I have a farm tractor with a positive ground. I hooked a 15A battery
charger to it, and had massive sparks shooting all over the place. Yes,
I did connect the polarity correctly, with the red clip on the batt +
post and black one in the batt - post.

Because of this, I removed one of the battery cables (to the
tractor)[the negative one], and reconnected the charger. The sparks were
so intense, they melted a small hole at the top of the battery post.

In all the years I have charged auto batteries, I have never had this
happen. Even touching the clips together on the charger dont cause such
intense sparks. What the heck could cause this?
My first thought is a shorted battery, but the tractor lights work fine.
There just is not enough charge to turn the starter over fast enough to
start the engine. I would think that if the battery was 'dead shorted'
it would not operate the lights, or make the starter turn slowly.

To insure the charger is not defective, I connected it to another
battery and it's charging properly, on both the 2A and the 15A settings.

Anyone have any clue what's happening. I sure dont!!