I found a way to charge the Motorola DTR radios from a 12V backup power system. It requires two parts, a 12V to 5V USB power converter and a USB charging cable for an older model of NexTel phone (both from Amazon):
Classic Straight USB Cable for the Nextel i860
I cut the cigarette lighter plug off and installed PowerPoles. The cable will plug into the charging base included with the radio, but doesn’t seem to intiate the charging cycle. It works fine plugging it directly into the radio though. If you already have the Gomadic cable, all you need is their Tip #14.
I would suggest buying tips for all your USB powered devices (cell phones, tablets, etc.) and be prepared for when the “lights go out”.