This is the essence of voltage drop: a battery (or voltage source) supplies energy for doing the work of moving charge. When current is flowing, components such as resistors consume energy, and the amount of work per unit charge associated with the current flowing through a given component is the component’s voltage drop.
Additionally, as a battery discharges, its internal resistance increases, which also contributes to a voltage drop. Finally, when a battery is heavily loaded, the active materials within the battery start to dissolve, which also reduces its performance. All of these factors work together to cause a voltage drop under load.
The amount of voltage drop will depend on how much current the battery is supplying. A 12 volts battery should read around 11 volts when under load. Keep in mind that this is just a general guideline and may not be accurate for all situations. If you are unsure of what the voltage should be, it is best to consult with a professional.
As more current flows through the battery, it becomes harder and harder for electrons to flow from one electrode to another. This increase in resistance causes a drop in voltage.
Both effects occur as a battery is drained. The open circuit voltage goes down and the internal resistance goes up. Note that open circuit voltage is specifically measuring just the voltage the battery puts out with the internal resistance taken out of the equation.
Battery A has a voltage of 6 volts and a current of 2 amps, while Battery B also has a voltage of 6 volts and a current of 2 amps. When connected in series, the total voltage would be 12 volts, and the total current would remain at 2 amps. Advantages and Disadvantages of Series Connections