My home-brew LED bike lights

battery-driven  |  generator-driven  |  Which type for which use?

Battery-driven bike light

Here's my dual-LED light designed for brevets and flèches.


The heatsink is overkill; I probably could have left it off, as I have good thermal conductivity from the LED stars to the entire outer casing, but it adds a certain flair.  My kids call the light Wall-E, I think of it as Wall-E after he joins a punk rock band.  The heatsink is probably over half the weight of the light.

This light plus a 4 amp-hour 11V Li-ion battery will give ~250 lumens for 12 hours or ~450 lumens for 5.5 hours, more than enough for an all-night ride.
Total weight of light + battery, very close to 16 ounces (450g).
To visualize its size, the square tubing is 1 inch (2.5 cm) on a side.  The light sits nicely in the palm of one's hand.

On my one and only flèche (24 hour ride), it compared well to my teammates' HIDs, at least when my light was on high mode.
One point:  The very white/blue light is, IMHO, vastly superior to the yellow light I got from a high-end halogen system.

The funky "foot" mates to a plastic quick-release mount from a busted $15 bike light.

Details:

The overall plan was cribbed and modified from Allen Chapman's site: http://bikeled.org/
He has lots of detail on case construction.  I'll only point out what ISTR I learned from him:  A disk sander really helps to get the corners square, which helps parts to mate well for better heat conduction.
parts list:

Generator-driven bike light

OK, I'm gonna embarass myself by showing you the unfinished product.  Please ignore the rubber bands and Saran Wrap (used in lieu of screws to hold the casing together and plexiglass covers for the lenses).  I built this first as a proof-of-concept and haven't yet finished it up like Wall-E above.  Actually, it works really well with the Saran Wrap!
                                                        More pictures below!

Early this year I built a super-simple generator-driven LED light as a test of concept.  No driver board or regulator of any kind is needed. Details are below.

It works great, generating something like 200-250 lumens from an old Soubitez sidewall 6V generator.  That's more than enough for very comfortable road riding, even with my poor night vision, all from a sidewall generator!

Total cost around $47 plus generator and lots of hobby time to make the housing; can be cut to ~$36 plus generator with a rectifier, see below.

I used the Soubitez 'cause I had one in a drawer.  I'm thinking seriously about a cheap dynohub; this is for my commuter, and I do in fact forget to charge my battery lights from time to time.

The concept I was testing was the current regulation apparently inherent in bike generators/dynamos.  What I'm told by people who seem to know is that such dynamos will pretty much top out at 0.5 amps, regardless of the load they are wired to.  The peak voltage rises or falls (or so I'm told) until the 0.5 amps goes out.  Drag at peak voltage (and the speed required to reach it) varies with the voltage your load requires for 0.5 amps. (Or so I'm told)

While I haven't put a meter or oscilloscope on the thing yet (front-wheel generators are hard to bench test w/o rollers), I get LOTS of light with only normal drag, and I haven't managed to burn anything out, even with some high speed downhills.

Parts list; see links above for mail order sources:
Wire two pairs of two LEDs each in series, then wire the two pairs front-to-back as a rectifier bridge (diagram below).  Attach to generator and go ride!

Note that with the under-$5 purchase of a rectifier, you could run the same design, and get nearly the same light, with only 2 LEDs in series.  That makes housing construction easier.  As it is, each LED gets a theoretical peak amperage of 0.5 amps for half the time; with a rectifier, that would be 0.5 amps all the time.  These LEDs are rated to 1.0 amps continuous duty, and with proper heat sinking I know they work well at that rating (see battery-driven design above). 

I'm looking into a standlight design built from a simple 1 Farad supercapacitor ($10?) so that I'll still have a headlight while waiting in the left-hand turn lane at intersections.

Circuit:
You can manage with much cruder housings, too:  Just mount the LED stars on an aluminum bar, and glue on the lenses with silicone.  Your lenses are subject to getting banged up, though.

More embarassing pictures: the first shows how the back is a transverse section of tubing.


The next shows the kludgey mount I made out of junk-drawer reflector mounts.  This one won't hold up in the long run.
I raised it a bit above the front wheel to keep it above the water the front wheel flings forward (and wind brings back).
You can see the Soubitez bottle generator at bottom.

Horses for courses, or which one do I want?

There are endless flamewars on rec.bicycles.tech about battery versus generator.  I think both have their places, you decide:

For ultra rides that require light weight and low energy expenditure, the battery design wins hands down, IMHO.  This will surprise lots of randonneurs, as the SON dynohubs are very popular with the randonneur crowd.  A Schmidt SON dynohub is the best generator available, performance wise, ignoring cost.  Per Schmidt's web page, the SON 28 weighs 570g, and the SON delux weighs 390g, both without skewer, compared to ~145g for a plain front hub of high quality (with steel axle, no skewer).  Taking the lighter SON, that's a 245g penalty for the SON.  245g worth of battery will run my dual-LED light for about 9 hours at 3.1W, or about 7 hours at 4W, which should compare favorably with the light output of high-end SON-driven designs, i.e. dual-LED.

So if my light needs are 7 hours or less, replacing the SON with a similar weight penalty of Li-ion battery gives me no net weight penalty, and no drag at all.  The SON is widely recognized as low-drag, but it isn't no-drag.  Schmidt's web page cites a 65% efficiency at 15 kph, but that will likely change at higher speed.  Giving the benefit of the doubt, let's say that at 25 kph, the SON is 70% efficient.  I use 25 kph as a reasonable randonneur speed (while actually cycling, not counting stops, where weight and drag are immaterial). 

At 70% efficiency and 4W output the SON requires 5.7W of input power.  Each hour that's enough work to raise another pound of batteries 15,000 feet (I have to raise the first 245g, SON penalty or batteries, regardless).  If that sounds absurdly high, it's also the same work as raising 180 lbs 83 feet, in other words, the work for someone my size to climb a modest hill.  Neither is very big: the work to drive the SON OR the work to raise the battery, and either is really easy to ignore, but if one encounters less than 15,000 feet of climbing in that hour, the energy expenditure penalty to carry an extra pound of batteries is less than that of running the SON.

So my 245g (8.5oz) of batteries incurs no weight penalty over the SON, and is enough for 7 hours of light (at 4W) with zero drag.  Each hour I actually run the SON requires an additional effort equivalent to climbing a modest hill.  You can run the SON at higher wattages, so it would take more battery weight to get comparable light, but that will also raise the drag penalty.  For multi-day brevets, carrying even 2-3 pounds of batteries actually incurs less energy expenditure than running the SON at night, but of course you could have spare 1-pound batteries at a bag drop and do even less work.

Finally, while the SON has admirably low drag when "unloaded," i.e. with the light switched off, there's still about a 0.75W penalty over a regular front hub at 25 kph (per Heine & Ohler's article "Testing the Efficiency of Generator Hubs").  One hour at that level of energy input is enough to raise a pound of batteries almost 2000 feet (or a 180 lb cyclist+bike 11 feet).  Again, these are both very small energy expenditures - but what is often forgotten is that carrying an extra pound of batteries for an hour over a course that has 2000 feet of climbing requires actually very little energy.

Batteries do require charging, which you must remember before the ride, and you gotta remember to turn the light off during daylight.  Batteries also run out if used long enough, so make sure you've brought a sufficiently large battery.  I can do that for "event" rides like a brevet or flèche. 

For commuting and spur-of-the-moment use: I used battery-lights for decades for my 2.5-mile commute.  But I've often forgotten to plug in the charger, and had to either sneak home in the dark or get a ride from my wife.  This is what generators were meant for.  Last fall my commuter light had both a battery failure (won't hold a charge after years of use) and the burnout of a no-longer-available obscure halogen bulb.  Time for the generator design above.  It's always ready, and LEDs have especially long service life (thousands of hours, if not over-driven).  As noted above, an LED rated for 1.0 amps should be perhaps-even-impossible to burn out with a 0.5-amp generator, if proper heat sinking has been built in.

Other obscure bike tips