Surface charging

The concentration of sulphuric acid is higher at the plates than in the electrolyte in a battery that has recently been charged. This is because the sulphuric acid formed at the plates during charging has not had time to spread into the rest of the electrolyte. If the stand-by voltage of the battery is measured directly after charging, a higher value will be obtained, giving an incorrect reading of the charge status of the battery. This is called surface charging. If the battery is left without load for a while the concentration of the sulphuric acid will even out. It is therefore important to leave the battery without load for at least 2 hours before measuring the stand-by voltage of the battery, allowing time for the concentration of the sulphuric acid to even out in the electrolyte. The measured voltage is then a good indication of the charge status of the battery.