I've run this for a few shows, and the internal batts don't last even two hours, i've maxed out around 1 hour 40 min...does anyone know of a low priced batt/power pack that would work with this? I've heard valence is a good alternative, but is there any word on how long/how much the small valence puts out/costs? Also, any cheaper alternatives? I have no problem running around an hour on internal batts, maybe more, so I really only need 2-3 hours runtime from power...suggestions?