RE: lipo to nimh cells number conversion
Because the Lipos will be so much lighter you can afford to lose a volt or two and still get comparable perfomance.
3.7X4 = 14.8 V Lipo
1.2X12 =14.4V NiMh
12 NiMh cells would be roughly equal in power, but the extra weight will make a difference. NiMh will be about 2.5 X the weight at 12 cells. If the motor can take the higher wattage, I would bump it up to at least 13 and probably 14 NiMh cells to account for the extra weight.