Calculating runtime- Mtbr.com
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  1. #1
    Steel is real
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    Calculating runtime

    Hi all. I have made a few led bike lights now using the cutter buck kits with the triple. I am thinking about different battery options as i currently run a 14.8v lithium ion. I would like to run a smaller voltage to reduce weight and cost. When i calculate the runtimes useing the led pro calculator it obiously gives me greatly reduced runtime. Now i dont know if i am doing this correctly. So the current system is using 14.8v 2600mAh battery with a buck drive. Then the new system would be say 11.2v 2600mAh battey with a boost driver. Can i use that calculator to get runtime or is it a different calculation all together... This was all brought about after using a friends mity cross which runs on a 3.8v battery . It put out a little more light than my triple at 1000mAh. whats the go here. How can i improve my lighting.....

  2. #2
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    Quote Originally Posted by mercman
    Hi all. I have made a few led bike lights now using the cutter buck kits with the triple. I am thinking about different battery options as i currently run a 14.8v lithium ion. I would like to run a smaller voltage to reduce weight and cost. When i calculate the runtimes useing the led pro calculator it obiously gives me greatly reduced runtime. Now i dont know if i am doing this correctly. So the current system is using 14.8v 2600mAh battery with a buck drive. Then the new system would be say 11.2v 2600mAh battey with a boost driver. Can i use that calculator to get runtime or is it a different calculation all together... This was all brought about after using a friends mity cross which runs on a 3.8v battery . It put out a little more light than my triple at 1000mAh. whats the go here. How can i improve my lighting.....
    Its the same calc.
    Vbat/Vled * mAh/mA
    Drop 20% for driver losses and battery manufacturer lies.

    I dunno if 11.2 is a good battery choice for a triple and a boost. Nice on a quad.

    A cutter triple should be brighter than the 350lm mity, though the wide beam might not look it. Are you sure you're driving at 1A?

  3. #3
    aka RossC
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    there is always this calculator too

  4. #4
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    calculator confusion

    ok thanks guys. Here's the thing though. I have used that calculator and if i enter the values of 1000mAh and 10.5 v VF with 2600mAh 14.8v battery at 85% then i get 3 hours runtime. if i then change that to the 7.4v 2600mAh battery i get 1.5 hours run time. I dont think its taking into account the boost driver factor. So i need to know if i can get a similar run time from a boost driver on a lower voltage battery as what i am getting on the higher 14.8v battery. If i can go smaller then i can save weight and cost. But here's the big question. Is a buck driver setup better than a boost. It would seem that most manufactured light sets out there would be running a boost driver system.
    What are your thoughts....

  5. #5
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    Those calculations are correct... the 7.4v battery at 2600 mah would provide half as much run time than the 14.8 at 2600 mah. Think about it this way... the 7.4v battery only has 2 cells, the 14.8 will have 4 cells... so you will have twice as many batteries providing power.

    In theory, if you ran a 14.8v 2600 (4cells) mah battery, you would have to have a 5200mah(4 cells) 7.4v battery to have the same run-time. Normally, boost drivers are less efficient than buck drivers, so the run-time would probably be less with the boost driver using the same cells, but probably less than 10% everything else being equal...

  6. #6
    aka RossC
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    Quote Originally Posted by mercman
    ok thanks guys. Here's the thing though. I have used that calculator and if i enter the values of 1000mAh and 10.5 v VF with 2600mAh 14.8v battery at 85% then i get 3 hours runtime. if i then change that to the 7.4v 2600mAh battery i get 1.5 hours run time. I dont think its taking into account the boost driver factor. So i need to know if i can get a similar run time from a boost driver on a lower voltage battery as what i am getting on the higher 14.8v battery. If i can go smaller then i can save weight and cost. But here's the big question. Is a buck driver setup better than a boost. It would seem that most manufactured light sets out there would be running a boost driver system.
    What are your thoughts....
    George from Taskled has posted his thoughts on CPF on this debate a couple of times. . Basically George feels that buck drivers operate on a more logical premise than boost drivers.

    i.e Buck drivers start with a couple of volts disparity between Vf and battery voltage. This helps minimize battery current draw and as the battery drains, the battery voltage and Vf come closer together providing an increasingly efficient driver.

    With boost drivers. You start with a couple of volts difference between Vf and battery voltage however, since the battery voltage is lower than Vf, the corresponding current draw from the battery is higher compared to a buck drive alternative. As the battery drains, the discrepancy between Vf and battery voltage increases and the driver becomes increasingly inefficient. There are also more stresses and heating on the components since the input current also grows as battery voltage falls.

    I am sure that some of this is George's opinions coming from a driver design standpoint. He has to deal with high currents and component failures during the design process. Since he takes care of all those problems for us, it seems there is little difference in the real world application.

  7. #7
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    all the maths in the world are only as good as the input details
    the best way is a real time run test if you have the time and a cooling fan.

    I have found led pro to be out by only 10 minutes on the test with a quad R2 and 2600 mah 14.8 pack but that was a brand new battery pack .

    another reliable way I think is to measure the current going out of the battery on your desired setting and divide that by the mah of the pack to get a fairly near answer.
    ( if this is wrong please correct me )

  8. #8
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    Quote Originally Posted by mercman
    Then the new system would be say 11.2v 2600mAh battey with a boost driver.
    I have 11.1 V LiPo and it is not so good in combination maxflex - quad R2. Full battery is 12.6V and MaxFlex is forwarding 12.8V to LEDs - difference is too small. Fortunately I have took bad connector and/or cable so voltage sag helps me
    On other side 7,2 V Lipo with Maxflex is heating significantly more than with 11.1V. I have burned one of two drivers when was testing MC-E at 1000mA.
    I think that buck drivers have no problems with heat looses

    14.8 V 2600mAh is made with 4 cells of 2600mAh and 7,4 is just 2 cells, so it is double in weight, capacity and in running time

  9. #9
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    thanks

    Thanks guys i thought that was the case. Someone told me that the runtime would be the same from boost driver as it would be from a buck driver. This just didnt make sense. Logically the boost driver is going to drain the battery's more quickly than a buck driver. So it would seem that the best combination is still a battery voltage higher than the VF. Its just more efficient.
    So that brings me back to another point. Perhaps the light produced in the professional sets is more engineered in the sense of optics...

    Mike

  10. #10
    Carbon8er
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    all the maths in the world are only as good as the input details
    the best way is a real time run test if you have the time and a cooling fan.
    spot on.

  11. #11
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    Quote Originally Posted by El34
    spot on.
    You're on vacation. No Posting until the 10th!!!!

  12. #12
    Carbon8er
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    hehehe,
    yeah I broke the vacation/holiday rule.
    I am in still out west in Utah, back later this week.

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