Follow up - I asked the moderators to kill my last note since I found a solution (battery isolators) for my problem after I posted my question to the list , but I have a follow-up question: I found an almost perfect and simple solution for my problem here: http://www.bagotronix.com/radiopwr.pdf I have two concerns about this circuit. First, my car radio can has a high current draw on the power lead, so I think IN4001 diodes would not be able to handle the current load. Can anyone suggest more proper power diodes I can use for this application? Second, I do not plan on using a 9volt ni-cad battery as my backup battery, but rather a motorcycle or other small lead-acid battery to keep the power to the radio for many weeks at a time in case the main battery shuts off. Would I still need a resistor in the circuit, and if so, what value? Besides these two issues, I think this is a great and simple solution. Thanks in advance. -Hank