Just checked battery university. It says that the more discharged a battery gets, the higher the internal resistance. It also suggests that if there's one weak cell in a pack, the internal resistance in that cell can be limiting. Might provide some keys to what happened to your tekkeon. (e.g. You mentioned recording another show with the tekkeons...where the batteries at or near being depleted?)
Nevertheless, throughout the battery university site, it discusses issues occurring when batteries are under heavy load. I'm wondering if you need to get a battery with a higher amp rating than the Tekkeon to power your 788.