Radar detectors never had batteries. The issue is as the voltage drops, their service range drops preciptously. Just hard wire connectors to each car you'll have it in. the easiest way that I've found is the wire harnesses in the rearview mirrors. For BMWs, green/white is switched positive and brown/black is ground. there are a million DIYs online.
The price of this is ridiculous for what you get, but this is the setup that I've had good luck with.
http://www.ebay.com/itm/18-Radar-Det...xX4p0v&vxp=mtr
Basically, strip and tin the wires and slide them into the back of the wire harness to mate to the pins of the hot and ground wire. Completely reversible, no splicing, etc. If you need more help let me know.