Dropping 18VDC to 6VDC

[tubesornothing]

I have some old 6V mountain bike lights with dead lead acid batteries.  I want to cheap out an ressurect them, but use my 18V LiIon drill batteries.  I am figuring out a way that I can drop the voltage without too much waste.  Here is what I have looked at:

1.  use a series resistor - too much power waste
1.  use a zener - too must power waste?
3.  regulator - I think 18V into a 6V regulator will  cause too much power waste
4.  DC-DC converter - good idea, but the cheapest is $80
5.  Run the two 6V light in series and overvolt them - hmm, I don't want to fry the lights.

Any other suggestions?

thanks

[DummyLoad]

three lamps in series - no waste.

with two lamps 6V wasted - or the power of one lamp.

with one lamp 12V wasted  - or twice the power of one lamp wasted.

[tubesornothing]

I only have two lamps though...  I don't really want to spend more money, if I can help it.

I did find a 6V regulator that will take up to 20V input with a 7A output for about $14 (kinda pricey) I suppose I could just get a couple of 6V 3A regulators, it might be cheaper.  Not sure, but do regulators have much waste heat with a large potential of voltage?

[tubesornothing]

OK, Wikipedia says linear regulators or inefficient cause "regulates the voltage by burning off "unwanted" voltage as heat" - not what I want.

[billcreller]

  Well, maybe the 2N3055 transistor and a pot for bias would be cheaper than a regulator etc. The transistor could heat sink off the bike someplace (??)
  Sorry....just another brain fart

  Oh yeah, once the correct R value was found for that voltage and load, a resistor could replace the pot ( that's two brain farts)

[tubesornothing]

I think I would be in the same boat - offloading the volateg difference as heat.  To much of a waste of precious power. 

After a lot of looking into this, I think I will switch to LED lights. Doug and others have put out a lot of info on how to roll your won.  There are also some inexpensive DC-DC converters, so I can leverage my drill batteries.

[EL34]

Two 3.6 lithium ion 2500mah batteries are $6.83 with free shipping
http://www.dealextreme.com/details.dx/sku.29030

7.2 volts total in series = 2500mah
4 is series/ parallel = 4800 mah battery pack


I am guessing you are trying to run Halogen bulbs?
They might not take 7.2 volts.