MP3 player? If so, I would consider making a 'dummy' battery that is really a wooden dowel the same size as the battery, with electrical contacts on each end with wires attached. I'd build a 1.5 VDC power supply that runs off the cigarette lighter socket and connect it to the wires. Another option would be to use a NiCad rechargable battery and power it off a standard battery charger that plugs into the cigarette lighter. But, you would have to still connect the wires to the battery (or battery holder contacts in the MP3 player) and to the battery charger. For myself, I'd carry extra rechargeable batteries for the MP3 player.