I am developing a bike light that runs off of a 3.7V Li-Ion battery. I am currently running 3*10mm Ultra Bright White LEDs (~25,000mcd each). They each draw 20mA.

I am relatively happy with the ratio between their brightness and their current draw...but I would like to get the lights a little brighter, without chewing up my 1200mAh battery to quickly. And these LEDs are not very expensive, either.

I am currently using through hole LEDs, but am open to SMDs or any other solution you may have.

  • \$\begingroup\$ Make a circuit which rapidly pulses the LEDs rather than keeping them on all the time. You can get nearly the same brightness at a fraction of the average current. \$\endgroup\$
    – Kaz
    Apr 22 '13 at 23:43
  • \$\begingroup\$ @Kaz: Do you have any references for that? I thought LEDs were most efficient at or near their maximum current. I can see how pulsing them would make them more efficient when running at less than full brightness, but does it really make them more efficient at full brightness? \$\endgroup\$
    – Johnny
    Apr 22 '13 at 23:47
  • \$\begingroup\$ @Johnny - Using PWM to control brightness is pretty common knowledge. In fact, at specific frequencies, the LED can appear brighter when pulsed at a lower current. Plus, the LED temperature stays lower than if it were on all of the time, so it has better characteristics and a longer life. \$\endgroup\$ Apr 23 '13 at 0:22
  • \$\begingroup\$ i bet this is bright, 70 dollars for 1 led! digikey.com/product-detail/en/PT-121-R-C11-MPB/1214-1059-ND/… \$\endgroup\$
    – skyler
    Apr 23 '13 at 0:51
  • \$\begingroup\$ @Kurt: I understand how PWM is an efficient way to dim the LED below max brightness while maintaining high efficiency, but for an application that needs maximum brightness, I wasn't aware that you can pulse the LED for better luminous efficiency than just running it at 100% of its rated current. This article seems to dispute that PWM leads to greater efficiency than running it at max current: electronics.stackexchange.com/questions/17528/… \$\endgroup\$
    – Johnny
    Apr 23 '13 at 1:04

I don't know if I can recommend an exact LED to use (I'm constantly looking for better ones myself), but maybe I can point you in somewhat of the right direction.

Essentially, you want three things out of a bike light:

  1. Be seen by others.
  2. See what's right in front of you.
  3. See what is far away.

To start, your source voltage of 3.7 limits you to using 1 LED with current limiting resistor per parallel string. This isn't a bad thing, it just means you have to use more resistors as opposed to putting a few LEDs in series for double or triple that source voltage. The good news is that most lower-powered white LEDs have a forward voltage of 3 - 4V. I have to recommend that you DO use a resistor in series with each LED, even if the rated LED voltage is 3.7V. The LED current will fluctuate as it heats up and the LED forward voltage changes which can damage it or the battery. The resistor is there to make sure that current is limited.

You mentioned using 10mm LEDs rated for 25000mcd each @ 20mA. I'm guessing that the viewing angle on these is pretty low, such as 15 degrees. These types of LEDs are good for distance, but they don't work so well to light up things right in front of you. Here are some LEDs like that: 100 x 10mm 120000mcd @ 20mA, 12 degrees viewing angle - $40 (0.40 each). There are many LEDs for sale like this online.

If you are looking to use fewer LEDs that are higher power, something like this has a decent luminous intensity as well as a wide viewing angle: 62000mcd, 125 degrees - $12.95. The downside is that it will get very hot (needs the heat sink) and use quite a bit more current than your standard LEDs. This may be an even better choice: 300 lumens, 125 degrees - $6.95, but again, it will drain your battery a lot faster. However, pairing this type of LED with a focusing lens will reduce the viewing angle, but greatly increase the intensity. With the right lens, you can get everything you want out of one LED.

Consider the difference between a focused and unfocused flashlight: focusing the light beam increases the brightness, but less of the room is lit up; unfocusing the light reduces the brightness, but more of the room is lit up. To get the best light using the least amount of power, I would suggest you use a grouping of LEDs with a very high luminous intensity (mcd) paired with a group of LEDs with a high viewing angle (degrees). The easier route is to pair a high power LED with an appropriate lens, but this might not be ideal for you since you have such a small battery capacity. Either way will ensure you can see things far away, and the path in front of you is well lit.

Here is an online calculator which can estimate the output in lumens from the candella and viewing angle attributes: http://led.linear1.org/lumen.wiz

If you are trying to figure out your battery life, it can be estimated by dividing the amp-hour rating by the total current draw. This leaves you with the total hours of operation. Although, it isn't entirely accurate, and will vary based on the age and quality of the battery as well as the actual amount of current being drawn - some batteries do better with high discharge, some not so much.


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