Cells can be connected in series to form a battery. The type of battery in general use in motor vehicles is lead-acid accumulator. The battery contains three or six 2 V cells to make a battery that delivers 6 V or 12 V. The word battery is sometimes used incorrectly for a single cell. For example, what we call a torch "battery" is often a single cell. In a lead-acid accumulator, each cell has one lead electrode, one lead (IV) oxide (PbO2) electrode and sulphuric acid as the electrolyte. When the battery supplies a current, lead and lead (IV) oxide are both converted into lead (II) ions. When the reactants have been used up, the battery can no longer supply a current; it is "flat". It can, however be recharged. When the vehicle engine is running, it rotates the dynamo (the alternator) which generates electricity and recharges the battery. If the dynamo is faulty, the battery will become flat. It can then be recharged by connecting it to a battery charger, a transformer connected to the mains. This reverses the sign of each electrode and reverses the chemical reactions that have occurred at the electrodes. Lead and lead (IV) oxide are reformed, and the battery is again able to supply an e.m.f.
