In the manual it states that they recommend that once a month you drain and recharge the batteries. How do you all completely drain the batteries? I've charged my unit when I received it a few weeks ago and it still shows full bars on the control box and coil. I guess I could leave the unit turned on until it shuts itself off but that might take a while and if I want to go detecting... then I may have less than adequate charge. Thanks for your help.