LeftieBiker wrote: As for the voltage theory, perhaps you can explain why the Leaf's Halogen Low beams have a nice, reasonably bright, full pattern.
The voltage "theory" is spot on. The dead giveaway is the color temperature of the high beams. It's quite yellow compared to other cars using the same bulbs. Color temperature has nothing to do with reflector design, but is a clear indication that the bulb is being under-driven. And, as a consequence, much less bright than it could be. Incandescent bulbs are voltage-dependent. The output is strongly affected by even a fairly small change in voltage. Per the link I give at the bottom,
"Headlamp bulb light output is severely compromised with decreased voltage. The drop in light output is not linear, it is exponential with the power 3.4."
Interestingly, bulb life is even more strongly (exponentially) affected. Check the following chart:
You can see that even a 10% voltage reduction from nominal can result in 30% loss of output. The good news is the LEAF's OEM high-beams will probably last forever
LEDs are not voltage-dependent, but rather current-dependent. They aren't affected by voltage drop (within reason) as long as they can get enough current to drive the LEDs to the desired output.
As to why the high-beams are more affected, I would speculate that part of the problem is in the wiring harness. Use of thin-gauge wire will cause a voltage drop, and this is dependent upon the load. Since high beams draw more power, the voltage drop will be more pronounced. I suspect the high beams could be improved by re-wiring their circuit with heavier gauge wiring.
Lots more info here about headlamp voltage drop and how to improve it:http://www.danielsternlighting.com/tech ... elays.html