cap had a small vent hole which allowed hydrogen gas generated during charging to escape from the cell. The cells are connected by short heavy straps from the positive plates of one cell to the negative plates of the adjacent cell. A pair of heavy terminals, plated with lead to resist corrosion, are mounted at the top, sometimes the side, of the battery. Early auto batteries used hard rubber cases and wooden plate separators. Modern units use plastic cases and woven sheets to prevent the plates of a cell from touching and short-circuiting. In the past, auto batteries required regular inspection and maintenance to replace water that was decomposed during the operation of the battery. "Low-maintenance" (sometimes called "zero-maintenance") batteries use a different alloy for the plate elements, reducing the amount of water decomposed on charging. A modern battery may not require additional water over its useful life; some types eliminate the individual filler caps for each cell. A weakness of these batteries is that they are very intolerant of deep discharge, such as when the car battery is completely drained by leaving the lights on. This coats the lead plate electrodes with lead sulfate deposits and can reduce the battery's lifespan by a third or more. VRLA batteries, also known as absorbed glass mat (AGM) batteries are more tolerant of deep discharge but are more expensive. VRLA batteries do not permit addition of water to the cell. The cells each have an automatic pressure release valve, to protect the case from rupture on severe overcharge or internal failure. A VRLA battery cannot spill its electrolyte which makes it particularly useful in vehicles such as motorcycles. Batteries are typically made of six galvanic cells in a series circuits. Each cell provides 2.1 volts for a total of 12.6 volts at full charge. During discharge, at the negative (lead) terminal a chemical reaction releases electrons to the external circuit, and at the positive (lead oxide) terminal another chemical reaction absorbs electrons from the external circuit. This drives the electrons through the external circuit wire (an electrical conductor) to produce an electric current (electricity). As the battery discharges, the acid of the electrolyte reacts with the materials of the plates, changing their surface to lead sulfate. When the battery is recharged, the chemical reaction is reversed: the lead sulfate reforms into lead dioxide. With the plates restored to their original condition, the