Melted ground wire
A short 10 gage wire should handle 55 amps without melting.
https://www.wiresizer.com/ws_userguide.html
Key off, all switches off, connecting the ground wire to the battery should not spark. Somewhere a circuit was complete.
With over 55 amps going through that wire, my guess is a battery hot wire was grounding.
If you looked at the ammeter, was it showing maximum disharge ? If not, the short is before the ammeter. (in other words between the battery positive and the ammeter)
I would NOT add bigger wires. I don't see fusible links, so would add them where they ought to be.
For hunting the problem, you may want to use a lamp and/or a circuit breaker.
Any of the wires shown here are hot with the battery connected.
Any of them touch ground and with the negative connected to the battery, the circuit is completed.
So these are the wires I would begin looking exposed conductors, hanging loose, and obvious signs of contact with the body.
Pinching is another favorite way for Murphy's Law to find a way to short circuits.
Further, since the body ground is the one that got hot, the short is unlikely to be the alternator or on the engine. Grounding to the engine should have resulted in the high current flowing through the big ground cable.
Something is seriously wrong.
A short 10 gage wire should handle 55 amps without melting.
https://www.wiresizer.com/ws_userguide.html
That means that there is a short.
Key off, all switches off, connecting the ground wire to the battery should not spark. Somewhere a circuit was complete.
With over 55 amps going through that wire, my guess is a battery hot wire was grounding.
If you looked at the ammeter, was it showing maximum disharge ? If not, the short is before the ammeter. (in other words between the battery positive and the ammeter)
Something happened during cooling off or when you pushed it so that the short was no longer touching.
I would NOT add bigger wires. I don't see fusible links, so would add them where they ought to be.
For hunting the problem, you may want to use a lamp and/or a circuit breaker.
Any of the wires shown here are hot with the battery connected.
Any of them touch ground and with the negative connected to the battery, the circuit is completed.
So these are the wires I would begin looking exposed conductors, hanging loose, and obvious signs of contact with the body.
Pinching is another favorite way for Murphy's Law to find a way to short circuits.
Further, since the body ground is the one that got hot, the short is unlikely to be the alternator or on the engine. Grounding to the engine should have resulted in the high current flowing through the big ground cable.