Batteries convert chemical potential energy into electrical energy to enable electron movement through a circuit. The battery's positive terminal's potential energy per charge defines its voltage rating. Measuring the voltage between a battery's terminals without current flow gives the terminal voltage.
Batteries are devices that convert chemical potential energy into electrical energy through a chemical reaction within an electrochemical cell.
This energy allows for the movement of electrons through a circuit, where the electrical energy can be used to power various devices by converting it into other forms of energy such as light and heat.
As the chemical reactants in the battery are used, they supply less potential to the moving charges until this potential falls below a useful threshold, signaling that the battery is flat or requires recharging.
The potential energy per charge at the battery's positive terminal determines the battery's voltage rating, compared to water pressure in a system.
The voltage from a battery is likened to the pressure that drives water from a high-pressure area to a low-pressure area, similarly driving electric charge to flow towards a lower voltage.
The battery performs work on the charge to raise it to a higher electric potential, similar to how a pump works on water.
When measuring the voltage or electrical potential difference between the terminals of a battery not currently powering a device, you are measuring the battery's terminal voltage.
However, it should be noted that the internal resistance of a battery can affect the electrical current flow and the terminal voltage, particularly as it decreases with lower temperatures.