38 Chapter 4 Lithium-ion batteries will also “self-discharge” or loose energy as it sits in storage. There are two basic forms of self-discharge, permanent and temporary. Permanent self-discharge means the battery will never be able to return to its original capacity. This is usually due to an increase in the impedance within the cell while it sits in storage. Temporary capacity loss is that capacity that is lost during the storage period but will return once the battery is cycled again. Different chemistries have different performances in this category. In the next sections we will begin delving into the basic calculations that are used in battery design.
Most of these are derivations of Ohm’s Law. Based on experimentation done by Georg Ohm back in 1825 and 1826, Ohms’ Law basically states that current is approximately proportional to electric field, in other words voltage is equal to the current multiplied by the resistance of a circuit (Ashby, 2009). This fundamental law forms the basis for almost all of the basic calculations that follow. This section will use this basis to define some of the core calculations described in simple terms that can be used in order to determine things like how many cells are needed in a battery, what capacity cells are needed, and so on.
Ohm’s Law and Basic Battery Calculations While there are many different formulas that can help you in your battery pack sizing calcula- tions, perhaps the most important is Ohm’s Law. Ohm’s Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical system (Ashby, 2009). And since voltage and current are two of the few things that we can actually measure in a battery (other than temperature, almost all other factors are calculated) they become critical in our calculations. Ohm’s Law then is stated as such: Voltage (V) is equal to current (I) multiplied by resistance (R) Which can be described as:
V = I Ч R Now the beauty of this equation is that from this one equation, if you have two of the three variables you can calculate the missing one. For example, the same equation rewritten to calculate current as current (I) is equal to voltage (V) divided by resistance (R): I = V/R And the third part, as you may now have guessed is that you can calculate resistance using the same functions. Resistance (R) is equal to voltage (V) divided by current (I): R = V/I In the previous Chapter, I defined these terms but let me now give you a visual example of the three in order to help you better understand them and know how they interact. The graphic in