It was a real pain that this year the voltage limit for Electric Vehicle was 7.2V, when lithium ion batteries are either 3.7V or 7.4V. Leaves us stuck with temperamental NiMH batteries when Li-ions are so much easier to use.
Lipos are also a safety hazard. NIMH is much safer to use.
Most Li-ion batteries I used were OK, although I did have one stop working. However, personally, I think the 9V battery is the best option, since it is lighter.
"Hold what you got and hit them where you can." --Ernest J. King, CNO
Materials Science 2/2/8, Chemistry Lab 1/2/4, Electric Vehicle 58/2/28, Astronomy 2/3/40, Wind Power 9/4/24, Optics 27/17/11, Robot Arm 57/1/X (EV, Robot DQ'd at regionals)
don't do builds