Observations about the universe, life, Lausanne and me

Friday, September 7, 2007

Butt-ugly DIY bikelight

A couple of days ago I decided that the light of my push-bike was not bright enough. After looking at the prices for better lights (i.e. at least 20 Watts halogen) in a catalog, I was convinced that building my own would be the way to go.

Parts List:

*) 20W, 8 degree halogen spot (5 CHF)
*) rubber pipe coupling(13 CHF)
*) PP pipe plug (1 CHF)
*) lead acid gel cell (48 CHF)

*) a couple of cables
*) a aluminium plate
*) a switch
which I filched from the lab, therefore free.

All in all 67 CHF, or about 40 Euros.
A note about the batterie: I got myself a whopping 7 amphours cell, weighting in at about 2.6 kg. This should give me about 3.5 to 4 hours of light. (20W @ 12V draw 1.67A, and 7/1.6=4.2h, remove 20% for low efficiency)

First I drilled a coupe of holes in the aluminium-plate - it will be mounted on the bike-frame, where the cup-holder thingie normally sits. I also drilled two holes in the end-cap, for the switch and the cables.

The halogen spot, was connected to the cables via a cable-connector (no clue what it is properly called - those things electricians use to connect mains-cables). This has the advantage that you can change the spot easily - to get one with a wider angle, for example. Or a 50W bulb. Which would still give me more then an hour of light. Hmmmm...

The spot and the end-cap fit perfectly into the coupling, which can be tightened up so they sit, well, tight I guess.

Then I mounted the whole thing in my bike with a couple of cable-binders. Which is ugly. But hey, light! Bright! Shiny! Also, nobody will want to steal my bike now!

I'll probably tinker a bit with the mounting of the battery tomorrow (waterproofing comes to mind), and I might build a nicer enclosure for the light at some point (aluminium tubing maybe?), but I am pretty happy with it. Now I need only a bit of darkness...


  1. Nice article, but that lead-acid battery won't last for very many charge cycles if you take too much power out of it each night.

    As a "rule of thumb", lead-acid batteries should only be discharged by about 20% of the total available, if you want them to last as long as they are supposed to.

    Your project will work fine for the first couple of weeks, (if fully discharged nightly and recharged), but after a while the battery won't last as long each night. After a month or two, it may not even last an hour.

    In this case, you should run that 20 watt lamp for no more than 50 minutes before recharging the battery. If you switch to a 10 watt lamp, you could go for 100 minutes before endangering the life of the battery.

    If you had used NiMH, NiCad, or Lithium-ion, then you can discharge the full amount and still expect the cells to last a normal life. It one reason why lead-acid batteries are less expensive than those others, as you must use five times the required amount to do the same job - if you want the battery to last the normal amount of time.

  2. I made this exact light about two years ago. Same 20W bulb, same monster lead acid battery. Often times I wouldn't charge the battery until the light started dimming drastically - 4 hours+ of run time. I would always use it for at least 2 hours without charging (since that's how long my night commute and standard midnight leisure ride took). It took over a year for my battery to crap out. Actually, to this day, I'm not sure if it was the battery or charger that died, but I'm assuming battery since I treated it so badly. For the last few weeks, I was still getting enough juice to last for my nightly ride, but I definitely had to charge it when I got home.

    Perhaps I bought a deep-discharge battery? It certainly wasn't labeled as such. For a cheap, low-tech solution, my lead acid battery performed quite adequately.

    good luck!

  3. Here's a better setup with nice directions: http://www.instructables.com/id/EGIJ2D3E6NET9K545H/

    Ultimate 18v Bike Light!

  4. Hey guys, thanks for the comments. I know the battery is no deep-cycle battery, but since my commute is only about 30min, it should hold up if I charge it each day.

    Maybe I'll get myself another one one day, but for now I'll see how this one holds up...