3 Watt LED Bike Light Experiments
February, 2008, Rev c
Michael Krabach

Contents

Prototype 3 - Double Cree with Individual regulators

To reduce the cost of the last prototype, I decided on a prototype with only two LEDs, and a cheaper regulator than the Buckpuck ($18). So I used what others are using, cheap regulators from China. This also gives redundancy in case one LED or regulator fails. Not having a large heat sink as in prototype 2, I used 1 ½ “ aluminum angle stock (available at most hardware stores) and added pieces of a computer CPU heat sink to increase the cooling surface. All bonding of the angle stock was with JB Weld epoxy, and pressed in place with steel C-clamps, then heated with halogen lamp to cure the epoxy faster in a cold room. The amount of epoxy used was as little as possible to minimize the thermal barrier for the heat transfer.

The LEDs and lens are the same as in prototype 2. The photos show the individual power wires attached to the star LEDs. I was still using JB Weld to bond the star LEDs to the heat sink. The 8 deg diffuser lenses are seen in the second photo.

The current drivers are specifically for 3 watt Cree LEDS and have an input voltage from 3 to 6.5 volts. (Read this forum thread before selecting cheap drivers.) Since each can only handle 3 watts, two DX SKU-4255 circuit boards were used. Each is supposed to drive a LED up to 700 ma, but actual readings were 550 ma each. For a power connector, a surplus PC connector (as used on floppy drives, etc.) was used only because that is what was available in my junk box. Due to limited space, the two clicky switches were glued on top of a perf board with super glue, and the regulators were glued on the bottom of the board with hot glue. The lenses were attached and sealed with GE Silicone II adhesive. The final prototype is in the last photo.

I also wanted a light that did not need a 5 pound lead acid battery. The lower power requirements are also met with 4 NiMH AA batteries. With independent lights, it is easy to reduce the light when not needed. The batteries will last about 1 ½ hours with both lights on, or 2 ½ hours using just one of the lights. The actual duration depends on the battery capacity and state of charge of the batteries.

Note that regular NiMH AA rechargeable batteries will self discharge over time and the high current drain usage on these batteries seems to accelerate the process. I have found with Energizer AA batteries about 1/3 of them will self discharge much faster than expected after extended use. If not recharging the batteries before using them, I recommend the new NiMH hybrid rechargeables. These have various branded names as Hybrid, Eneloop, or labeled “Ultra Low Discharge”. These do hold a useful charge for months and are definitely preferred if the lights are stored and not used often. The current capacity for regular NiMH AA batteries is about 2500 ma hrs while the Low Discharge AA batteries is only abut 2000 ma hrs., so there is a trade off. For longer duration a standard 6 volt alkaline lantern battery can be used.

Lantern batteries are not designed to handle heavy current drains, but they are cheap enough and reliable at about $8 each. There are major differences in types and brands of lantern batteries. While shopping I noticed that when picking up two different brand batteries, they weighed appreciably different. The Eveready Energizer battery was heavier than Duracell battery. An Internet examination of manufacturer specifications (or what I could find, which leaves a lot to be desired) reveled subtle differences. The Eveready Energizer Alkaline 529 battery has 26,000 ma hrs capacity, but the industrial version MN529 only has 20,500 ma hr, and the Eveready Super Heavy Duty 1209 has only 12,000 ma hrs capacity. The Duracell alkaline MN908 is rated at 11,500 ma hrs capacity, but the MN918 (looks the same) has 24,000 ma hr. Note that this is for a standard industry discharge rate, which is to 0.8 v/cell at 25 ma continuous drain. This is much less drain than 1 amp, so the real capacity running this light will be less. But with the Energizer alkaline you should have 10 hrs of light and about half for the others. Intermittent use will extend the battery capacity. The only high capacity lantern battery that I have found retail is the Energizer 529, at Walmart.

A heavy duty water bottle holder works well holding the lantern battery as seen in the first photo below. The second shows the style of water bottle holder that the battery will not fit into. A Radio Shack 4 AA battery holder, seen below, works very well and is $2. I also purchased the “heavy”duty connector. This cheap battery pack can be dropped into a handlebar bag or taped to the bike as convenient. For longer duration carry extra batteries either loose or buy another battery holder and just switch battery packs when needed. It is prudent to lay a piece of webbing under the batteries as you insert them so that you can remove them out easily. A good smart charger should be used to charge the batteries such as the Maha MH-C401FS.

I have found that this dual clicky switch prototype works nice changing between low power and high power while riding. It is also cheaper because the current regulators used are about $2 each. The 1 ½ “ aluminum angle stock seems to work adequately as a heat sink and is a little over sized for a current drain of 1.2 amps. This is my favorite light for bike path riding.