Understanding capacity retention rate in Lithium-Ion Batteries
What is Capacity Retention Rate?
Capacity retention rate is a critical parameter in evaluating lithium-ion batteries and other secondary batteries. As these batteries undergo various battery evaluation tests, such as cycling tests, float charging tests, and storage tests, they experience deterioration, leading to a decrease in their capacity from the original value, measured in mAh or Ah. The capacity retention rate is calculated as the ratio of the current capacity (mAh or Ah) to the initial capacity, multiplied by 100. In simpler terms, it represents the remaining capacity as a percentage of the initial capacity, initially set at 100%.
Example of Capacity Retention Rate in Cycle Testing
During cycle testing, which assesses battery degradation through repeated charging and discharging, the capacity retention rate serves as a key evaluation metric. The graph demonstrating capacity retention rate in this context typically shows the percentage of capacity retained on the vertical axis against the number of cycles on the horizontal axis. A battery that sustains a high capacity retention rate after numerous cycles is considered to be of high quality.
Factors Influencing Capacity Retention Rate
Several elements impact the capacity retention rate of a battery apart from cycle count. These include the charging and discharging rates during cycles, ambient temperature, and others. The decline in capacity retention rate can be estimated using degradation coefficients such as Kc in cycle tests and Kf in float tests. The formula is: reduction in capacity retention rate = Kc x square root of cycle tests + Kf x time in storage.
Key Considerations
One important factor in cyclic testing is the depth of discharge (DOD). A larger DOD leads to more significant expansion and contraction of the active materials during charge-discharge cycles, increasing contact resistance and affecting the capacity retention rate. Another caution is the electrolyte amount in the battery. Insufficient levels can lead to drying up (liquid depletion), severely affecting the capacity retention rate.