Your safety comes first when working with battery chargers. Safety precautions: Do not touch the clamps together to see if your battery charger is working. Tapping the clamps together will cause large voltage spikes at the rectifier. While the rectifier will handle a surprising amount of over current, they do not like high reverse voltage click the link above for more information
A/C recovery is a complicated process these days espcially with the new R1234YF refrigerant.
Always make sure you are using the right machine for the job, make sure you have enough room in the internal/external recovery tank to handle the refrigerant you are about to recover. More information is available by clicking the above link