32 Chapter 3 generally used in relation to measuring at the pack level. DC resistance describes the magnitude of the difference between voltage and current (assumes no difference in phase shift between voltage and current). Secondary Battery—A secondary battery is simply a battery that is rechargeable; exam- ples include lithium-ion, nickel-metal hydride, and lead acid. Separator—The separator is a thin sheet of material, often a single- or multilayer plastic (polypropylene) or ceramic based material, that separates the anode from the cathode in order to prevent them from touching and creating a short circuit. The separator also must allow the lithium-ions to pass between anode and cathode.
Series—A series configuration refers to a grouping of cells connected in series (e.g., negative to positive, etc.). Connecting cells in series increases the voltage of the overall system. The example below represents three cells: let us assume that they are 3.6 V and 5 Ah similar to our previous example, in a series configuration. This configuration would end up with 10.8 V (3.6V Ч 3 cells) but will remain at 5 Ah. Putting lithium-ion cells in series is similar to connecting multiple garden hoses end to end, just as the garden hoses are in series so are the batteries.
The difference is that in batteries connecting them in series increases the voltage but not the capacity—the same effect does not occur when connecting garden hoses in series ( Figure 7 ). Figure 7 Lithium-ion cell in series connection. Short Circuit—A short circuit occurs when the positive and the negative poles or elec- trodes of a battery are connected. In essence, a short circuit creates a circular connection within a cell, driving all of the current back into the cell or pack which will eventually, and usually very quickly, lead to a catastrophic failure.
This could occur inside the cell due to the growth of dendritic materials that grow between the anode and cathode con- necting them electrically. This could also happen if a microscopic piece of debris gets assembled in the jelly roll, which could eventually pierce the separator and connect the two electrodes. If it happens inside the cell, it is an “internal” short; but if the electrical connection is made between the poles outside of the cell, it is referred to as an “external” short (e.g., outside of the cell). State of Charge (SOC)—While DOD measures how much of the battery is being used, SOC measures how much is left at a specific point in time.
SOC is usually measured from 0% to 100%, but it is measuring against the DOD. Basically the SOC tells you how much energy or power you have right now, it is like the gas gauge in your car ( Figure 8 ).