My newest prep is a USB battery pack by TOMO. For $20 on eBay you can get this unit that has dual usb outputs (one 1amp and one 2amp port), recharges via microUSB and has a nice green information display on the front. This unit does not come with batteries. It runs on 1 to 4 18650 lithium ion batteries. The only con I see to this unit is while it’s operating, the backlight for the screen stays on full time (a small waste of energy). The unit will auto shutoff in 20 seconds if your not charging anything or recharging the internal batteries.
I would reccomend to use unprotected 18650 batteries as this unit has protection built in and for an easier fit in the unit. You can also use 18650 batteries from old laptop batteries like I have. One word of caution though, if you have never opened up a laptop battery pack yet, do some research first. Even a dead laptop battery pack opened has the potential of high amerage power that can easily cause burns or venting batteries.
Youtube user “Bunker Squirrel” has found an undocumented feature on this unit. Once it is either charging a cell phone or your recharging the unit, you can turn off the green backlight by holding the power button for a half second. This will help lengthen your battery time a little bit by not wasting power on the LCD backlight.
You can find this unit on amazon here: