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I am grappling with an electrical question - The metal nipple on the tender wire coming from my positive ground battery managed to touch the metal door sill, creating a spark and a hot wire. This is something I had never seen from a negative ground battery. When the battery is connected all the way to the tender, there is no problem and it charges well. I called the manufacturer but they just told me to keep the rubber cap on when the tender is not being used so that the metal in the wire would not touch another metal part but did not suggest anything else. Is there any other way to correct this or is it just due to the polarity of the positive ground battery?

If you are speaking of the short cable with a two pole rubber plug, the exposed conductor is the negative lead and would be connected to the negative terminal of the battery. With a positive ground, the exposed conductor touching any part of the metal would be a direct short across the battery. The fuse is on the positive side on the cable so unfortunately is of no use since it never sees the current. Some solutions would be to keep the cap on as the company suggested or else put another fuse in the cable on the negative side. If you have a battery disconnect switch, placing it in the ground or positive side and turning it off when the tender cable is exposed would be another way to prevent the problem.

You are correct that the problem wouldn't happen in a negative ground car which is undoubtedly what the tender was made for.

As Howard notes above, the exposed pin on the connector is negative and should it touch metal anywhere on a positive ground car you'll get the sparks. What I did was cut the wiring going to the battery terminals and reverse-spliced them back together. This gives you a positive-grounded exposed pin so the fireworks won't happen. This doesn't affect the tender's performance at all. Just be sure not to reuse that wiring on a negative ground car.

Thank you, Howard and Don!