If two plates of lead are immersed in a dilute sulphuric acid solution and a current is passed between them, the lead plate connected to the positive terminal of the driving cell acquires a layer of lead oxide (PbO 2 )on the surface. If the lead plates are disconnected from the driving cell and connected to a lamp, the lamp will light for a while. The lead plate – acid arrangement will drive a current through the bulb in the opposite direction to the current supplied by the driving EMF.
In the 1860s, a French scientist Gaston Plante found that if the charging and discharging provcess was carried out a number of times, the thickness of the lead oxide layer gradually increased and the arrangement was able to supply current for a much longer period.
In modern arrangements the lead plates are replaced by grids of lead antimony alloy filled with past under pressure. Red lead oxide is used for the positive plates and lead oxide (PbO) for the negative plates. On first being charged these become PbO 2 and lead respectively.
Six of these are packed side by side into a case to make a 12V car battery. Lead acid batteries have the advantage of having a very low internal resistance, so are able to supply the large current needed to turn and and start a car engine. The capacity of a car battery is measured in Amp hours (Ah). A battery rated at 80 Ah will supply a current of 8 A for approximately 10 hours or 2 A for approximately 40 hours. The approximations are needed because the capacity is greater when the battery supplies a small current than when it supplies a large one. So that batteries can be compared they are often quoted for 10 hours.
As the battery discharges both sets of plates change to lead sulphate and the acid becomes more dilute. The density of the acid falls, meaning that the state of charge of the battery can be estimated by measuring the density of the acid with an instrument called a hydrometer. A fully charged 12 V battery actually has an EMF across it's terminals of about 13.2 V and the acid is about 1.25 times the density of water though these figures both vary with the temperature.
Water gradually evaporates from the battery. It is important that the level of acid in the battery is inspected periodically and that the level is topped up with distilled (pure) water. On top of this the battery is best left in a fully charged state, and when charged, they should be charged with a charging current specific to the battery, recommended by the manufacturer. Over discharging and shorting the battery terminals also should not be done. Over discharging changes the lead sulphate to a white crystalline form which cannot be changed back into lead oxide (PbO 2 ) and lead while shorting buckles the plates. The battery must then be replaced. When batteries are left unused the slowly discharged, should be given a topping up charge periodically.
When fully charged, explosive gases – oxygen and hydrogen - are given off, so the battery must never be examined with a naked flame.