Which one actually has more cranking power? I'm having problems with my Braille (it won't hold a charge). So I'm thinking about getting a PC680 to put in while I send my Braille back for them to look at. If they replace it under warrantee, I'll probably just sell it. BUT, which battery is actually better during cold starts and which will hold a charge for longer? And is the PC680MJT basically just a PC680T with a metal jacket for extra protection?