I recently bought an external pack (can't remember the brand or mAh) that is supposed to recharge cell phones and other devices. I thought it was just an external battery but the directions say that it must be disconnected once the phone (or whatever) is fully charged. There is no indication that the battery has been fully charged other than looking at the battery level on the device, so that needs to be monitored. I haven't used it other than one time to charge a cell phone. Is this a common set up?