i lent my wii to a mate and he returned it with Lithium Energisers. since i got it back, i've been playing battle revolution, pangya golf and metroid prime a fair bit as well as mucking around doing other stuff, rumble on, speaker on full volume and it hasn't even dropped down 1 bar! It's amazing, although I fear i'll need a bulk recharger to get it back up.
Anyway, I couldn't recommend it higher.