This is as portable as you can get, but not very powerful. You can get probably get it cheaper locally. I had one for a long time. The plastic whisk detaches and the whole thing is about the size of a pocket flashlight. It's smaller and lighter than an Aerolatte frother.
It's powerful enough to scramble 2 eggs (slowly and laboriously) and mix protein shakes (also slowly, and add liquid gradually).
http://www.amazon.com/Portable-Drink-and-Formula-Mixer/dp/B000FNH01E/ref=sr_1_1?ie=UTF8&qid=1431203172&sr=8-1&keywords=battery+mixer