Battery Management System Controls 95 Another option for the PCB slave board is the use of a flexible circuit board or flex circuit. This solution is essentially the same as the hard PCB board but instead of using a hard substrate, it uses a flexible plastic substrate, or a combination of the two. This is most often used in the CSC boards that mount directly to the cell modules. Balancing One of the other features of most BMS systems is their ability to maintain the cells in a pack at the same SOC, this is referred to as balancing the cells.
The reason for balancing is that lithium- ion cells, just like any manufactured component, are manufactured to within a specification range but are not all exactly identical in their SOC or voltage when they are shipped from the factory. Additionally, lithium-ion, like most all batteries, suffers from “leakage” or “self-discharge” over time. In other words, if a cell is shipped at 3.7 V and 100% SOC, by the time it reaches the pack manufacturer it may be down to 99.5% SOC (purely for explanation purposes). So for a large lithium-ion pack that is made up of hundreds or thousands of cells, the cells may all arrive at the pack integrator at very slightly different states of charge.
While that may not seem to be a major concern because the variations are so small, it can create some major problems as the system begins operating. This is because the ability to charge and discharge is limited by the cell with the lowest (on discharge) cell SOC. A simple example may help to clarify this. In the example below ( Figure 5 ), we see three cells with two at about the same SOC and the third at slightly lower capacity. This is an unbalanced group of cells. When the cells are fully discharged, cell #1 will be fully discharged before the other two and the pack will stop discharging as any further discharge will damage cell #1.
This means that there will always be some remaining charge that is essentially unusable in the other cells as it will never be able to be fully discharged ( Figure 6 ). Over time, this variation in the cell SOC will grow as the battery is cycled and eventually will lead to premature failure or end of life of the system. Additionally, the cell that is the weakest in this example will get more “use” than the others, resulting in it premature aging. In other words, this first cell will work harder >ŽǁĞƐƚ ĂƉĂĐŝƚLJ ĐĞůů Figure 5 Imbalanced cells at beginning of discharge.