Is it safe to use car chargers?

Using an iPhone 6s plus with iOS 10.3.1.

I am wondering the safest means of powering the phone in the car.

The car has three options for USB.

1. A USB port direct into the media dash that powers the phone but it has no labelling at all.

2. A cigarette lighter plughole.

3. A power outlet in the central shift console that fits a cigarette lighter style USB outlet - it receives a lighter plug charger.

The car has 12V sockets. I think that means the central console power outlet and the cigarette lighter, not sure what the dash USB is. There's no info on amps.

I have two dual USB output plugs. One of them has a 1A and a 2.1A USB output. The other has two outputs that are not marked.

Any information on the amp or voltage output through these ports would be greatly appreciated.