Week 8 Multi cell Battery Pack

Study – How weakest cell limits the usable capacity of the battery pack? What is the solution?

Batteries of HEVs and EVs are made of large number of cells, which are connected in series and parallel connection to achieve high voltages. Greater the battery, greater will be the chances of failure, because all cells are not equal in terms of capacity and state of charge. After a certain number of charge and discharge, all cells are not going to work on same capacity and they will have different SOC as well, there are some reasons for that, we are going to take a look at later stage. Now, Cell, which is performing bad is called as weak cell, it may become cause of battery failure.

Weak cells have lower capacity then good one, so it discharges more quickly than others and it will at full charge, while others are still need some charge, so that extra charge will put extra stress on weak cell, that will lead to permanent failure. what if a cell has capacity more than others, that will be again, dangerous, because we will not be able to charge it at full capacity, it will remain uncharged in every charge cycle. With every charge and discharge cycle weak cell will become more weaker than before.

Now look some possible cases of weak performance of cell or how does weak cell limit the usable capacity. After it, we will see some method, which is used to maximize the battery performance regardless weak cells.

1. All cells are not equal. Manufacturer can not predict the exact capacity, even they are using advance technology. Even batteries of same chemistry, same size and shape can have different capacities.

2. Uneven temperature distribution. More heated cell will be bear more stress than good one, which ultimately lowers the capacity.

3. Weak cell supplies higher voltage, which is again, affects the usable capacity.

4. Supply beyond the rated voltage causes the high degradation rate.

Cell balancing –

If cell is overcharged and overheated, they tend to degrade. Here cell balancing comes into picture, cell is one solution to snub weak cell and other problem related to it. Cell balancing is a process to equalize the voltage and state of charge among the cells when they are at full charge. There are two methods of cell balancing, they have their own unique advantages.

1. Active cell balancing – In active cell balancing excessive charge is redistributed and further, it can be used. Capacitive and inductive shutting transfer the charge. Active charge balancing method is complex and uses costly components, but it is more efficient than passive balancing because it uses extra energy while passive balance dumps that.

Capacitive shutting method uses a flying capacitor, which is connected to each cell series. Capacitor picks up charger when it is grater than average and it dumps the charge when it is below average.

Inductive shuttling charge distribution method uses a transformer. Primary winding is connected to battery and second is switched across individual cells. It averages charge level by taking pulses from fully charged battery.

Active cell balance during discharge-

Assume, battery is fully charged (90%) at initial.

After certain discharge and charge cycle some cell will become weak, so they loses capacity and drains charge faster than others. below, figure is showing discharged cells.

Active balancing transfers charge and all the cell are now at the same SOC (30%).

Active cell balance during charging-

During charging weak cell reaches to full capacity before other. Again, weak cell limits usable capacity. Active balancing redistributes the charge and other cell reaches at full capacity.

2.Passive cell balancing – Passive balancing allows the cells to have same capacity regardless of weak cells. Passive balancing method uses a resister, which is attached parallel to each cell and it is used to bear extra charge. If weak cell has more charge than others then it will be allowed to pass extra charge to resister, till it gets same capacity. Passive balancing method is not efficient because it wastes energy and takes ling time to equalize, but it is a low cost option.