Basic problem with the Battery Tender products is that they don't have a current/thermal shutoff... So if something is shorted the Battery Tender should shut down, rather than continuing outputting a voltage and eventually overheating/shorting out.. In my personal case, I caught it early enough before it started a fire. The product is built in China and its basic design should be modified to include a current/thermal shutoff.. But then its price would likely double..
