In testing the Nite-Ize conversion in 2, 3, 4, and 5 D-cell flashlights, I have several comments:
1. Using 2 cells produces considerably less brightness, and TRIPLES the current consumption (compared to 3+ cells).
2. Going beyond 3 cells produces no significant increase in brightness or current consumption.
Conclusion: It's a VERY poor choice in 2-cell flashlights. Best results are in a 3-cell flashlight, with little additional benefit in using 4+ cells.
BTW, the 2-cell MAGLED flashlight is significantly dimmer than the 3+ cell versions, and has shorter run-time, too. Again, the 3-cell version seems to provide the best combination of brightness, size and run-time.