Negative battery terminal spark

[44boggers]

Hi guys, I am sure this question is going to seem very but here it goes.

So I am just starting to put the basic electrical back in my truck. Complete frame off resto, so all wires, terminals and battery are new. I only have a 3 wires hooked up right now.

I have the positive going to the starter

I have the ground going from the battery to the back of the cylinder head.

I have a ground strap going from the same ground on the cylinder head to the frame.

That is it, 1 positive and 2 grounds.

So normally you would connect the positive terminal first and then the negative on the battery. But when I do this it the negative terminal sparks so aggressively it will melt the terminal.

But if I connect the negative first and then the positive there is absolutely no sparks.

Isnt this a sign of a short somewhere? but that seems impossible since there are only three wires hooked up to the chassis/block/battery and all new wires.

Only other thing to add, is the first time i tried this tonight had a battery tender still hooked up. So I thought maybe the battery tender was creating some sort of connection. So I took the tender off and still the same effect.

Would a freshly charged battery (hot off the tender) do something like this?

Thanks

Paul