Battery Pack Design Criteria and Selection 39 ƵƌƌĞŶƚ Ŷ Ŷ Ŷ Ŷ ƚ ƚ sŽůƚĂŐĞ ZĞƐŝƐƚĂŶĐĞ Figure 4 Ohm’s Law described. Figure 4 above is simplistic, but should do to help us understand these three items and how they are interrelated. In Figure 4 we see a large water tank, the size of the tank is analogous to the amount of energy that is in the battery system. The voltage is analogous to the amount of water pressure at the bottom of the tank that is forcing the water, or in this case, the electrons forward. The current in this example relates the size of the pipe through which the water is flowing out. So the larger the current the more power can be released.
And finally the resis- tance is a bit more difficult to describe, but let us think of it as the friction that exists within the pipes, which tends to cause the current to run slower This can also be described in electrical engineering terms as an equivalent circuit model, which is shown in Figure 5 . As you continue on in this chapter you will quickly see that this set of formulas becomes critical to understand in order to calculate virtually all of the other formulas that follow. Note that in the following calculations I am generally using nominal values at the beginning of life (BOL) unless otherwise stated. In most cases this is adequate for the basic battery pack sizing exercise.
Once you begin delving into the details of the operation of the pack, you can begin applying factors such as internal resistance, depth of discharge, state of charge, tem- perature, and other factors to accurately size and determine the end of life (EOL) of a battery system; but as our purpose here is to provide only a brief and simple overview we will focus only on the “basics”. s Z / Figure 5 Equivalent circuit model.