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Hi Dave,

This becomes more important in a car where the battery is in close proximity to the chassis. One could argue that a battery that is installed close to an un-insulated ground system on a boat could have the same issue but here's what the problem is using the example of a car. If you connect the negative cable first, then you have made a connection from the negative of the battery to the whole body of the car. Now you connect the positive cable cable and may see a spark as you do so, but this is not really an issue unless there are some combustible fumes kicking about. Next you take your wrench and start tightening up the nut on the side of the positive terminal and in doing so accidentally touch the chassis/body of the car. What you have done here is create a short from the -ve to the +ve of the battery. Remember the -ve is already connected to the car so all of the body and chassis is -ve, so your wrench from the +ve terminal touching the chassis as you tighten the nut is as good as taking the spanner and putting it straight across the terminals. What tends to happen next is that the spark generated is big enough to weld the wrench to the body of the car so now you cant get it off, it begins to glow red and if you're really unlucky the battery will explode covering you in boiling battery acid. As I mentioned, less of an issue on a boat where the battery is usually in a battery box of its own and there is no chassis to worry about, but a good question and still to be considered on boats.

Best regards,

Captain Ian Fagg