Hi, C batteries are not that common, so today I went buying 8 of them.
And I was shocked by their cost: without going to expensive long lasting batteries, the cost for a pair of C batteries was 4.5€, which means that 8 batteries cost 18€ (22USD). And that's enough for an hour of usage.
Use it 30 times, and the cost of batteries will be more than the cost of the ROV itself: not to mention the ecological impact of disposing that amount of batteries.
Probably an easy solution would be to use standard rechargeable Ni-Cd or Ni-Mh batteries, but since those are 1.2V, we'll end up with 9.6V. To get 12V we would need 10 of those, so we should make the battery tube longer. These cost around 10-12€ per pair: will be 120€ for a set, but this is the cost of less than 7 usages of normal batteries.
Or even cheaper, would be using LiPo: a pretty standard LiPo costs around 10€, a good battery around 20€ and super- quality one around 30-35€. They are 12V, so we would need one of them, or maybe to increase the duration, 2 of them wired up in parallel (also to keep the ROV balanced, with one per side of the ROV).
I don't know how long the ROV would run with LiPos, but even having 4 standard LiPos (2 as spare) would cost less than a full set of Ni-Mh (40-80€ depending on the quality of the LiPo vs 120€).
Running on LiPo have the additional problem of having to redesign the batteries' tube as normally LiPos come in the form of a parallelepiped.
Room for thoughts :)