There have been a number of recent threads on this topic. I recommend reading through
this thread for more information.
In general, alkaline batteries are not recommended for vehicle applications. The leakage rate is just too high, especially at high temperatures. Therefore, I would not recommend using the Mag D in the car.
Good quality CR123s should hold up fine in a vehicle, but the high temperatures will shorten their usable life. In fact, any battery stored in a vehicle will have its life shortened because of the repeated exposure to high temperatures.
Of all the batteries available, the Energizer Ultimate Lithium AA seems to hold up the best at high temperatures, and you will generally find that people recommend using these. Of course, that does not help you since you already have a flashlight that uses CR123s. My recommendation is to use good quality, name brand CR123 cells. Panasonic makes the Surefire and Duracell branded cells, and Sanyo makes the Energizer branded cells. Go with any of these and you should be fine.
Better yet, use this as an opportunity to get yourself another flashlight that runs on AAs! One can never have too many flashlights!
Cheers,
BG
