Quote:
Originally Posted by Rapt0r9
I worked at a gas station for about two years. During that time I have replaced many car batteries. The one thing that I remember my boss telling me is to always connect the positive line first. He said is was because the line would spark is the ground was connected first and sometimes fumes accumulate around the battery which will ignite. I have never tested this theory but it hasn't failed me yet.
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This is common practice but why would you want to connect both at the same time? There is no big reason to connect the positive lead first. If there is going to be any sparks, either lead will produce them. It might occur if a high current load were still energized, i.e. the lights. During that small period of time when you are placing the connector on the battery terminal, some sparking might occur. During charging, lead acid batteries do produce hydrogen. In most modern batteries in good condition, the hydrogen is contained in the battery, our sealed batteries especially. If you are placing a new, off the shelf battery in the car, it is unlikely there will be enough gas to cause an explosion. In any battery with a crack in the case or defective vents or the fill tubes open, there might be some hydrogen present. As you know from chemistry class and the Hindenburg, hydrogen is a volatile gas.