Terminals on batteries can melt for a variety of reasons. The battery in the picture above accidentally had a tool or cable dropped on it. The terminal originally looked like this:
Not only did the terminal heat up enough to melt the lead capturing it, but the hot terminal then melted its way through the case. That was one hot bolt. Be very careful with cables and tools close to large batteries! Use insulated wrenches and wear eye protection when connecting or disconnecting large batteries.
How the above failure happened was obvious to the mechanic who accidentally shorted the battery. The failure of the terminal in the battery below was less obvious to the customer:
This is another case of the terminal overheating, but it happened perhaps a month after the battery was put into use. What happened here was a loose cable connection in the first month of operation. In this electric cart, the connection likely wasn’t tight enough, and/or it vibrated loose. When a cable connection is loose, that greatly increases the electrical resistance. Combine the large current are flowing out of the battery to power in this case an electric cart with increased resistance produces enough heat to melt lead. We see this regularly both in golf carts and motorcycle batteries when the connections have not been appropriately torqued. After installing batteries in any equipment with vibration (especially golf carts and motorcycles), recheck the tightness of all the battery connections after the vehicle or device has had some use. If connections work themselves loose, tighten them.
Battery connections need to be torqued to be appropriately tight. Loose connections can lead to melted connectors. Melted connectors are an installation or maintenance issue, not a warranty issue.