As David so rightfully pointed out that the correct way to do this is to pull out the code from the button click handler that you need to call from the other non-event driven method and put that into a separate method.
However, if you are pressed for time and need to do a quick and dirty test to see if it works, you
could (and I am not saying that this is right) call your button click handler with dummy parameters.
btnVoltsCurrent_Click(Nothing, Nothing)
But you can only do this if you do not need the sender or the event arg parameters.