82 750csr twin melted battery cables and + post

hi i have a 82 750 twin which is all stock and has ran great for over a year that i have had it. i recently noticed that it was getting hard to start i even had to kick start it on occasion because it didnt have enough juice to turn it over. i found that the battery had lost its electrolite and had shorted out so i bought a new one filled it and charged it for a little over 14 hours on a 2amp motorcycle charger per the instructions with the new battery. i checked the volts and they were 12.98 exactly as the instructions said they would be. i installed the new battery in the bike and it roared right to life and ran great. i let it idle in the driveway for about 5 min then hopped on to take it for a spin around the block. before i even put it into gear it died. i tried to restart it but there was no electric at all it was flat dead then i smelled smoke so i hopped off and saw smoke rolling out from under the seat by the time i opened the seat i saw it had fried all the insulation off the neg ground wire and melted the positive post on the brand new battery. i cant see any obvious spots where the battery or cable would have grounded to the frame and i ensured the battery was the right size prior to installing it. my question is this..... has anyone had this happen to them or does anyone have any ideas about what caused this. i am at a loss and really want my inexpensive commute bike back asap. thank you for your help. i do ok with mechanical issues and even simple electrical issues but have almost no experience working on motorcycles so any help would be a huge help

ouch! :sick:
id start checking all the connections on the bike including ground cables for corrosion/dirty connections,then id go through the charging system

It's almost always the ground wires that melt when the battery shorts to the frame. In your case there is even more evidence in that the + terminal is melted. Somehow, either the battery + shorted against the frame, or possibly the seat pan. Sometimes someone will leave a wrench or screwdriver where it can short.

This is just the most likely cause, there may be other causes, but I've never seen anything but a hard short melt a battery cable. Even a shorted rectifier won't likely be able to melt a battery cable.