I use the battery extender from
www.semsons.com, it works really well and I have an Audiovox Maestro. If you check there site, you can see if it is compatible with your device. As far as the voltage, mine can only take 5 volts as well, but I use NiMH batteries instead of alkaline, so the voltage only equals aroun 5.6 volt which my Maestro can handle. But im not sure about your device.