24v to 12v

Can you get two of those headlights and wire them in series? Two 12v lights in series requires 24v to run.

The other options are:

Tap 12v off one of the batteries in the pack.. This is bad because it makes the pack imbalanced.

DC-DC converter.. This keeps the pack balanced but requires buying a DC-DC converter

First, you need to measure the resistance of the headlight. If it draws 2 amps at 12v, than it should be somewhere around 6 ohms. To supply the light with 2 amps off 24v will require a circuit resistance of 12 ohms, or 6 ohms added. The power dissipation will be 2 amps * 12v, or 24 watts.

Check here: http://www.allelectronics.com/matrix/Power_Resistors.html

There's a 10 ohm 25 watt resistor. Two of these could be put in parallel for a resistance of 5 ohms.

Another option is to put three 2 ohm 10 watt resistors in series for a resistance of 6 ohms.

:) ;) :)
Andrew , I COMMEND you that was a very good explaination 8) 8) Later

One thing though: The headlight won't measure 2Ω when it's off. It will measure much lower, because the filament needs to measure 2Ω when it's white hot.

Also make sure those resistors have good ventilation. They're going to get quite hot.

:jawdrop:
I am sorry Mr. Linkofhyrule , You are wrong 1. the resistance of a filiment remains the same off or on, the amperage changes 2, yes the resisters will heet but 12 watts is a very small amount

actually, he's right, Mr. thinks-he-knows-it-all-strawhistle, I work in the lighting industry and we have to allow for the full rated amps of the bulb on inrush (right when you turn it on). Yes you're right, the current drops after the filament warms up, but if you have 12V, and the current goes from High to Low, what does resistance do? V=IR. Voltage is constant, so if current drops, then resistance INCREASES because the voltage is CONSTANT. You can't defy the laws of physics.

Don't argue with someone unless you know you're right.

From Wikipedia:
The actual resistance of the filament is temperature dependent. The cold resistance of tungsten filament lmaps is about 1/15 the resistance when the lamp is lit. For example, a 100 watt, 120 volt lamp has a resistance of 144 Ω when lit, but the cold resistance is much lower (about 9.5 ohms).

http://en.wikipedia.org/wiki/Incandescent_light_bulb

Ok, next topic at hand:
Why use a resistor and lose XXX Watts in heat. Thats 50% efficiency (50% to the bulb, and 50% to the resistor lost in heat). Only half the power is going to the bulb, and they're even less efficient. If you got a cheap DC-DC converter like this one:
Vicor DC DC Converter Module VI-212-CX 24V IN 15V OUT

The efficiency is likely between 80-90% and only 10-20% is lost in heat. I have the 48V version of this exact one from the same seller. Its 75W 15V on the output. The output can be trimmed to 90% of the 15V, so 13.5. Most automotive/motorcycle stuff that is 12V, is actually made to run around 13.8 (12V is just something people use to estimate). So, trim the potentiometer to 13.8, plug into the 25V, and voila, 75W 15V output (5A). for 12 bucks, its not a bad price, and it'l do the job right.

:jawdrop:
I am sorry Mr. Linkofhyrule , You are wrong 1. the resistance of a filiment remains the same off or on, the amperage changes 2, yes the resisters will heet but 12 watts is a very small amount

actually, it'd be 24W, and yes, it is alot, ever felt what 25W feels like? Its quite warm, and its all being lost forever. Its not a small amount, its the same power that the light could be using to stay on for 50% longer.

Awesome find Frodus. I also noticed that seller carries a 12v to 15v DC converter which I might use for something.

I figured that the price of a DC-DC converter might run a lot, and it's hard to beat $2 for power resistors. But, at that price, the DC-DC converter definitely makes more sense. And, it'll save considerable energy for range depending on what he's driving.

Mr. frodus I stand corrected !!! ti just shows to go you 1. no one can know everything ,2. old dogs can learn new tricks if you show them ,3 take everything with the grain of salt 4 believe only half of what you hear!! thank you for pointing this out to me !!! }:)

Anyway, is there any reason you can't/don't want to find a 24V bulb?

For now I'll just use resistors, it's not that efficient but I only ride 15min or less at night. Will definitely get a dc converter soon.

Riding without lights on at any time is not a good idea, it gets you killed more easily by soft drink slurping car drivers holding a phone in the other hand....

Mr. Mik

still, at 12 bucks, a dc converter is definately worth the money...

Just in the energy savings alone in a couple months, you could pay for it.

the resistors do the job, but not a great solution.