I recently bought a portable 1900mah battery pack for my iPhone 4 but having a bit of an issue with it, or rather my phone. When I plug it in, my iPhone recognizes it, but shows a charged icon. Not the lightening bolt, but the plug icon, which means that it's plugged in but not charging. Does anyone have any tips on how to get it to charge? This is happening when my phone has even 40% or less. Occasionally it works, but for some reason it only charges for a couple minutes. When plugged into my MacBook or the wall, the iPhone charges fine. It's just with this thing I'm having issues.