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Discharging and charging of batteries

A battery is hooked up to a device such as a small lamp, motor, or pump, and the voltage across the battery is measured as a function of time. If the system is allowed to operate for an extended period, a decrease of the battery voltage is observed (Figure 1). In the end, the battery stops working.

If the battery is analyzed, it is found that it consists of chemicals which, during its operation, undergo reactions. At the end of the life of a battery, all the original substances have been used up, and new chemicals have been produced (see Chapter 4 on chemical processes and more on electrochemical devices).

There are battery types that can be “recharged” with the help of an electric power supply. During recharging, the chemical reactions observed during operation of the battery are reversed.

Interpretation

A battery is a chemically driven electricity pump. Chemical reactions release energy which sets up an electric potential difference (a voltage) across the battery, and drives a charge current through this potential difference (Figure 2). Charge flows from lower to higher potential through the battery, i.e., it is “pumped uphill.” The voltage that can be set up by the chemical reactions depends upon the chemical state of the battery. That is why the voltage decreases with time as the battery gets discharged (Figure 2, left). Note that discharging does not mean a loss of electric charge but a decrease of the store of energy.

When a battery is charged, it is not charged or “filled” with electric charge; it remains electrically neutral since the current of charge flows through the battery. As the chemical substances return to their original state, the battery is charged with energy (Figure 2, right).