Car audio question

I would never use a battery charger to use to check any electrical device, rather it would be a gas gauge or radio or even a head light. Most newer battery chargers will not charge unless it scenes some electrical feed back from what you hook the clamps to. Safety feature built in, if you happen to pull the clamps off of a battery or if they disconnect themselves, you don't worry so much about full current arcing. Batteries take DC current to charge, a cars charging system does an excellent job in doing so using diodes to make a steady stream of DC current. Unforgettably an average battery charger does not produce a steady stream of DC current, it produces a semi-stream of DC current just enough to charge a car battery. Yes you can check this with a meter, and it will show DC current, but using an Oscilloscope you will see indeed it has not a flat signal wave such as a battery. Some electrical devices may or may not check out properly using a battery charger instead of a car battery.