I bought a Uniden marine hand held. Can I have it on while its in the charging cradle on my boat and listen for transmissions? Or will it be strictly battery operated. I have a DC adaptor for it, but I think its only for charging the batteries while the radio is in the off position. Is there an adapter I can buy to be able to run the radio off the boat's accessory battery, and save the radio batteries for emergencies. And, How long will new radio batterys last while the radio is on? Thanks