Roland's Homebrew Bike Lights

I was shocked at the price of high power bike lights typically in the range of $100 for a very basic single light, and up to $450 for a fancy HID system so I decided I'd look at building them myself, the more I looked around the web the more motivated I became to attempt building a system myself.

The original, the Wooster Mark I

The Wooster Mark II

Lamp Head, Switch, and Cable improvements

The soldered RCA/Phono socket on the back of the lamp in the Mark I design was causing too many problems. Although it never failed while in use, almost every other time I wanted to ride I would find it broken. Also using the cable as a switch was a major pain, this led to the switch enabled Mark II.

The lamp head changes were a simple upgrade, requiring no more than some speaker wire, some shrink sleaving, a phono plug and a push button switch, total cost to upgrade from Mark I about $5, and I have a bunch of spare plugs left over.

Basically the speaker wire is soldered to the back of the pins on the lamp, then one of the wires is cut and connected to the two tabs on the pushbutton switch. Then on the other end of the cable is a phono plug that connects to the water bottle power supply. I used simple black electrical tape to hold the switch in place. It has not had a problem with heat, dirt, rain, or vibration. Note, the lamp is attached as in the Mark I design with the pipe clamps

NiMH Battery

The 50 minute run time of the SLA battery in the Mark I was far too limiting. I found someone selling a couple of 12v (10 cell) 2700mAh battery packs on Ebay, as pictured. The dimensions of the battery pack was just perfect to fit two of them into a regular size water bottle. Thus, by wiring them in parallel I have a 12v 5.4Ah battery supply. I was also very hopeful that NiMH would give me better efficiency than SLA at a 20watt load.

The results were phenomenal. The battery weight was down from 1135g to 860g. And the efficiency of the larger NiMH on the same load was 100% (or better) versus the 70% I was getting from the Sealed Lead Acid. The biggest surprise was that I was able to get a 3h30 run time from this battery pack (better than even the specifications indicated!)

NiMH Charger

NiMH charging, done quickly is very difficult, and requires microprocessor control. However, there are some very simple shortcuts that enable the creation of a 24 hour charger in an extremely trivial manner. There are some simple rules to NiMH charging:
  • For extended life don't charge at more than C/3.33 (this is the capacity of the battery divided by 3.33, thus for my 5.4Ah battery this would mean limit charging to 1.6A.
  • Once the battery is hot (during charging) do not continue to charge at anything more than C/10
  • Trickle charge at no more than C/20, somewhere between 0.05C to 0.03C is best.
  • Cease charging if the battery voltage (per cell) rises to 1.78v (note nominal voltage is 1.2v)

So, with my laptop charger that is about 17v on no load, this solves the last of these, and by simply putting a 5 Ohm resistor (actually I used two 10 Ohm resistors in parallel, resulting in 5 Ohms) this limits the current to the battery so that all of the rules of charging above are met. NiMH batteries should be charged when they reach 1.0v per cell. Thus even if I take the pack down to 10V, the difference is at most 7v, and 7v across a 5 ohm resistor is a maximum of a 1.4A charge rate, the battery quickly charges a few volts, also the power supply gets pulled down to lower than 17v once on load, and basically after about 20 hours the battery is charged and sits in a trickle mode charging about 0.24A, which is 0.04C, perfect!

This also made the "charger" VERY simple to build, simply a phono socket, a laptop style power socket, and a 5 ohm resistor in series.

Part List:  Black for Lamp assembly, Blue for Battery Parts, Maroon for Charger Parts

PartStore Part NumberCost (USD)
GE Halogen MR-16 Lamp, 12V,
with protective glass lens
Home Depot20 watts, 40 degree Floodlight 8.37
Hose ClampLowe's1/2"-1" Clamp (2 pack) 1.18/0.59
Hose ClampLowe's3/4"-1 1/4" Clamp (2 pack) 1.18/0.59
Old Inner TubeN/AA road tire inner tube was the best size 
Old regular size water bottleN/A   
Duct TapeN/A   
24 gauge speaker wire 6ft Lowe's $0.06 per foot.0.36
Right angle Phono plug (4 pack)RadioShack You'll only need one1.99/0.50
12V NiMH 2700mAh battery packs Ebay 2 Packs 28.00
Automotive Inline Fuse Holder RadioShack 270-1238 1 1/4" by 1/4" Rated 7A 1.79
7A Slow blow fuses (4 pack) RadioShack You'll only need one, plus spares 1.99/0.50
5 Ohm, 10 Watt Resistor RadioShack Used 2 * 10Ohm, 10 Watt 1.99
Chassis Mount Phono Sockets (4 pack) RadioShack You'll only need one 1.99/0.50
Power connector RadioShack 5.5mm * 2.5mm 1.99
Project box (plastic box) RadioShack Mounts the sockets and resistors 1.99
  Total $52

Roland Wooster, 2007. All rights reserved.