You can also just let your transmitter sit empty for 10 minutes or so. I generally just do this instead of dealing with putting the battery in backwards, unless you are in a big hurry. From Marshall's site:

"Also, the timer will automatically reset (remaining charge in the transmitter will dissipate to zero) if you remove the battery and let the transmitter just sit empty for 10 minutes."

Check out the Q & A section on the website for the PowerMax under "I put in a brand new battery, but it's still double-beeping.":

http://marshallradio.com/en/north-am...ax-transmitter