I remember my first experience with using two batteries in a 40% Aerobatic plane a few years back. I used a product called "The Battery Backer". Product was supposed to isolate each battery from the other so if one failed the other would continue to power the plane. It worked great, until one day, the Battery Backer failed! Inspection after the crash showed two fully charged, working batteries, one dead Battery Backer, one dead 40% Aerobatic plane!

Moral of the story: SHAIT HAPPENS In this hobby, you'd better get used to it, cause any electronic product, made by man, can & will fail! Although, with me it's the electronic brain to thumb connection that usually goes first

David S.