Rob, at the moment mine run off 3 x AA sized Duracell batteries, although there's no reason why they couldn't be replaced with rechargable cells.
Glad you're enjoying the Neptune mate...my first one was a disaster, it was replaced FOC, the replacement wasn't much better, so that went back...the early ones suffered from electrical problems. The third one which turned up was much better, it never let me down once, I had great fun with that...a chap from France saw it, wanted it, and drove all the way over here to collect it.
This one is yet to go in the water properly, once the weather warms up I'll get out for it's maiden voyage. (cold weather, rheumatoid arthritis and COPD don't go together). I'm just in the process of changing the radio, I want to use my FC-16 with it.
Rich