Well, keep in mind that a battery puts out 1.5V, give or take. So you could use 10 battery operated lights wired in series (or five, if they are all two-cell devices that use the batteries in series for an output voltage from the battery pack of 3.0V) for operation on an automotive electrical system. I'm using 14.7V as an estimate of the system voltage, not 12V, because the former number is likely closer to the voltage that will actually be provided while the engine is running, and you likely won't notice the difference when the engine is off.
nate