[JoshTriplett]

There's no standardized regulation or filtering; devices typically see it all. And it's much more than the alternator voltage; a device might see transient 24V or more, and as little as 9V.

See https://en.wikipedia.org/wiki/Automobile_auxiliary_power_out... for more.

Modern chargers tend to be really forgiving, and they're often powering a device that has its own battery, so there are multiple levels of regulation between the car and the device.

On the other hand, I've used automotive adapters that do a simple DC-DC conversion with no regulation, and pass through to a barrel connector to a device that would otherwise be powered by standard alkaline disposable batteries. Those aren't nearly as robust, and I've seen devices fail or power-cycle due to undervolting.