melted battery terminal

Good morning
I have a 24v system running off of 2 12v batteries.
I have recently added a new circuit off of one battery to power 12v heater plugs.
Since then, in the last 500 miles maybe, the battery terminal had melted on the negative of the second battery only? What could this be?

Thanks

any heat in the electrical system, or in your case melting, is usually caused by a bad connection, or cable which is not strong enough to carry high current.

Heater plugs can take a hefty amount of amperage. It could be a bad battery clip that isn't making a good connection on the terminal. Everything powered in the vehicle looking for a negative path is going to be arcing across a bad battery clip.