John, most of my batteries are Tenergy brand, 9.6v, 2000mah, NiMh (3.9"x1.18"x1.18" sq). I was getting them from Tenergy but they seem to have doubled in price. Amazon still has them for $15.99 plus there are other brands. Tenergy has 2 9.6v, 2000mah, flat battery packs plus a charger for $37.99, the battery pack is 4.09"x2.32"x0.63". Too wide but you could always put it in at a slant.
I've found 9.6 volts to be plenty to power my engines, they go just as fast as they did under 18vac. 11.1v 2200mah LiPo batteries, from what I found, have a lower starting voltage. I think that is because LiPo batteries have a lower internal resistance so more power is supplied initially, but I only found that necessary on 1 or 2 engines. LiPos take a different type of charger than NiMh. Tenergy has a good website that will show you what they have, but like I said their prices are high. I actually got a couple off Ebay from Tenergy for less than what they were charging thru their own store.
I normally get 2+ hours of run time and it takes equally as long to recharge at 1amp/hr.