First, what happens when a battery is charged?
When a battery is being charged, the flow of electrons and the chemical processes that occur during discharge are reversed. The result of the charging process is that the lead sulphate that is formed during the discharge process is once again converted into lead dioxide, lead and sulphuric acid and this restores the necessary chemical energy that will be converted into electrical power during future use. An optimum charge voltage is important for charging a battery. If the voltage is too high, the water will be electrolysed. This reduces the electrolyte level over a period of time. If the voltage is too low, the battery cannot be adequately charged and this can also reduce its service life.
Second, what happens when a battery becomes discharged?
The electrolyte is diluted sulphuric acid that permeates the pores of the plates and separators. It fills up the voids in the cells. The sulphuric acid component is responsible for turning pure water conductive so that it can be used as electrolyte.
If a device (for example, a lamp) is connected to the terminals of a lead-acid battery, the difference in power between the poles results in a flow of electrons leaving the negative pole and flowing through the device and on to the positive pole.
This flow of electrons transforms the lead dioxide on the positive plate and the spongy lead on the negative plate into lead sulphate. This chemical process consumes the sulphuric acid and yields water. The specific gravity of the electrolyte is thus decreased and it is because of this that the state of charge of the battery can be determined, by measuring the specific gravity of the electrolyte.