24VDC forklift battery charger to adjustable (or 12V) output

Discussion in 'Power Electronics' started by SparkysLab, Apr 30, 2018.

  1. SparkysLab

    Thread Starter New Member

    Apr 30, 2018
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    Hello brethren. I am more of an industrial electrician than an electronics person. I have tinkered in electronics building simple projects and doing simple repairs. Please forgive me if this is the dumbest idea ever, or if it's been covered so many times and I couldn't find the thread.
    I have 3 electric forklifts that have seen various jobs around my workshop, and plan to turn one of them into parts for other projects, including a variable voltage high amperage DC power supply.
    Ideally, I'll be able to adjust the DC output for various voltages, but may limit it to around normal truck charging voltages.
    The chargers have the capability of between 140 and 300 amps DC at 24V.
    My idea is to have a potentiometer mounted to an scr or PWM controller to control the DC output so it can be adjusted for different charge rates, and probably a timer mounted in there too.
    Has anyone done this, and/or know of a good solution to the design?
     
  2. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    Given the size of the chargers, it would indeed seem that the individual parts will be large enough to work with, so that is a definite plus. But given that a forklift battery charger is a single purpose device it may be a challenge to change the output voltage. The first step will be to obtain a circuit diagram of the charger that you want to modify, because understanding how that particular system works is required in order to make any useful changes. With 300 amps out I am presuming that the input voltage is either 240 volts of possibly 480 volts, and probably three phase as well. That implies a transformer and some big rectifiers, and no room at all for mistakes. So the first ordwer of business will be getting the circuit diagram.
     
  3. SparkysLab

    Thread Starter New Member

    Apr 30, 2018
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    Thanks for the reply MisterBill.
    I have at least one of the charger's schematics.
    I look at it's single purpose as a 24VDC power supply... Not as a forklift charger. So my question as a very simple tinkerer - Is there a commonly used way to take a higher voltage, high amperage power supply, and drop the voltage roughly in half and maintain the ampacity and do so OUTSIDE the power supply?

    As a thought experiment - You have two 2/0 copper cables coming into your workshop at 24VDC and capable of supplying 100 amps.
    Is there a common circuit that those two cables can be attached to that output 12VDC and maintain 100 amps?
    Of course, I am hoping that the circuit does not involve big resistors that drop half the power by making heat. hah


    I am not opposed to making changes inside the box, but trying to find a circuit to build outside of the charger.


    2 of the chargers are 240V single phase.
    I had considered simply dropping my input voltage from 240V to 120V, but I am guessing that that would monkey with all the control circuitry. I probably still will try that, but since the regulated voltages are around 24VDC (depends on whether float, equalize, etc) the regulation is going to be as high as the transformer can supply at 120VAC.
    The schematic that I have scanned is of the 3 phase charger, and I think that will be the last choice until I have figured out whether I can modify the DC outputs.
    I am grateful for your reply and any advice.
     
  4. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    Unfortunately the scanned schematic does not appear to have been posted successfully.
     
  5. MisterBill2

    Well-Known Member

    Jan 23, 2018
    1,725
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    A single phase supply may be simpler to modify, presuming that a modification would be a better choice. a 100 amp liner regulator can be made simply by using a lot of pass transistors in parallel, I actually had one like that for a while It had about 15 of the 2N3055 transistors as the pass transistor, another as the driver, and a big blower to cool the heat sink. A switching regulator would be a lot more efficient but beyond my design experience, other than to caution that switching regulators are a lot more complex than they look. And if the charger has different modes it will be important to understand what it is doing in each mode.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You many also need to add some huge filter capacitors to reduce the output ripple voltage.
    Do you have an oscilloscope to look at the voltage?
     
  7. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    A good linear regulator should get rid of most of the ripple, adding a choke input filter should do the rest. But they would be physically big parts at 100 amps. A lower current regulator would be simpler to build but only provide a lower output current. The only real issue is that such a setup would be rather inefficient. But that is the trade-off for simple and cheap.
     
  8. SparkysLab

    Thread Starter New Member

    Apr 30, 2018
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    It didn't occur to me to worry about the ripple... It is already a battery charger, and rather high end.
    I've seen lower-end retail battery chargers that use a single diode for rectification, and they still charged.
    Yes, I have a scope to check for ripple.

    MisterBill, I didn't try to post the only schematic that I have scanned, because it is a 3 phase and not the first one that I planned on trying this with. I'll open the other two chargers and see if they have schematics under the cover.

    How inefficient are we talking about here, MrBill?
    Are we on the same idea about taking the 2 leads from the 24VDC 100A power supply and connecting them to a new item to drop it to 13.6ishVDC without using the rest of those VAs as heater fuel?
     
  9. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    Actually, your assertion about heater fuel is totally correct. That is how most linear regulators work.
    Consider that a linear regulator system with a 24 volt input voltage and delivering 12 volts out is dropping the other 12 volts in what amounts to an electronicly controlled resistor. so half of the power is being converted into heat, meaning 50% efficiency. That might be entirely acceptable i some applications however. I am not saying that is a poor choice or a bad plan, just that you need to be aware of it in advance.

    A switching regulator could be a lot more efficient, and I have just come across an interesting new scheme that uses capacitors. Consider a circuit that charges two capacitors in series across the 24 volts, and then connects them in parallel to deliver 12 volts.In theory at least, the only power dissipated would be in the capacitor ESR and in the switching losses. The main downside is that the output would be about half of the input, which may not be enough, unless you are able to get a bit more than 24 volts out of the existing charger, which my thinking is that to charge a 24 volt battery you do need more than 24 volts. So the capacitor switching system seems to merit a lot more examination, I believe.
     
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