Mr. Norton gave you some good advice. Unfortunately, most wall warts are not very elite units (made as inexpensively as possible) and may be as simple as a half wave rectifier. Most are simple full wave rectifiers with no filtering so put out a pulsing DC voltage which, when open circuit, provide quite a bit higher peak voltage than the battery being charged. They depend on the internal resistance of the transformer windings to limit the current. That is why they always get warm when you use them. Also, the current measured with a normal ammeter will not read the true current flowing because of the high ripple content. The only way to be sure of the current is to use a true RMS meter or an oscilloscope to monitor the output.