I'm looking to get a large portable battery. Something between 10,000 mah and 30,000 mah, for around or less than $150. I would prefer the battery to be the form factor of an iPad. I basically want to have something I could stack with a Macbook Pro, or an iPad, and carry around to extend battery life. If it's cube shaped, or more chunky, that is fine too if that shape makes functional sense. Brand / quality matters as well, since I've had some bad experiences with mobile external batteries in the past just crapping out. Any recommendations?