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Thread: What is the maximum current from 3 D batteries?

  1. #1

    Default What is the maximum current from 3 D batteries?

    I just modded my first Maglite using a SSC P7 and this driver and it's only drawing .6 amps using 3 D-Cells. Is this normal?

    I would like to mod another light. Is there a way to get more brightness (and amps if I understand correctly) from 3 D cells?

  2. #2
    Flashaholic* think2x's Avatar
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    Default Re: What is the maximum current from 3 D batteries?

    D cell alkalines are only good for 1 amp at best. The driver you linked too needs more voltage to drive a single emitter and the P7 should handle 3 amps fine if heatsinked right. Instead of 3 D cells you can add a 1" PVC pipe cut to the proper length to use as a sleeve, flip the tail spring and run 4 C cells in it to get adequate voltage for that driver or you can remove the driver (3 D cells) and run it direct drive.

  3. #3

    Default Re: What is the maximum current from 3 D batteries?

    Quote Originally Posted by think2x View Post
    D cell alkalines are only good for 1 amp at best. The driver you linked too needs more voltage to drive a single emitter and the P7 should handle 3 amps fine if heatsinked right. Instead of 3 D cells you can add a 1" PVC pipe cut to the proper length to use as a sleeve, flip the tail spring and run 4 C cells in it to get adequate voltage for that driver or you can remove the driver (3 D cells) and run it direct drive.
    Could you explain what you mean by there being insufficient voltage? It lights up quite well, I'd guess around 200 lumens.

    Edit: I just wanted to add that I'm a total beginner. I'm not trying to be argumentative, just curious!
    Last edited by LivingDaylight; 03-03-2012 at 06:48 PM.

  4. #4

    Default Re: What is the maximum current from 3 D batteries?

    Quote Originally Posted by LivingDaylight View Post
    Could you explain what you mean by there being insufficient voltage? It lights up quite well, I'd guess around 200 lumens.

    Edit: I just wanted to add that I'm a total beginner. I'm not trying to be argumentative, just curious!
    200 lumens is almost like its just dark.


  5. #5

    Default Re: What is the maximum current from 3 D batteries?

    The DX driver you're using is only good for 925ma. Try one of these 2.8 amp linear regulators and 3*NiMH D cells. Craig sells the same driver in the marketplace. The nice thing about that setup is you can still use alkalines batteries if your regular NiMH are in the charger. Alkaline D cells actually do put out more than 3 amps in a pinch. They just don't last very long.


  6. #6
    Flashaholic* Techjunkie's Avatar
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    Default Re: What is the maximum current from 3 D batteries?

    The voltage of alkaline cells sag under load, but they are capable of at least 3A. Last year I made two lights as gifts for some non-flashaholics. One was a 4D pushing the full 2.8A to a regulated MCE. The other was a 2D that pulled ~2.5A from a 6AA serial carrier filled with ordinary alkaline AA cells to feed a boost driver running 3S XRE @800mA. I once left my old analog multimeter on current test when I meant to test the voltage of a D cell and the straight short registered between 6-8A on the meter before I realized what I'd done.

  7. #7

    Default

    My P7 pulls over 4 amps on fresh charged cells... And its somewhere around 900ish lumens... Get rid of the driver, even on alkaline cells it will be around 500-600 lumens..

    My SST90 3d mags are direct drive and pull over 10amps...

  8. #8

    Default Re: What is the maximum current from 3 D batteries?

    Quote Originally Posted by Al Combs View Post
    The DX driver you're using is only good for 925ma. Try one of these 2.8 amp linear regulators and 3*NiMH D cells. Craig sells the same driver in the marketplace. The nice thing about that setup is you can still use alkalines batteries if your regular NiMH are in the charger. Alkaline D cells actually do put out more than 3 amps in a pinch. They just don't last very long.
    If the one I have is only good for 925, and I'm getting 600, doesn't that mean something else is going on? I only plan to use alkaline batteries - could that be the cause?

  9. #9

    Default Re: What is the maximum current from 3 D batteries?

    See Silverfox's alkaline battery shootout thread and examine the curves for D cells.

    Yes, D alkalines can deliver some decent current. But Siverfox's 3A discharge curve shows that alkalines are very poor at holding their voltage under high current drain. A D alkaline will quickly sag below 1V after around 10 min of run time.

    Do you know what your LED's forward voltage is at 1A drive or 3A drive? If the forward voltage is high, then you typically need more input voltage from your battery stack to get the driver to run in full regulation. It's a circuit thing.

    Direct drive is certainly simple and with 3xD alkalines, you have very little risk of overdriving your P7. The downside is that the output is unregulated and the light will start out bright, sag very quickly, and continually head downhill as the cells drain.

    If you use a 7135-based linear regulator at 2.8A output, I would use a 4D Mag, not a 3D, if you plan to use only D alkaline cells. The 3D will work, but the run time in full regulation will be short (probably less than 5 min). The 4D probably can hold full regulation for closer to 20-30 min. Alternatively, you could get a 1050mA 7135 driver and a 3D Mag probably can run in full regulation for close to 100 min.

    The problem with your DX buck driver is that the driver requires a certain minimum input voltage to run in full regulation (meaning it delivers the constant 925mA that it is design to provide). At 925mA, and Seoul P7 LED could have a forward voltage of about 3.25V (based on the P7 datasheet). That's why it helps to know what the actual forward voltage of your LED is at the driver's rated drive current of 925mA. That DX driver typically needs about 0.6V of extra voltage above the LED's forward voltage to reach full regulation. The lower your LED forward voltage is, the lower the required input voltage to reach full regulation. Based on the datasheet, you may need at least 3.85V input from your battery stack, or close to 1.3V per cell. Of course, a real flashlight system has parasitic resistances in the circuit arising from things like cell-to-cell contacts, spring contacts, and switch contacts. So you might need more like 4.0V input, or around 1.35V per cell.

    So one possibility to explain why your battery stack is delivering only 0.6A is that you are not running in full regulation and the combination of battery sag and system resistance results in what you get.

    Another possibility is that maybe you do have a very low forward voltage P7. Let's say it is 2.9V at 925mA drive current from the driver. If the driver is 85% efficient as a guess, then based on power delivered to the driver = power output to the LED,

    Vbatt*Ibatt*driver efficiency = Vf*If

    Let's assume Vbatt is about 1.35V (I picked off an early time voltage from Silverfox's 1A draw curve for D cells since I assume that you made your measurement right at the beginning, and not an hour after turning on the light). Vf is 2.9V. If is 0.925A.

    Thus,

    (3*1.35V)*Ibatt*0.85 = 2.9V*0.925A
    Ibatt=(2.9V*0.925A)/(3*1.35V*0.85) = 0.78A

    So we are in the ballpark. However, it's tough get to a calculated 0.6A, which is what you measured. I'd have to increase the battery voltage to 1.4V per cell, drop the LED forward voltage to 2.8V, and increase the driver efficiency to 90%. And I still get an Ibatt of 0.66A. Thus, to get to 0.6A, I think that I'd have to assume some unreasonable set of parameters, meaning most likely the reason your battery current draw is so low is probably because of 3xD alkalines aren't delivering enough voltage to reach full regulation. Your light is essentially running in direct drive and delivering 600mA of drive current to the LED, instead of the full 925mA.

    So to use that DX driver effectively, I would suggest driving it with 4xD alkalines.

    Otherwise, I'd switch to a 7135-based driver that delivers 1050mA drive current (such a driver will have four 7135 chips on-board, each delivering 350mA nominal) and use that in your 3D Mag mod. I think that higher drive currents are not practical if you plan to use only alkaline cells.

  10. #10

    Default Re: What is the maximum current from 3 D batteries?

    Thank you, Justin for that reply, I learned a great deal from it!

    Would this driver be better? I plan to do another mod soon.

    Also, could I string some AA batteries together somehow to get a higher voltage and full regulation?

  11. #11

    Default Re: What is the maximum current from 3 D batteries?

    The DX regulator SKU.6190 you selected this time is still only 1000mA. You need a 2800mA regulator for a P7. If you took an SKU.6190 or even better an SKU.7612 and wired up an SKU.1886, you'd have the same board I mentioned in post #5. Here is the original thread on how to make a sandwich. I don't believe DX sells the sandwich board but KD does. Also DX is currently out of stock on SKU.1886 slave board. That and the fact that SKU.1886 is a package of 15 regulator. A little expensive if you only need one. The SKU.7612 board has room for a 4th AMC7135 is why they're better. But why go to the trouble of making a sandwich board when so many folks are selling them that way? There's no real savings in money.

    Take a look over here at the Driver board list. You might find something more to your liking. It's in the sticky 'threads of interest' over in the Flashlight Electronics - Batteries Included forum. The first line of that post lists his off-site page where he maintains the list now.

    You mentioned something in your other thread, "Do I actually need the driver?" That's actually a good question. Many people have made direct drive P7's. Since you have a meter that measures current, that will be the best answer to your question. Just be aware that the resistance of the test leads and the shunt in the meter itself, will make the current reading artificially low. Also it might read and work OK initially, but the LED's Vf drops as it heats up. It might heat up in 10-20 minutes and draw too much power. That won't be a problem if you use alkaline batteries. In 20 minutes it will be time to replace them. If you see the LED start to turn 'angry blue', turn it off. That's usually the last thing you see before it dies.


  12. #12

    Default Re: What is the maximum current from 3 D batteries?

    Quote Originally Posted by Al Combs View Post
    The DX regulator SKU.6190 you selected this time is still only 1000mA. You need a 2800mA regulator for a P7.
    Thanks for the response and bearing with my lack of knowledge.

    My understanding is that it will still work well at 1000mA, just not be as bright as it could be? Is that correct? I have two maglites, (including the one referenced here I will probably redo), and I would like to have:

    #1 that is 200-300 lumens with really long runtime - I thought that the board I've used would do this, but if I understand correctly the voltage/current will not be sufficient to give long runtime since I am using the wrong driver.

    #2 that is as bright as possible - this would be the 2800mA one, correct? How long would 3 D batteries last at the level of current?

    Thanks!

  13. #13
    Flashaholic* think2x's Avatar
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    Default Re: What is the maximum current from 3 D batteries?

    Quote Originally Posted by LivingDaylight View Post
    Could you explain what you mean by there being insufficient voltage? It lights up quite well, I'd guess around 200 lumens.

    Edit: I just wanted to add that I'm a total beginner. I'm not trying to be argumentative, just curious!
    From driver description.
    needs to be 0.5~0.9V higher than total LED Vf

    3 alkaline cells are 4.5v and some of the P7's have a high Vf so the difference between the emitter Vf and battery voltage may be less than the quoted 0.5-0.9v. I built a 3 cell P7 with a linear regulator and it wouldn't reach it's rated current until I added a 4th cell for the same reason.

    Justin Case,
    Very well written explanation.

    Last edited by think2x; 03-04-2012 at 08:57 AM.

  14. #14
    Flashaholic* think2x's Avatar
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    Default Re: What is the maximum current from 3 D batteries?

    Another option I would consider is to replace the P7 with a newer emitter. An XM-L on an 8mm-14mm board will fit on the heatsink pedestal and has a much lower Vf with far better efficacy and would be very noticeably brighter with the same current.

  15. #15

    Default Re: What is the maximum current from 3 D batteries?

    Quote Originally Posted by think2x View Post
    Another option I would consider is to replace the P7 with a newer emitter. An XM-L on an 8mm-14mm board will fit on the heatsink pedestal and has a much lower Vf with far better efficacy and would be very noticeably brighter with the same current.
    I like that idea. If not for this build, possibly a future one.

    Would this one work? If I understand correctly it's for 3.35 Volts max so I would need a driver to match?

    Edit: I just tried it in direct drive, and I got 2.4 Volts, 1.2 amps by measuring it on the back. Does this make sense? Shouldn't D batteries give more than that?

    Individually the batteries give 1.5 V, so I don't think they are dead.
    Last edited by LivingDaylight; 03-04-2012 at 09:50 AM.

  16. #16

    Default Re: What is the maximum current from 3 D batteries?

    +1 to think2x's suggestion of upgrading to an XM-L. However the one you picked is a bare emitter. That would involve both finding and then reflowing the emitter onto the star. I would leave that for a future project when you get a little more experience. Both places I linked to in my first post sell XM-L's already mounted on stars. Again as think2x mentioned, 14mm or smaller. Anything larger and you're going to have to cut more than the cam tube off the Mag reflector for the LED to focus. The hole in the standard incan Mag reflector is about 15.5mm. So a 16mm star is already too large.


  17. #17

    Default Re: What is the maximum current from 3 D batteries?

    I did some testing recently with Copper tops and RayoVac D cells. With the battery short-circuited into my ammeter, they will provide about 7 amps, temporarily. This is not recommended behavior, but I am an electrical enginner and I took the proper safety precautions. So the maximuim short-circuit current is about 7 amps.

    I then measured internal resistance, but not like the battery manufacturers do. I used open-circuit voltage over short-circuit current and then also short-circuit voltage and short-circuit current and came up with roughly .23 to .29 ohms depending on method . How that is used is thusly: with the cell delivering 2 amps and starting out with 1.6 volts open-circuit, the 2 amps thru the .3 ohms will cause a ( 2 x .3) .6 volts voltage drop internally and the voltage at the battery terminals is then 1.0 volts.

    With the battery only delivering 1 amp, you end up with 1.3 volts at the battery terminal.

    So voltage sag with moderate current is indeed an issue with alkalines.
    Last edited by moodysj; 03-06-2012 at 01:23 PM.

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