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USB Charging with 12V taken up?

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I have a USB key in the port closest to the passenger side, a Lightning cable in the other and I plugged in a dashcam in the 12V cig lighter spot.

All of a sudden the Lightning cable is providing no power to the iPhone to charge it. Is that expected? The dashcam isn't hardwired yet as we were just testing to see if we wanted it or not, so it's using the 12V spot right now. However, if we wanted to plug in other items in the 12V, then it sounds like we can't charge our phones. Seemed a bit odd.

The Lightning cable works fine if there's nothing in the 12V and the USB key and Lightning cable are the only items connected.
 
I have a USB key in the port closest to the passenger side, a Lightning cable in the other and I plugged in a dashcam in the 12V cig lighter spot.

All of a sudden the Lightning cable is providing no power to the iPhone to charge it. Is that expected? The dashcam isn't hardwired yet as we were just testing to see if we wanted it or not, so it's using the 12V spot right now. However, if we wanted to plug in other items in the 12V, then it sounds like we can't charge our phones. Seemed a bit odd.

The Lightning cable works fine if there's nothing in the 12V and the USB key and Lightning cable are the only items connected.

I have heard, though I don't know if it is true, that only one of the two USB slots charges at 2.1 amps, while the other charges at 1 amp. Is it possible the iPhone (or the lightning cable) just isn't recognizing the lower (or the higher) charging amperage? What happens if you switch the USB key and the Lightning cable?
 
There are 12V splitting adapters available to let you plug more than one thing into a single cigarette lighter outlet. Search Amazon or eBay and you'll find lots.

That may be, but that's not what the poster I was responding to suggested.

I have no idea whether or not it would be advisable to use something like that in the Model S. Someone else can address that.