Your lipo will work fine, it's because the cells have different voltage ratings. A 6s nimh is supposed to be 7.2 volts, even though they are much higher when fully charged, and a 2s lipo is supposed to be 7.4 volts, even though they're about 8.4 when fully charged. Basically, Nimh and ni-cad use a different type of cell than lipo, li-ion, li-fe, Etc. And if you think batteries can be confusing, try researching small brushed motors. You need a freaking college education understand those things.