most automotive bulbs are designed with 200-400 hours bulb life(some are out of these ranges, but this is a ballpark), I'm guessing they are counting on a 13.8V system with some voltage losses from resistance/etc, you can probably count on some voltage sag from the pack and from the resistance of the connections as well, so for simplicities sake we drop that factor off both sides of the equation since they will semi-cancel each-other out. So, to get that bulb operating at that really brilliant white you get pushing in the <20 hours life range, we solve the following equation:
[(13.8/X)^12]x250=20
The equation says, basically: bulb is designed for 13.8V and will last 250 hours at design voltage, X represents the voltage required to reduce bulb life to 20 hours.
If my algebra is still good, the result is ~17V required to reduce bulb life to 20 hours assuming a 250 hour bulb. 14 NIMH cells would be a pretty good starting point IMO. Personally, I would pick up some cheap bulbs to play with (order online, they are out there for under $5 each). Start with maybe 13-14 cells and work up maybe as many as 15 cells. Keep in mind it's going to depend on how well the cells hold up under that load, and how long you rest them after they come off the charger. Keep in mind that on-paper estimates do not always hold true in reality. Every brand of bulb will have a slightly different behavior, and different bulb life, and handle overdrive slightly differently.
Eric
