There is going to be Cell to Cell Variation in production and in use. Both of these topics are significant and important to understand as the result can be an under-performing or short-lived battery pack.
Variables in Cell Production
When design and assembling the cells there are a number of parameters that need to be controlled to reduce the variation in cells. Typically this is with respect to capacity, but internal resistance is also a significant variable in pack production.
Pack Design and BMS
Cell-to-Cell Variation and Deterministic Pack Effects
Due to random cell-to-cell variation and deterministic effects of thermal gradients, the lifetime of battery packs is uncertain. As strings go to higher voltages, or thermal gradients increase, not only does the average lifetime decrease, but the uncertainty in lifetime increases, causing a double effect on warranty lifetime, reliability and up-time. Using Brill Power’s BrillCore technology, these effects can be mitigated, increasing the commercial lifetime of batteries.
Cell Matching
What level of cell matching do you do prior to assembling a battery pack? Assuming the battery pack will be balanced the first time it is charged and in use. Also, assuming the cells are assembled in series.
- none, force the cell supplier to deliver cells matched to within +/-0.02V
- none, gross balance the pack during first charge once built
- preselect and group cells prior to build
- pre-charge/discharge all in-coming cells to a set voltage/SOC
- average-balance cells in parallel group prior to building in series
- average top-balance cells in parallel group prior to building in series
Cell Balancing
A set of cells connected in series will all see the same current, whether they are generating it when discharged or accepting it when charged. If all of the cells have the same initial SoC and the same capacity then they should charge and discharge equally.
However, if one cell has a slightly lower capacity it will be discharged further in SoC and it will go slightly overvoltage each time it is charged up. Over time this will result in a further reduction in it’s capacity.
See also: Passive Balancing, Active Balancing Topologies, Cells in Parallel