Using rechargeable batteries to power circuits is a proven method for providing energy to mains-independent equipment. A major disadvantage of this is that the battery usually turns out to be empty at the most inopportune moment. As a user, you are unexpectedly confronted with the fact that the circuit suddenly doesnt work any more. Sometimes this is only a minor inconvenience, but at other times it can be a catastrophe. For instance, just imagine what happens to a model airplane if the radio receiver stops working in flight due to an empty battery. We can assure you that the consequences are anything but pleasant.