USB cable powering options

Ok I'm new here so if this is in the wrong forum I'm sorry... Anyways... I have a portable dashcam for my car that runs on USB and it included a cigarette lighter plug that converts to USB and everything works as it should.. Now the problem is when the cars ignition is cut off so is the power and my camera internal battery only last 15 min without a power source... So I bought a USB battery pack that holds like 20, 000 Mah to keep my dashcam rolling while the car is off, Now the question is.. Instead of me having to manually disconnect the cam from the cars cig USB plug to my battery backup... Is there some sort of USB cable or switch where I could have my cam plugged into both sources and it would automatically detect if there's no power on this source to switch to another one? Or even just a simple switch that I could manually press in order to make it switch back and forth. I've asked RadioShack frys best buy and they have no clue what I mean it sounds so simple but yet I've found no products that allow me to do this so any help would be greatly appreciated thanks

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Might be a whole lot easier to have the cig socket

rewired to be ON when the ignition is off.

GMC wire their "accessory" sockets, PC for Cigar/Cigarette lighter socket, so that they are ON when the ignition is off.


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Joe Shadetree (mechanic) here.

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