What is depth of discharge and why does it matter?


The process of releasing stored energy from a battery, whether passively or actively, is referred to as discharge. The total amount of energy removed from the battery during this process, relative to the battery’s total capacity, is known as depth of discharge. 

For example: with a 100kWh battery, 80% depth of discharge is 80kWh. With a 64kWh battery, 80% depth of discharge is 51.2kWh.

We often think of depth of discharge as a fraction of the total capacity, but we can also think of it as a difference between two states of charge. If we start with 80% and end with 20%, our depth of discharge is 60%. 

Discharging a battery is an essential function, but the specific rate and frequency of discharge will determine the impact on battery health. The more frequent and significant the discharge is, the shorter a battery’s lifespan will be. It is generally not recommended to completely discharge an EV battery, a process that can dramatically shorten the battery life.

Research suggests that higher depths of discharge result in faster battery degradation, both in power output and useable capacity. However, not at battery chemistries respond the same, with LFP batteries showing incredible resilience to aggressive discharge.

For frequent short trips, such as daily commutes, minimizing the depth of discharge is a great way to promote battery health. For example, let’s say we use 20% of battery capacity get to work and back, and we start Monday morning with 80%. Instead of waiting to charge on Thursday when we have only 20% remaining, resulting in a depth of discharge of 60%, we could charge daily, returning the 20% to the battery each time. This limited depth of discharge across thousands of cycles will yield a healthier, more functional battery. 

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