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The first starting and charging systems were designed to be 6 Volt and positive-ground systems, with the vehicle's chassis directly connected to the positive battery terminal. Today, almost all road vehicles have a negative ground system. The negative battery terminal is connected to the car's chassis.
Cars used 6 Volt electrical systems and batteries until the mid-1950s. The changeover from 6 to 12 Volts happened when bigger engines with higher compression ratios required more electrical power to start.
6.3 Volt valve heaters appeared in the USA in the 1930's to facilitate the growth of car radios. 6.3 Volts being the on-charge voltage of a six Volt lead-acid storage battery or accumulator used within vehicles.
An vehicle battery is an example of a wet cell battery (three for a six Volt system and six for twelve Volt. Each cell of a lead storage battery consists of alternate plates made of a lead alloy grid filled with sponge lead plates (cathode) or coated with lead dioxide (anode). Each cell is filled with a sulphuric acid solution, which is the electrolyte. Initially, cells each had a filler cap, through which the electrolyte level could be viewed and which allowed distilled or deionised water to be added to the cell. The filler cap had a small vent hole which allowed hydrogen gas generated during charging to escape from the cell.
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