Lithium-ion and other Lithium type batteries carry a lot more energy per gram than the other types. If you bring a battery made of AA or AAA cells, you might carry less weight than a Li-Ion battery, but it will have less charging capacity. That doesn't matter, though, because either will probably be acceptably light, even if it has enough capacity.

One advantage of using AA or AAA cells is that you can get them at any corner store. This is not a great strategy if you plan to use the battery a lot, because of environmental and financial cost, but it is a real winner for unexpected needs.

Then again, if you bring enough capacity -- or even more than you expect to need -- in Li-Ion batteries, life will be simpler.

As Francis warns, don't let the Li-Ion battery overheat. It can happen from charging, and it can happen from exposure to the sun, so insulate it in the summer.