Negative battery terminal sparking on contact

Henry Harmell-McBride said

It was a large enough spark that it left a noticeable gash in the battery terminal. Not sure if that's considered a welding type spark.

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You have something dead shorted somewhere.....OR you have the starter electrically engaged. Either would provide a substantial spark....but didn't you say the starter engaged when you connected the battery? That tells me you have the starter engaged "somehow" whether through a starter defect, or by operator error. lol Happens to the best of us.