Capacity Loss In Lithium Batteries Due To Depth Of Discharge — Why?

Internal Chemical Reactions

When lithium batteries are deeply discharged, the lithium ions are pulled out of the cathode material, leaving behind a solid-electrolyte interphase (SEI) layer. This SEI layer grows thicker with each cycle, increasing internal resistance and reducing the battery’s capacity. A typical lithium-ion battery will see a 10-20% capacity loss after 300-400 cycles, with each cycle representing a full charge and discharge.

Impact of Depth of Discharge

The depth of discharge (DOD) has a significant impact on lithium battery capacity loss. For example, a battery that is deeply discharged to 20% state of charge (SOC) will experience more capacity loss than one that is only discharged to 50% SOC. As a general rule, lithium batteries should not be deeply discharged below 50% SOC to minimize capacity loss. In fact, a study by the US Department of Energy recommends avoiding discharges below 40% SOC for optimal battery health.

Cycle Life and Depth of Discharge

The number of cycles a lithium battery can handle before capacity loss becomes significant is also affected by the depth of discharge. For example, a battery that is deeply discharged to 20% SOC may only last for 300-400 cycles, while one that is only discharged to 50% SOC may last for 1,000-2,000 cycles. To extend the life of your lithium battery, it’s essential to monitor its state of charge and avoid deep discharging.