discharge from 100% to 5% led to a 20% capacity reduction after the same number of cycles.
- Graphs from Preger 2020 illustrate that as depth of discharge increases (with blue representing the smallest and red the largest), most battery chemistries degrade faster and have shorter lifespans. Notably, LFP batteries show the least sensitivity to depth of discharge, which is part of the reason that Teslas equipped with LFP batteries can be charged to 100% regularly.
Practical Takeaways for EV Drivers
The main lesson is that less depth of discharge is generally better for your battery, as long as it fits your driving habits and lifestyle. At the end of the day, your EV battery serves you, not the other way around.
But, if it works for your lifestyle, consider the following changes to prolong your battery life.
Instead of using 50% of your battery before recharging, use 20%, recharge, and then using another 30%. For example, rather than going from 80% to 30%, you could go from 60% to 40%, recharge back to 60%, and then discharge to 30%.
This will work best if you’re only driving 20 - 40 miles a day, which is what most American drivers typically do.
