36v battery in a 24v application

Discussion in 'General Electronics Chat' started by NewbiefromFL, Nov 30, 2007.

  1. NewbiefromFL

    Thread Starter Guest

    I bought an electric bike that uses 24v. It has an internal controller set in epoxy inside the motor casing. Exceeding 24v will fry the controller in short order. I have an unused 36v battery sitting around that I would like to take with me to swap out if the stock (24v) battery runs out of power and strands me. Is there a battery power management board, regulator, or step-down device that would allow me to use the 36v battery? Thank you, in advance, for all input!
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    This question seems to be inside the wrong topic. Why don't you register and ask later?

    You can use the LM7824 voltage regulator. Perhaps you will need to use an additional resistor if you load consumes current above 1A. The regulator is suitable for voltages up to 38V. Make sure your battery will not give more than that.
     
  3. GS3

    Senior Member

    Sep 21, 2007
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    This is a complex issue and simple answers for complex problems are almost always wrong and worthless.

    There are two different issues here. One is the controller and another is the motor. If you just want to be able to replace the 24 V battery with your modified 36 V battery, without modifying anything in the bicycle, then that is going to take some serious modification because you will need a DC to DC stepdown adapter for your battery which can handle the entire load. When you are talking about a power motor you can forget a linear regulator so we are talking switching here. In any case we would need to know the power requirements.

    Now, if you can tinker so that you can supply the motor and the control unit separately, then that simplifies things a lot because the motor, assuming it is strictly electro-mechanical (which needs to be confirmed), can be fed a pulsed 36 V without problem which is fairly simple to do and then we can deal with the lower power requirements of the control unit separately.

    We would need to know the power consumed by each part. In reality, to give a good answer we would need to see the schematic of the wiring and know what motor it uses etc.
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    If you wish to continue this discussion, can I ask that you sign up - the Comments and Suggestions Forum is for people to provide feedback and corrections to the site, hence our decision to allow unregistered members to post here.

    I have moved this thread to the General Electronics Chat forum.

    Dave
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Vehicles typically draw considerable current. I doubt a linear regulator would suffice. An SMPS might be suitable, but without additional data from the scooter's owner, who can say for sure?
     
  6. David Bridgen

    Senior Member

    Feb 10, 2005
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    Nevertheless, a number of high-current diodes (17 to 20) in series may do what you want.

    Might be costly and they would probably need a bit of heat-sinking.

    If you know, or measure, the current which the motor draws you can soon find the price of suitable diodes.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    As a variation on David's suggestion and since this solution is an emergency backup power source, you may be able to use a 12 volt power Zener diode to drop the 36V volts to 24V. A heatsink would definitely be in order on this one.

    hgmjr
     
  8. GS3

    Senior Member

    Sep 21, 2007
    408
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    Using diodes in series is really not a solution. Firstly, any significant voltage drop in the diodes depends on the current going through. With no current being demanded at the motor the current will be much higher than when the motor is demanding max current. If the control module cannot withstand the voltage range it can be damaged. not to mention that 1/3 of the power would be wasted in the diodes and needs to be dissipated as heat. I really do not see this as a practical solution.

    I am presently using 3 diodes to drop 5 volts from USB to 3 V for a GPS receiver and it works but the voltage stability is not that good ranging from 3.2 to 2.9. If the current was greater the range would be even greater.
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Complex is relative, and no, that is not always true.

    You can use an additional power transistor (series pass) with the LM7824. This is sometimes called a boost transistor. You can get 3A or more by doing this and without the need of using an expensive 3A regulator.
     
  10. David Bridgen

    Senior Member

    Feb 10, 2005
    278
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    Neither do I except as a possible get-him-home solution, as long as the caveat about the control's voltage limit is observed.


    That crossed my mind too but I think the cost of the several rectifier diodes may be less than one God-almighty sized zener.
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Point well taken. He would probably have to get the zener diode and the attendant heat-sink its own set of wheels.

    hgmjr
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Regulation with a zener is not a good solution, especially for high currents. Also, the rectifier diodes in series will do a pretty bad regulation too. The voltage drop can vary from 0.6V to 0.9V for a typical rectifier diode. Just see the datasheets, especially the V/I curves:
    http://www.fairchildsemi.com/ds/1N/1N4001.pdf
    http://www.fairchildsemi.com/ds/1N/1N5400.pdf
     
  13. chuckey

    Well-Known Member

    Jun 4, 2007
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    One quick and dirty solution would be to use "super zener", i.e. a hi-power NPN transistor with the +36 going to the collector and the cathode of a 1.5W 12V zener, the other end of te zener goes to the base, the emitter of the transistor is now your hi-power (10A?) + 24V output. Mount the transistor on a heat sink, cos it will dissipate 12 X motor current = 120W? A decoupling cap across emitter to -, to stop noise/motor spikes would be a good precaution.
    Frank
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    This would only work if the motor is 1/36 HP or smaller.:eek: When I say "vehicles take a lot of current" I mean "a LOT of current!" A "small" electric bike can be up around 1 HP. I've seen one at 50 HP.
     
  15. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    There is no transistor for that. A horsepower is about 750W. 50HP is the power of a lift motor (and a big one). So, a motor of about one ton in a bike? Nah!
     
  16. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Not one ton. Closer to 75Kg, if I remember correctly. The whole bike was only a few hundred pounds. That bike went fast, but not very far.:D

    It would be good to know the size of the OP's motor.
     
  17. JamCarJohn

    New Member

    Dec 16, 2007
    3
    0
    Hey, you might want to drill into the battery where you can get 12v from 1/3 of the battery, 12v from the next and then 12v from the last 1/3. Insert a long screw to contact the plates. Then, simply series two of these together for 24v and you'll have a spare 12v for cellphone chargers etc. Happy cycling!
     
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