I've found some Do-it-yourself guides to making a 4-AA battery gadget charger, which are basically four AA batteries connected to a cut up usb cable:
Such a device uses the AA batteries to recharge your gadget.
This looks like a useful project, but I was wondering, is this safe to use long-term, for gadgets like smartphones/android devices/mp3 players/tablets/etc.? Assuming it is connected with the right polarity, and lower-voltage rechargeable batteries are always used, it should always be around 5 volts, which is standard for all usb, but isn't there supposed to be a circuit to prevent overcharging? Is the worst case here simply not charging, or bricking the device?
Also, what would keep this from reversing? If the AAs went dead, would plugging in your phone actually drain the phone's battery to the rechargeable AA's?
A 5v regulator with red battery connected to input, red usb connected to output, both blacks connected to ground, doesn't seem to be charging when the battery is dead. I'm wondering why this recommends connecting green and white to ground. Wouldn't this make voltage return through -, d+, and d-, which is not recommended?