I'm not an expert on this, but my good friend Romain Klassen in Victoria B.C. states that this is quite safe. He maintains that ALL his (u-boats) run w/ external batteries. Makes sense to me. We don't run these models in saltwater so there's not a problem with the conductivity,radio interference, or corrosion from the saltwater.
You need to be able to utilize the biggest and best battery your boat can handle. The reason I say this is you need maximum runtime from your batteries If you need more guidance, please contact me here via the forum.
Subcommittee member #2348