I made the mistake of leaving two esc's turned on in Celestia once, took a 6v battery down to 1v! and there it stayed, costing me a new battery!.

one thing that I did notice with my Najade, using an electronise esc capable of swallowing 24 volts and giving 40 amps, running it with two lead-acid gel cells in tandem to give 12v 4.5AH I gound that they would both get very hot, which for a gel cell, probably isnt doing it any good, so the move was made to Ni-CD/Ni-MH cells, dropping a couple of volts to 9.6, I found that these batteries wired in parrallel, the Najade was riding better and planing, where as with the gel cells she was getting the RPM but their extra weight was offsetting that.

even with 12v ni-cd cells, performance was only marginally faster than with the 9.6v. you may find she runs better on 9.6v and sub-C cells are comming now in somewhere around 3700mah - 5000mah, they can also be fast-charged where as lead gels can take offence to high power charging and rapid discharging.

result, Try the lead gels but you MAY find performance is improved with a paired set of 9.6v batteries