The storage battery or cell is a device that coverts chemical energy into light energy when required.
During charging of battery, there is some chemical changes in the battery and absorb energy. The absorbed energy is converted into electrical energy when connected to an external load.
Hence, the correct answer is "Storage battery or cell".
Upon comparing a solution with a pH value of 4 and that having a pH of 7, you will realize that it has a pH difference of 3 although the concentration of ions of hydrogen having a pH of 7 is 1000 times greater than being at a pH of 4.
