<P> Operation of a lead - acid battery may, in case of overcharge, produce flammable hydrogen gas by electrolysis of water inside the battery . Jump start procedures are usually found in the vehicle owner's manual . The recommended sequence of connections is intended to reduce the chance of accidentally shorting the good battery or igniting hydrogen gas . Owner's manuals will show the preferred locations for connection of jumper cables; for example, some vehicles have the battery mounted under a seat, or may have a jumper terminal in the engine compartment . </P> <P> Jumper cables should not be used to interconnect between different voltage systems . Connecting 6 V and 12 V systems together may cause damage . </P> <P> If the dead battery is physically damaged, has a low electrolyte level, is decayed or frozen, a jump start will not repair the battery . A vehicle with a frozen battery should not be jump started, as the battery may explode . </P> <P> A hand - portable battery, equipped with attached cables and charger, can be used similarly to another vehicle's battery . A self - contained jump box contains a battery and connects directly to the battery of the engine that needs a boost . Portable boosters may automatically sense the battery's polarity prior to sending power to the vehicle, eliminating the damage that can result from reversing the connection . There are various portable jump starters that are multi-functional and can be used to charge other electronic devices as well . </P>

When​ jump-starting a​ vehicle which of these connections is the last connection to be​ made