I have been going nuts trying to come up with a lighting solution for winter.
Last winter, I bought a NiteRider Road Rat for my after dark training rides. I felt that it was plenty of light for my needs.
The problem is that the battery only lasts 2 hours, and my commute may require up to 4 hours of light.
I read the Geekiness thread for a few hours. It occurred to me that a nice compromise between hacking up yard lights and spending a fortune on a fancy Li-ion light set would to retrofit the RoadRat with a big enough battery to get me through the day.
So, I figure that I can buy a 6V 10ah NiMH battery pack and smart charger from BatterySpace.com The upgrade costs $80. If I have figured this out correctly, I should be able to get almost 6 hours of run time out of my 10 watt halogen light.
If this project works out well, I may buy another Road Rat for $65 and battery pack for $50 and throw in a 20W halogen for a secondary light.
This seems like a cost efffective approach to get a decent amount of light on the road and a run time of 4+ hours.
You have the right idea. Niterider just buys the battery and marks it up for huge profit. If you can solder, its easy to make whatever battery pack you want. You could also consider buying a 12V lamp and running a 12V or 13.2V battery pack. 12V batteries are more common and may be cheaper, and you can get brighter bulbs as well. Compare watts (amps x volts) to find a similar brightness bulb. With a 12V You'll only need 1/2 the current rating to achieve the same brightness and run times. No need to buy the bulb from Niterider either. I believe they use a standard MR-11 bulb which is very common, but you should check on this.
Interesting thought about going 12V. Has anyone changed their NiteRider 6V system to a 12V system? I know that the bulb is different, but I wonder if their are any other issues in switching to 12V, like heat or wire specifications.
Which brings up a good question, why are most of these lights built around 6V systems rather than 12V?
Wattage determines heat, so a 10W 12V bulb will generate the same amount of heat as a 10W 6V bulb. Just about any electrical wiring has insulation rated over 200V, and higher voltage allows you to use thinner gauge wire for the same amount of wattage. In other words, its actually beneficial to use a higher voltage.
I suspect niterider uses 6V bulbs on their lower end systems because the bulbs generally have a lower wattage range (5-15W vs. 10-20W for 12V). The 12V systems are for the brighter/more expensive systems. To me, the housing looks similar on all of them, but as mechBgon said, check to make sure the 20W doesn't melt the housing if you want to go brighter.
You might be able to find a MR-11 12V bulb at your local hardware store. It may be too bright at 20 or 35W, but you could check to make sure it fits.
Pull your bulb out and take it to a good building supply store like Lowes and compare it to a replacement MR-11 bulb. If your bike light has this style and size standard bulb you can replace it with a very bright LED bulb. The current draw is considerably less and will greatly extend your night riding time. These replacement LED bulbs are 12 volts so you will need a 12-volt battery as well. I built my own by purchasing a track light fixture for MR-16 bulbs. I simply removed the track light fixture base and added a clamp to fit the handlebar. I liked the looks of the larger MR-16 light fixture as apposed to the tiny MR-11 fixture. I use a 12 volt 4.2 AH Gel Cell battery I purchased on clearance sale at Wal-Mart for $4. Here is where you can purchase the LED MR-11 and MR-16 LED bulbs. http://autolumination.com/mr16.htm