In article ,
Frosted Flake wrote:

When I hook up a voltmeter to a power plug in my car, I read a bit over
13VDC without the engine running. Wit the engine running, I get
somewhere around 16VDC. I have a device that uses a wallwart rated at
12VDC out and it reads, no load, about 13VDC, like with the auto motor
off from the power plug. If I hook the device to the power plug in the
auto, should I be concerned about the 16VDC when the motor is runing?
If so, any suggestions as to how to keep the power to the device closer
to 12-13VDC?

Your volt meter or alternator is busted. Older cars should read about
14.4V at normal temperatures. New cars may vary it from 12.6V to 14.5V
due to various tricks to save gas. 16V at normal temperatures will turn
your battery into a bucket of lead paste.

The problem with automotive power is usually very high power surges.
Cranking the car can make a 40V spike, brake lights 10V, and there's a
constant 0.5V of background noise. Devices meant for a wall wart will
need protection or their power supply filter capacitor will eventually
explode from too much current.

A very simple resistor can do the trick. The inverse of the maximum
current draw will be in the right area. 2 Ohms for 0.5 amps, 10 Ohms
for 0.1 Amps, etc. You can also buy a noise reduction filter meant for
running an old base station CB/ham radio in a car. The base station CBs
had similar circuits to car CBs but without surge protection. As others
have suggested, there are also small DC-DC inverters that can output 12V
regulated.

Don't run a battery charger off 12V unregulated from your car unless it
was meant for that. So much electrical noise can confuse it and cause
damage to the batteries you're charging.