The LED's "shouldn't" roast the alternator but there may be any number of reasons for the failure. Check all of your ground leads - they are responsible for all kinds of gremlins when they get fouled up. Check the main battery lead for signs of corrosion. Also take a look at your alternator. Always a chance someone swapped it for a low amp unit or there is actually something wrong with it like a cooked diode. It might be time to beef up your charging system with a higher output alternator anyway...