96 Chapter 8 hŶƵƐĂďůĞ ĂƉĂĐŝƚLJ Figure 6 Imbalanced cells at end of discharge. than the others, which will lead to an early death for both the cell as well as the complete battery system. So it is vital to ensure that the cells are as closely matched as possible. However, this is exactly what BMS balancing is intended to resolve. Cell balancing is essen- tially the act of making all of the cells the same SOC. As Davide Andrea describes it, balancing is the term used for the process of bringing the SOC levels of cells in a battery closer to each other, in order to maximize the battery’s capacity (Andrea, 2010, p. 23).
One additional thing to note here is that when cells are assembled into parallel configurations, they will automatically balance to each other. Each of the groups of parallel cells will still need to be balanced, but the cells within the parallel group will self-balance. The other question that comes about when talking about balancing is when to do it. Most of the current BMS systems on the market today balance the cells while the ESS system is charging. This is done for a couple of reasons.
First, balancing usually takes a significant amount of time and the battery must essentially be out of use when it occurs in order to accurately measure the capacity and voltage of the cells in the energy storage system. Alternatively, for an HEV application, balancing could be done during a long freeway drive when the battery is essentially unused. However, this may generate some challenges while the BMS decides that you are on the freeway and can balance. This may be done by monitoring the speed of the vehicle and the duration that it remains at high speeds. Of course, this also assumes that you are on the road long enough to complete the balancing. Alternative definition:
another way to look at cell balancing is to imagine that you are at a party and a new game is being played. In this game, there is a table with three glasses on it. We need to fill and then eventually empty all of the glasses. But there are some rules we must follow. The first rule is that you have to use the special funnel device that allows all three glasses to be filled at the same time and with the same amount of water. The second rule is that you must stop filling the glasses once one of them is completely full, regardless of how full the others are. The third rule is that you must drain all the glasses at the same time using another funnel tool.
And finally, the fourth rule is that you must stop emptying the glasses once one of them is completely empty. Ok, now that we have the rules. Let’s take a look at what it might look like ( Figure 7 ).