How do you measure a battery's performance?
There are two parameters that measure battery performance: voltage and capacity. In very simple terms, the voltage is the force propelling each of the electrons coming out of a battery and the capacity is the number of electrons that can be obtained from a battery.
The voltage of a battery cell is determined by the materials used in it. The typical open circuit voltage of commercial lead acid battery cells is around 2.13 volts but the cell is said to have a nominal voltage of 2.00 volts. This means lead acid batteries with nominal voltages of 2v, 4v, 6v, etc. are possible.
The capacity of a cell is essentially the number of electrons that can be obtained from it. Since current is the number of electrons per unit time, cell capacity is the current supplied by a cell over time and is normally measured in ampere hours.
Batteries consist of multiple cells that are electrically connected. The way that the electrical connections are made determines the voltage and capacity of the battery. If the negative terminal of one cell is connected to the positive terminal of the next and so through the battery the result is called a series connected battery. The voltage of this type of battery is the sum of the individual cell voltages. For example, a 12 volt automobile battery consists of 6 2 volt lead acid cells. The capacity remains the same as for a single cell.
The other way to connect cells within a battery is to connect the negative terminal from one cell to the negative of the next cell and to connect the positive terminal to the positive terminal. When this is done throughout the battery, the result is the parallel connected battery. Here the capacities of the individual cells add to make the battery capacity but the battery voltage remains as the voltage of the individual cell.