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Bike light design - will this work?

  1. Aug 24, 2009 #1
    I'm planning on building a mountain bike light. I thought it'd be a good little project to get me into understanding electrical circuits and I want to know if my parts are compatible.

    Can anyone see any flaws?

    Light Specs
    Supplier WelchAllyn
    Part Number M10P004
    Cost $46.00
    Voltage 80 V
    Wattage 14 W
    Current 0.18 Amps
    Colour Temp 7800 K
    Lumens 1000 Lumens
    Life 500 h

    Supplier WelchAllyn
    Part Number B10R001
    Cost $30.00
    Voltage 14.8 V
    Wattage 14 W
    Current 0.95 Amps
    Efficiency 85.00%

    Battery Calcs
    Supplier Battery Junction . COM
    Part Number TENERGY-18650-148-6600-PCB
    Cost $119.95
    Voltage 14.8 V
    Capacity 6.6 Ah
    Run Time 6.63 h <-- ???

    Would a 13.2V, 4.4Ah battery work instead of the 14.8V listed above? Like this one?
    https://shop.graupner.de/webuerp/servlet/AI?ARTN=7663.4 [Broken]
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 24, 2009 #2


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    It will certainly be a bright light. I think a 40 watt fluorescent gives about 650 lumens.

    The lamp needs 80 volts but the ballast specs do not mention any voltage like this.
    The two are supplied by the same company, so why not ask them if they are compatible?
    What sort of light is it?

    As for getting you used to electronic circuits, I don't think it will help much.
    You are just joining 3 preassembled modules and you will have no way of knowing how they work. Also, at $196 or so it is a pretty expensive headlight.

    If you have spare power maybe you could consider some downward lighting on the handlebars as well as the forward looking headlight. This would give you a chance to avoid rocks etc that did not show up in the headlight.
  4. Aug 24, 2009 #3


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    Staff: Mentor

    Two problems I can see -- sensitivity to shock/vibration, and that sure sounds *heavy*

    At least the way I ride my mountain bike, anything I put on it better be light and very durable. An LED-based design would seem to be a better choice. Look at what is on the market today for mountain bike lights. I'll bet many of them are LED-based now.
  5. Aug 24, 2009 #4

    According to the ballast assembly drawing on their website, the light and ballast are compatible.

    HID headlamp assemblies typically retail from $200 - $1000. So I thought it'd be a fun project to build my own and get exactly what I want as opposed to comprising. I want to eventually branch off another light, just as you suggested aimed just a bit in front of the front tire.

    For now, I chose this particular light because of it's brightness. Although its a projection light, it also has a fairly wide spread to it. I think this is exactly what I need.

    My biggest concern is matching a battery pack to this system. Based on my research, I want to go with a Lithium based battery because of it's capacity to weight ratio. According to my "rough" calculations I'm estimating that the 14.8V battery will last around 6.5hrs. This is much more than I need and a smaller battery pack would be ideal. So I sourced a 13.2V battery from https://shop.graupner.de/webuerp/servlet/AI?ARTN=7663.4 [Broken] that has an appealing form factor.

    Will the 13.2V battery be sufficient for the system?
    Last edited by a moderator: May 4, 2017
  6. Aug 24, 2009 #5
    Yes, I agree. There are many LED systems out there, but nothing compares to the brightness of an HID. Nothing.

    With regards to weight, these lights appear to be quite small (1 - 1/2" to 2" in dia.)
    Do you know of any other HID light suppliers that might be smaller? Remember that $200 is "sort of" my budget for the battery & light.
    Last edited: Aug 24, 2009
  7. Aug 24, 2009 #6


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    Gold Member

    Wow. $46.00 for a lamp that only last 500 hours. Hope you don't ride too much.

    Seriously, LEDs.
  8. Aug 25, 2009 #7

    I'm not looking for criticism, I'm looking for support.
    The bottom line is that I need a bright light. My rides are typically 2-3 hours long. So this lamp should last for 160 - 250 rides - that's three to five years worth of "night" riding. It's plenty.

    What I really need to know is how to calculate how long the battery will last each ride. Let's assume a 4.4Ah rated battery and pushing 13.2V. The ballast draws 13.2V@14W - so that's 1.06Amps, right?

    So the battery should last 4.4Ah/1.06A = 4.15h, right?
  9. Aug 27, 2009 #8

    What I really need to know is how to calculate how long the battery will last each ride. Let's assume a 4.4Ah rated battery and pushing 13.2V. The ballast draws 13.2V@14W - so that's 1.06Amps, right?

    So the battery should last 4.4Ah/1.06A = 4.15h, right?
  10. Aug 28, 2009 #9


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    Staff: Mentor

    I didn't run the numbers. You calculate it based on energy supplied and energy used, plus some conversion efficiency. What is the effiency of your DC-DC? It will vary based on the input voltage, generally. Make a graph.
  11. Sep 1, 2009 #10
    Yes, it will work and as a reference this bike light uses that exact bulb and ballast from Welch Allyn.

    http://www.niterider.com/prod_slickrock.shtml [Broken]

    The efficiency of the ballast is 85%, but you don't need to use that since they spec the electrical input to the ballast on the data sheet. The electrical input is 16.9 watts on high and 11.5 watts on low. Since it's a regulated balast, the wattage will stay the same as the battery voltage changes.

    The easiest way to calculate the runtime is the compute the watt-hour rating for each battery.

    Batt 1: 13.2v x 4.4ah = 58.08 Watt-hour

    Batt 2: 14.8v x 6.6ah = 97.68 Watt-hour


    Batt 1 will run the light 58.08 Watt-hour / 16.9 watts = 3.4 hours

    Batt 2 will run the light 97.68 Watt-hour / 16.9 watts = 5.8 hours

    The battery specs typically assume a slow discharge rate in the 10-20 hour range so you won't actually get those exact run times. But you could expect a solid 3 hours with the smaller battery and 5:15 - 5:20 out of the bigger battery. You can repeat the calculations for the low setting.

    Since both of the batteries you listed are a form of lithium based batteries, you should really have some electronics between the pack and the light. You need something to shut the light off when the batteries get down to 3V per cell. If you discharge a lithium based battery below that point then it'll damage the battery. Then on the next recharge the battery will go into thermal runaway and catch fire / explode. The bigger Tenergy battery you list does include a protection circuit that'll shut the battery down. The smaller battery does not. You also need a charger specifically designed for charging the type of lithium battery you're using.

    Building a case for a light like this is also a little challenging. The bulb and ballast generate a fair amount of heat that must be dissipated by the case.
    Last edited by a moderator: May 4, 2017
  12. Sep 2, 2009 #11
    Hi crank-
    Ballasts (meaning inductors in my definition) are AC devices, so if you are using batteries, is there a chopper or inverter circuit somewhere in your design? Using a current limiting resistor in a dc circuit wastes energy.

    [Edit] The ballast certainly is a switching ckt of some kind. See Figs 7 and 8 in
    http://www.walamp.com/lpd/files/applicationnotes/80014411 - APPLICATION GUIDE.pdf
    It is switching currents up to 5 amps or so on a millisecond time scale, on what looks to be a random walk.
    Bob S
    Last edited: Sep 2, 2009
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