Two taillights and a generator
 

I have a fender-mounted taillight, a bottle generator (B+M) and an LED headlight (IQ fly). I'd like to add a second (rack-mounted) taillight.

Apparently, two headlights can be used with a hub generator, but would produce too much resistance (and thus slipping) with a bottle generator. I wouldn't think an extra .6 watt would be a problem, but am not sure how to go about it.

The generator should be a constant current device, according to this website: http://www.m-gineering.nl/son12vg.htm. This means, as far as I understand it, that the voltage, and thus power, will increase, while the current will stay at .5 amp.

Since a typical 3 watt system has the headlight at 2.4 watts and the taillight at .6w, it seems that the resistance for the headlight should be 15ohm, and the taillight 60. I can't measure, since my multi-meter is broken.

In order to make everything work with two taillights, it seems that I would want a total resistance of 14.4ohms. Wiring everything in parallel would produce 10ohms of resistance, which wouldn't produce enough power. Wiring everything in parallel and adding resistors so that the headlight circuit would total 21.6ohms, and each of the taillights 86.4 ohms would seem to make everything work out on paper. Is this right? I found a website selling 6.6 ohm resistors, but can't find any at 26.4 ohms (not surprisingly). Is there any reason why 25 ohms wouldn't be close enough?

And does my plan to add the resistor in the middle of the wire using heat shrink tubing make sense?

You are well off the track as the current LED lights are not ohmic devices. Just connect everything in parallel and spare yourself this type of considerations. LED taillights commonly take right now 0.2W at full power and my bet is that IQ Fly will already operate fine at 0.5-1 W. Even if you operated two front lamps, for LED they'd likely work fine in parallel.

Thanks, that makes it easier. So can you direct me to some more details about how the headlights and taillights do work?

I do not think I can direct you. I do my own measurements since I use a circuitry that intermediates between the dynamo and lights and have to know what it takes to operate those lights or, for that matter, nearly any lights I may end up using, since I want the circuitry to be versatile. The LED taillights tend to be very efficient and B+M LED headlights have even surpassed them in terms of efficiency.