Battery Pack Design Criteria and Selection 41 In this example, let us assume that we are using a 3.7-V nominal NMC cell and 96 of them in series to get a pack Voltage of 355 V and in order to achieve the desired energy we are using 70 Ah of cells. This could be accomplished by using two 35 Ah cells in parallel or one 70 Ah cell, or some other parallel combination that leads up to 70 Ah. So we get: 355 . 2 V Ч 70 Ah = 24 . 864 kWh Also keep in mind that the required capacity is generally determined by the electric motor current requirements and energy is typically determined by the end user based on the application type.
For instance, if the end-use application is an all-electric vehicle that is targeting about 75 miles of driving range with a system that is capable of providing 3 kWh/mile efficiency, then: 75 kWh ч 3 kWh/mile = 25 kWh battery size Also note the units used in these calculations, I have taken the liberty of using 25 kWh in my calculations instead of 25,000 Wh. But if you are running these numbers on your calculator along with the book you will find that the results do not match and the decimal point is off! To be fully accurate in your calculations you will find that you need to use 25,000 Wh/350 V to get the 71 Ah target.
For the sake of brevity I am making that calculation automatically for you here. Now one last thing to note here goes back to my earlier comment about how much current the electric motor or system will need to draw. A 71 Ah cell will by definition be able to supply 71 A for a 1-h period and will be able to offer higher power pulses at different rates. But this may not meet the current requirements of the overall system. Many PHEV and EV systems for instance may require sustained current draw of 30 A, 40 A or up to around 400 A depend- ing on the application and the system design.
So in order to achieve, for this example, a 90 A continuous current (89 A rounded up) you would need to either use a single 90 Ah cell, two 45 Ah cells in parallel or some other combination of lower Ah cells in a parallel configuration in order to achieve these targets. This leads us to a brief discussion on “C-rates.” A C-rate is a measurement that is provided by the cell manufacturer to describe how much current the cell is capable of providing over some period of time (t). So if you have a cell that is 70 Ah at a 1C rate it will provide 70 A of capacity over a 1-h period.
But that cell may be rated up to 5C for a period of 10 s which means that it would provide 350 A (70 A Ч 5) over a 10-s period. The same calculations can be used for charging and regenerative braking. The battery must be able to accept the C-rate charge that the system will deliver otherwise the energy will not be accepted in the battery and is often transformed into waste heat during regenerative braking for instance. Alternatively, if you know how many cells are to be used you can multiply the total number of cells times the energy in watt hours of each cell to get an estimate of the total pack energy.
This can be a useful tool if you conduct a packaging exercise and are able to determine how much