In order to obtain an electric current in a circuit, you need a source of electrical energy (a battery or a generator) connected to both ends of the circuit. Batteries have two terminals (+ and -), which are the external part of cells where two different chemical reactions take place. Click on the following link if you wish to learn more about the inside of batteries.
The electric battery does not maintain a constant voltage between its terminals (the voltage decreases as time goes by).
Furthermore, batteries do not always supply the voltage which is written on their sides, as the voltage depends on how the battery is connected in the circuit. The greater the intensity circulating through the battery, the smaller the potential difference between its terminals.
When a battery is connected in a short circuit (by connecting both terminals directly), it discharges completely in a few minutes and its voltage decreases significantly in the first few seconds.
The following visual shows the variation of the potential difference between the terminals of an electric battery as the light bulb consumes the energy supplied.
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