We looked at Mipow's Power Tube 2200 Universal Mobile Charger back in June, and the company has now expanded the line to include Power Tube 3000 ($49) and Power Tube 4000 ($69) battery packs. Rather than the previously cylindrical shape, these new "Tube" models are boxier, with built-in connectors. At one end of each unit is a cap with a USB port; this can be used with the included cable to charge devices that use Micro USB. Removing the cap reveals…
