One of the biggest economic challenges in the use of lithium-ion batteries is the fact that each time a battery is discharged and recharged, it loses a tiny amount of its capacity to hold electricity. Over repeated charging cycles this decrease in capacity becomes noticeable and ultimately enough capacity is lost that the battery is no longer useful (most people are familiar with this phenomenon in their cell phones or laptop computers). The “end of life” is typically defined as the point when the battery can only hold 80% of its original capacity, and the number of charging cycles achieved before that point is reached is referred to as the battery’s cycle life.
The loss of some capacity during each charging cycle is unavoidable, but the amount of capacity that is lost can be minimized through proper thermal management. Two concepts are important here. First, at the cell level, the hotter an individual cell gets during the charging cycle, the more capacity is lost. Second, at the pack level, the weakest link theory applies. The capacity of the entire battery pack is limited by the lowest capacity cell within the pack. AllCell’s proprietary PCC™ thermal management material effectively limits maximum temperature and ensures temperature uniformity, which improves cycle life by 50% or more compared to a pack with no thermal management.
Limit Maximum Temperature
In the same way that an ice cube keeps a glass of water at 32° Fahrenheit, AllCell’s PCC material limits the maximum temperature of the battery pack to the melting point of the wax within the PCC. As the battery pack heats up, once the temperature reaches the melting point of the wax, all additional thermal energy is absorbed by the melting process rather than continuing to raise the temperature. By choosing different raw materials, AllCell can set different maximum temperatures depending upon the type of cells used.
Ensure Temperature Uniformity
AllCell’s PCC material not only limits maximum temperature but also ensures temperature uniformity. The graphite within the PCC has excellent thermal conductivity properties. As each cell discharges electricity and begins to heat up, the graphite distributes the heat evenly throughout the pack, avoiding hot spots and keeping the temperature of hottest cell no more than 3°-5° C higher than the coldest cell.
Increase Cycle Life by 50% or More
The result of limiting maximum temperature and ensuring temperature uniformity is a dramatic increase in cycle life. The graph bellow illustrates the typical increase in cycle life driven by adding PCC to the battery pack. If the battery pack in this example cost $250, the capital cost per charging cycle would be reduced from $0.63 to $0.35.