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Battery to power an LED for a LONG time?

Discussion in 'General Electronics Chat' started by Mark JD, Feb 10, 2018.

  1. AnalogKid

    AnalogKid Well-Known Member

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    Here is how I would approach it. First, tighten up the requirements.

    Does the light have to be on continuously when it is operating, or can it appear to blink or flash?

    Is the light to operate 24/7, or can it be off at night?

    How bright? This might be a number of lumens, or the voltage and current of a device you have that is "good enough".

    With a lumens number, you can go shopping for LED types. With that, you can figure the voltage and current.

    With the voltage and current numbers, you can calculate the total energy needed to run the light through the time frame.

    With that number you can go shopping for batteries and try to find one that meets your physical requirements.

    After all of that comes whatever circuits are needed to manage the energy to the light.

    ak
     
  2. Mark JD

    Mark JD New Member

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    Ron, where do I get the ones you designed and make sure I'm getting the right size replacements? Also, to insure there's enough power and that lights stay on the whole time, I would probably use 3 of those Lithium Thionyl Choride D size batteries in each unit, if that would work. PWM?...is that Pulse-width modulation?
     
  3. Mark JD

    Mark JD New Member

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    Kid, my big problem is time...there's other works going there also, and a myriad of things that must be done, in a short time frame, to pull all this together.
     
  4. dave miyares

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Where do you live. Fill in the "location" under your profile.
    Here is an example:
    "D&D cell" = Pulls more current than "AA cell" Most are related by how much energy is used in the LED. 1 watt, 3 watt, or 0.5 watt
    "2 to 6 cells" Works from 1.8V or maybe 2.0V all the way up to 10 volts
    Because of PWM you will find, at two cells (3V) it pulls a certain current. Sorry I don't remember. But at 4 cells (6V) the light will pull 1/2 the current.
    It works slightly better at high voltage.
    upload_2018-2-11_12-17-46.jpeg
    "The 1-Watt LED Upgrade Kit converts most C and D (2-6 cell PR style flange) incandescent flashlights into super-bright, long lasting (50,000 hour) LED flashlights."
    I believe the 50,000 hours was with 6 D cells.
     
  6. Mark JD

    Mark JD New Member

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    Ron, I updated the profile...I live in Saint Louis...thank you for this help! I'll be writing a post to all the members here later...you guys are really great!
     
  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The reason for "Location" is that you can get a LED flashlight bulb at Home Depot, Walmart, etc but some one in India has different options.
     
  8. dave miyares

    Dave New Member

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  9. Mark JD

    Mark JD New Member

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    Ron, I just wanted to let you know I have some of the LED Upgrade Kits (74 Lumens) on the way.
     
  10. Mark JD

    Mark JD New Member

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    Mike, although it looks like the best thing for me to do is to go with Diver's LTC batteries and Ron's LED replacement bulbs...I just wanted to follow-up with you, since I said I would...attached are images of the Multimeters I have available to me (they're not great ones, but they have been very helpful around the shop, when problems arose).

    IMG_0218.JPG IMG_0219.JPG
     
  11. unclejed613

    unclejed613 Well-Known Member

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    i used to make LED bulbs like that. in a standard 2D flashlight, the battery life was about 6 weeks before the LED went to half brightness. actually, for white or blue LEDs you need 3 batteries.
     
  12. Mark JD

    Mark JD New Member

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    Jed, the Mag-Lites in the boxes are 3 batteries each...I'm hoping that using 3 of the size D (LTC) batteries that Diver suggested earlier #17, they will stay lit for the entire 7 months.
     
  13. unclejed613

    unclejed613 Well-Known Member

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    i think you will find the "50,000 hours" is not how long a set of batteries lasts, but is the average MTBF (mean time before failure) of most LEDs, some of them i've seen rated as high ad 100,000 hours. the MTBF of a standard incandescent bulb is about 150-200 hours (less if you drop the flashlight while it's on). that's the reason i was able to make LED "bulbs" back in the 1990s like the one ronsimpson had a picture of, and make a bit of extra money selling LED sets to preppers. i still have a flashlight around here somewhere with a complete set. there were 3 IR bulbs, one red, one orange, one yellow, one green, one blue, and one white. the IR ones were for use with night vision goggles. the white one wasn't very bright, but you needed white for reading maps without missing important features (like "danger" areas, printed in red, which if you used the red or orange LED, you wouldn't see it on the map)
     
    Last edited: Feb 12, 2018
  14. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Three cells @ 1.5V = 4.5V To get reasonable luminosity out a white flashlight LED, the I'm guessing the current through it must be >~25mA. That is P=IE = 4.5V*0.025A = 0.1125W.

    A LR20 D-cell battery has a capacity of about 22Wh, so three will last 3*22Wh/0.1125W = 587 hours = 24days...

    Use your multimeter to actually measure the LED flashlight replacement bulb with 4.5V, and redo the calculation above with a better number.
     
  15. Pommie

    Pommie Well-Known Member Most Helpful Member

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    The batteries linked to earlier were Lithium Thionyl Chloride batteries which have a nominal voltage of 3.6V and a capacity of 18Ah. Three in series is around 200Wh. However, even with these cells the LED can only draw 200/(7*31*24) ≈ 38mW. Assuming a 2V forward voltage the current can only average 19mA. That's not going to be very bright.

    Mike.
     
  16. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    My local Battery store sells a 4pack of Alkaline D-cells for $9. The MN1500 (data sheet here) have almost the same capacity as the Lithiums, however, the voltage on the Alkalines during discharge isn't as flat as the Lithiums.

    Making some series-parallel array out of MN1500 is a lot more affordable...
     
  17. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The LED bulbs I know use the 1 watt or 3 watt LEDs. (other power levels available)
    Most have heat sinking that tries to dump that heat into the walls of the flashlight.
    Most use the LEDs at near their rated power. 75% to 90% depending on design
    Some high end LED bulbs watch the LED temperature. One uses 1.5 watts until the LED gets hot then backs down the power to keep the part at a good temp.
    It is typical that a 1 watt bulb pulls about 1 watt from the battery and delivers about 0.9 watts to the LED. If the bulb is operated within its voltage range; then the battery current will be what ever it takes to get 1 watt.
    The PWM changes power to power. (battery voltage X battery current) = (LED voltage 3V X LED current 0.33amps)
    ...LED is probably not operated at full power
    ...PWM is 80 to 95% efficient. Most efficient at higher voltages.
     
  18. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Say you start with a 12V storage battery. If you use an 90% efficient 12Vinput LED driver to drive a 1W LED, then here is the capacity calculation.

    1Wled/90% = 1.11Wactual

    I = P/E = 1.11W/12.5V = 89mA

    7months*30days/month*24h/day = 5040h

    Capacity = 5040h*0.093A = 448Ah

    which takes 5 of these.

    Isn't it just more practical to plug it in?
     
  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    This is interesting because i wanted a light that could run for a very long time too in case of emergency.
    What i did was use a 12v lead acid battery and two white LED's and the estimated run time was about 3 months if i remember right.
    A 6 month run time is a big challenge.

    Batteries drain according to their Ampere Hour rating. The higher the AHr rating, the longer then run a given load. This means that for a particular battery and particular current load you can predict the approximate run time.
    For example, for an 8 AHr battery and 20ma load, the estimated run time is:
    8/0.020=400 hours, which is around 17 days.
    Because this is such a light load on such a big battery, we'll get better than that, maybe 1.5 times that or around 25 days but let's estimate 30 days which is one month for now.
    This means that if we want to run for 3 months we need a battery that is three times that, or 24 AHr.

    The alternative is to drop the LED current and thus the brightness.
    At just 2ma, we'd get 10 times the run time. So instead of just one month we'd get 10 months.
    8/0.002=4000 hours which is around 170 days which is about 6 months.

    Is there any way around this?
    Unfortunately there is no way around this unless you move to a different technology.
    For example, a solar panel can recover some energy from the room lighting.
    Les Jones had a good idea about a proximity detector, as long as the detector does not draw too much current.

    Another idea is to use a smaller battery, but have someone CHANGE the battery every month or every week or whatever you need. That way you can get more brightness. Make the battery easy to swap out.
    I did this with a wall clock. We had the clock mounted on the wall but it was screwed in place, so when we had to change the battery we had to unscrew it and replace, then screw it again. I replaced the screws with a hinge, so we could just tilt it up to replace the battery. It now takes about 20 seconds to replace the battery when before it could take up to 15 minutes just to replace it.

    So i vote for a new technology added to the current project, or make the battery easy to replace and replace it when no one is looking.
     

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