If you connect a current meter across a power source then it is shorting the power source. The power source and/or the meter will be destroyed!
Instead, current is measured by connecting a current meter in series with the load. Same for measuring AC current.
Maybe you want to see how much current a power source can produce when it begins to smoke? Then you overload it with a huge resistor, measure the voltage across the resistor and use Ohm's Law to calculate the current. Not good.
A power source is supposed to have a maximum allowed continuous output current rating printed on it or in its datasheet. Look for it.