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  1. #1
    MUFC - Champions!!!
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    battery advise.. please

    Im currently running the following

    2x cree xr-e Q5 leds
    1 1000ma buckpuck

    what battery will be enough to power them? Also what sort of runtime would I expect from a 12v 2400 ma battery pack

    Cheers in advance Ben

  2. #2
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    3S 18650 Cells should be enough.

    I'd also like to know how to calculate runtime when using a source with higher output voltage than required.

  3. #3
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    Quote Originally Posted by rool
    3S 18650 Cells should be enough.

    I'd also like to know how to calculate runtime when using a source with higher output voltage than required.
    I tend to use the rough formula bandied around on here of:

    0.8 x Vbat/Vft x mAh/mA

    which in this case gives

    0.8 x 11.1/7.4 x 2400/1000 = 2.88 hours or 2 hours 53 mins or so.

    There are actual runtime calculators around but most experienced heads around here (not me) will tell you it is only in actual use that you will get a true value for runtime. In my limited experience however it has been surprisingly close for the lights I have built so far.

    The 0.8 value is a rough factor to allow for battery variances and driver inefficiency.

    Hope that helps.

  4. #4
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    cheers for that

  5. #5
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    Here is the runtime calculator I have been using... seems to be pretty close

    http://www.jtice.com/led_pro/led_pro.htm

  6. #6
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    Quote Originally Posted by tamen00
    Here is the runtime calculator I have been using... seems to be pretty close

    http://www.jtice.com/led_pro/led_pro.htm
    If you put the following data into the input fields of LED Pro:

    Desired LED Current = 1000
    LED Vf Voltage = 7.4
    Battery Input Voltage = 11.1
    Battery mAh Capacity =2400
    Converter Efficiency = 80

    You get surprise, surprise



    Runtime in Hours = 2.88

  7. #7
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    Exactly the same calc you use... but dumbed down so I can figure it out...

  8. #8
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    I use the online calculator. Which produces exactly the same numbers.

    JZ
    It's not about speed, it's about lack of control.

  9. #9
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    Quote Originally Posted by Jim Z in VT
    I use the online calculator. Which produces exactly the same numbers.

    JZ
    Nice to see we are all singing off the same songsheet!


  10. #10
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    Don't forget to factor in shorter run times if you ride in very cold conditions.

  11. #11
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    or if you do not charge your batteries all the way up... runtime will be shorter... (I am pretty funny sometimes)

  12. #12
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    Quote Originally Posted by yetibetty
    Don't forget to factor in shorter run times if you ride in very cold conditions.
    We need an ambient temperature factor in the equation!

  13. #13
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    The batteries (our fav Li-ion) work poorly under 40F, much cooler and performance is down by 30-40% or better. Much below freezing and they are maybe at 10-15% of room temperature performance. I think the .8 (80%) factor in the equation is a good room temp fudge.

  14. #14
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    Quote Originally Posted by OldMTBfreak
    The batteries (our fav Li-ion) work poorly under 40F, much cooler and performance is down by 30-40% or better. Much below freezing and they are maybe at 10-15% of room temperature performance.
    This confuses me. Years ago when I was into backpacking, I was constantly reading how you had to have lithium batteries because they were the only ones that work at winter temperatures. Maybe our rechargeable Li Ion batteries perform differently in the cold than disposable lithium batteries?

    I just did a very brief web search (just a few minutes looking at the first 3 sites that google found) and the info was vague. Basically it said everything was crap in the cold, Li Ion being the least crappy and alkaline being absolute s***.

    Out of curiosity I just checked the voltage of my 15.6v NiMh pack. After my last night ride (2 weeks ago) I charged it up, and it's been sitting out on the back porch in the cold since then. The temp. is 10F (-12C) right now. The voltmeter says 17.35v, so still at 1.33v per cell. I don't think that's much lower than if it were 2 weeks off a charge at normal temps. But voltage with no load is probably not a good indicator anyway.

    I guess I need to go for a 2 hour night ride and see how long it lasts.

    JZ
    It's not about speed, it's about lack of control.

  15. #15
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    JZ the capacity will be lower in the cold not the voltage. Same power just for a shorter time.

    My status LED for medium (bFlex) usually comes on after 2 hours, the other night in the freezing it came on after 1 hour 15mins. that's with a Li-ion pack.

    Strange that the best way to store a battery is in the cold, yet they don't like the cold when in use.

  16. #16
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    Always store all your spare batteries cold. I have a box in the reefer where I keep all the spares. Like the site you found said, Li-ions suck the least. All are poor in the cold; some are worse. If the weather is awful, you might want to keep the battery next to your body.

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