42 Chapter 4 room for cells you have in the system. It is not uncommon to do a quick packaging study to find out how much room is available for the battery. With this information you can quickly estimate how many cells will fit into the space and determine what size battery you can offer. For example, if we are using 96 Cells of 259 Wh per cell (3.7 V Ч 70 Ah per cell) it equals 24,864 Wh or 24.9 kWh of total pack energy. Again, this also assumes that you have a specific cell in mind to be used in this application. This method is best if you already have a portfolio of cells that you can use rather than if you are trying to determine which cell is best.
The prior method is better suited to determining what size cell is needed. Now I would like to just quickly relate what we have reviewed already back to Ohm’s Law here. We looked at a couple of formulas for calculating pack energy and voltage using Ohm’s law. Remember that Ohm’s Law states that R = V/I. In this case we have used this same formula to calculate the following three formulas for capacity (current), energy and voltage: 24 . 864 kWh/355 . 2 V = 70 Ah 355 . 2 V Ч 70 Ah = 24 . 864 kWh 24 . 864 kWh/70 Ah = 355 . 2 V So to quickly summarize what we have determined with this example, we need:
96 cells at no less than 3.7 V in series in order to achieve the pack voltage One 70 Ah cell in order to achieve the pack current requirement of 70 A A total of 96 of these 3.7 V and 70 Ah cells Alternatively you could achieve the same system performance results using two 35 Ah cells in a parallel configuration and a total of 96 of these 2P cell groupings which would require a total of 192 cells to the system Calculating Pack Energy at End of Life Of course all of these calculations assume that you can use 100% of the battery to achieve that range, in reality you may only be able to use about 80–90% of that battery depending on the cell selection and usage profile.
That means that the 25 kWh must be the amount of energy that is usable in this system design. In other words, we must be able to remove 25 kWh of energy from the pack and, therefore, if we are to maintain the safety margins on the top and bottom and only use 80% of the total energy, we will need to determine how much total energy is needed. That means another quick calculation by dividing the usable required energy by 80% (assuming here that the system will use only 80% of the total energy) gives us: 25 kWh/80 % = 31 .
25 kWh total battery pack energy needed This means that in order to remove 25 kWh of energy from the pack to achieve the 75 miles range, the battery will really need to be sized up to over 31 kWh in order to achieve these goals. Of course the most important question to ask your customer when talking about pack