B&H Photo has a great deal on a 14,000 mAh USB battery for the 1st of April: $16.95. This is the type of battery I use to power my Tascam DR-60DmkII recorder for more than a full production day, that is, more than 12 hours. To make things manageable, I just bought some velcro with adhesive on the back and attach the whole battery to the back of the Tascam recorder. Since I've done that, I've never had to pause a shoot to change out batteries. Not a bad deal if you're tired of swapping out those AAs every three hours or so.