For powering an USB device in your car, you need a 12V car adapter. Such an adapter plugs into the cigarette lighter socket and provides an USB socket. You also have to specify the current, since USB by default provides 0,5A but for charging purposes can deliver up to 2,1A. Then you have a clear functional description.
I have an Anker adapter and one from Huawei which came with my Huawei E8278 LTE stick. Both adapters deliver sufficient current, so from a functional point of view, both adapters are equal.
Unfortunately one works fine, the other one not.
Here engineering comes into play. Your car does not provide 12V. It might provide 12,8 when the engine is off, 14,1V with a running engine, and maybe 9,5V during cranking. Furthermore there might be voltage dips when head-lights or seat heating are switched on. How are these conditions handled by a 12V car adapter?
My car adapter is used to power the LTE stick in the car, which is actually a portable hotspot in USB stick format. This hotspot needs about 25s to boot, then shortly thereafter the sat-nav in my car connects to it’s WiFi network and pulls detailed traffic info from the internet.
So far, so good. LTE stick plus Huawei 12V car adapter in the back of the car where the second 12V socket is located and off you go. Then by accident I noticed that the sat-nav lost it’s WiFi connection all of a sudden. After a memory rewind, it occurred to me that this happened shortly after I waited for a traffic-light. Since my car has a Stop-and-Go system, the engine shuts down during traffic light stops. Well that explains, during engine cranking the voltage on the cigarette lighter socket dips and the LTE stick reboots, causing a 25s WiFi outage…
I expected to search for an alternative solution, in other words, additional functionality like a battery powered hotspot. To investigate the voltage dip further, I plugged the LTE stick into the 12V socket below the dash, so I could see what’s happening. In this socket I had the Anker 12V car adapter, to be used for charging phone and pad. Surprise surprise, during Stop-and-Go the LTE stick remained powered on and did not reboot.
So both adapters are not equal! Each adapter reacted differently to voltage variations.
Next step, measurements: The Anker adapter switched off at 4,7V and on at 5,6V. The Huawei adapter switched off at 8,6V and on at 9,5V. So the Anker adapter most likely uses some form of DC-DC step-down (buck) converter and the Huawei most likely a linear one, since requiring 3V difference between in and output. Note that these are static measurements, the dynamic behavior most likely is different. The Anker adapter was able to handle the voltage dip caused by engine cranking, the Huawei one was not.
This is engineering stuff. Both Anker and Huawei did not specify any of this with their product. The “premium” price on the Anker adapter was a good investment. Huawei tested their product with a bench power supply: 12V input, 5V on the USB output: all according to spec… but not suited for use in a car…