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24 v battery charger vai 12

Discussion in 'General Electronics Chat' started by kinarfi, Sep 17, 2017.

  1. kinarfi

    kinarfi Well-Known Member

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    I am getting a new wheel chair and in preparation for a planned trip, I'm building a charger for it that gets it's power from the 12 volt charging system of my motor home. The sims are basically the same except for the capacitors in parallel with the load, the4r 24 volt battery system.

    I've always thought that a battery acted electronically as a very large capacitior and to simulate the charging of a battery, I put a large capacitance in series with battery with a paralell resistor to prevent a simulated over charge which happened with out the resistor.

    So, my question is why are the simulation lines so different if the battery acts like a huge capacitor, or am I wrong about that? I realize that SMPSs oscillate according to the size of the charge cap, so I guess the load capacitance doesn’t meld with the charge cap, any comments as to what's happening?

    I also noticed that the battery and cap charged quicker with the smaller charge cap.

    Jeff
    charger.png
     

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  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    A battery and a capacitor are VERY different things.

    What are C2 and R3 supposed to be doing?
     
  3. kinarfi

    kinarfi Well-Known Member

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    Spice doesn't have a discharged battery, so to simulate a discharged battery, I show each 12 volt battery at 11 volts and a series cap (C1) to accept the charge but I found out that if I don't add R3, it doesn't act like a battery
    Jeff
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I normally have the UC3845 watching the current in the MOSFET and inductor. That is how the part was designed. I think it will not be stable if it is watching the battery current.
    upload_2017-9-17_20-33-4.png
    Please attach your SPICE file.
     
  6. kinarfi

    kinarfi Well-Known Member

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    originally, I did have the uc3845n watching the FET /inductor current, but it seems that watching the battery current works well as that is what's important and it is more exact. the inductor current is on and off and the duty cycle varies. Battery current is constant and limits voltage until voltage limits current at the end of charge.
     

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  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I suggest you try doing electronics and not computer games :D

    I'm not a fan of simulators, and as you've found they very often don't work.
     
  8. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    You are just lucky.
    The UC3845 has a maximum duty cycle of 50%. (some of the other versions will run to 99%. UC384x)
    To boost from 12V to 24V you need 50%.

    Normally the current in the inductor starts at 0 and runs up to some value. This ramp is what sets the duty cycle in a current feedback PWM.
    Because you are watching the battery the current does not ramp up and the IC is probably stuck at either 50% when the battery is charging and 0% when the battery reaches voltage.

    In real life, not in SPICE, when the inductor goes into saturation there is no means to open up the MOSFET and save the supply.

    Cleaver idea. But I think you supply can not be stable. Probably does not matter so much for a "battery".
     
  9. kinarfi

    kinarfi Well-Known Member

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    It looked clever in my computer game, as Nigel calls it, but in doing the electronics, I couldn't get the current I wanted, 5 amps is my starting goal. My computer game, Spice, shows a solid 5 amps charge, then it ramps down after the voltage hits it's set point, my electronics doesn't seem to work that way, damn, I hate it when you guys are right. Can you offer any suggestions?
    I hate the idea of going to my backup plan of mounting a 12 dcv to 117 vac inverter and just using the 117 vac charger that comes with the chair, that's no fun!
    Any help would be appreciated, my scope looks like I'm stuck at a 50% duty cycle and going to a lower Henry value just kills FETs. I'm actually using a 47μh, a 33μh, 2 47μh in parallel and a 7μh inductor, the 7μh has cost the life of a Fet or 2.
    Jeff
     
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    A low inductor, 7uH, allows the current to build up very fast.
    A high inductor, probably saturates the core and then the current goes up very fast.
    With no current feed back the PWM will not know there is trouble.
    I would get two 12V chargers. Connect one to the bottom battery and another to the top battery. Treat them as two independent batteries.
    -----edited-----
    Now I remember you want to charge from your auto. (12 go 15 volts)
     
    Last edited: Sep 19, 2017
  11. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I don't have the "UC3845" so I used a LT1241 which is almost the same.
    I used Q1 to watch battery current.
    upload_2017-9-19_14-48-0.png
    upload_2017-9-19_14-48-42.png
    Regulates at 5A and 27.5V.
    I changed C2 from 0.1F to 0.01F to speed up the simulation. (slow computer)
     

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  12. kinarfi

    kinarfi Well-Known Member

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    uc3845
     

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  13. Colin

    Colin Member

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    A battery is more like a zener diode.

    Buy a 24v 5 amp transformer and connect a 5 amp bridge and a lamp.
     
  14. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    He only has DC as a source of power.
     
  15. Colin

    Colin Member

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  16. kinarfi

    kinarfi Well-Known Member

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    Power source 13.6 VDC from Motor home engine and available only when engine is running.
    http://www.ebay.com/itm/150W-DC-DC-...d=110961596417&_trksid=p2047675.c100005.m1851 I wonder what the output waveform looks like on this as the battery charges up.
    My plan is to be able to stop and see some attraction and then load the chair in the trailer and drive to the next and have the chair battery charge while driving.
    I built ' wheelchair battery charger-ron.asc' and it was running well, recharging batteries and then it quit, so I quit for the day and will figure it out tomorrow, THANK YOU FOR YOUR EXCELLENT HELP RON!!!!!!
    JEFF
     
  17. kinarfi

    kinarfi Well-Known Member

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    got it built and running, in the process of changing parts around, I created a bad solder joint, fixed that and fired it up and it worked great, only problem is that I could never get any more current than about 1.2 amp by using progressively lower micro henry inductors, ending with 7μH, Wurth 7443630700, I think my UC3845 was stuck at 50% duty cycle as fore told by ronsimpson
    Ron, did you mean to say , "you need 50%" or did you mean to say MORE than 50%
    Thanks,
    Jeff
     
  18. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    There are several modes of operations. In continuous mode 50% will double the voltage. (minus one diode drop and minus MOSFET loss, ect)
    The part you chose works from 0 to 50%. It will not go past 50%. There is another version that will go up to 90%.

    Probably you are in continuous mode. As the boost passes 2:1, then the IC will switch to dis-continuous mode. It is interesting that you need about 50% and the IC limits to 50%. I am not certain if that is good or not. Probably this helps your design. (with out current feedback on the MOSFET)
     
  19. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The simulator is working fine.
    It's the interpretation of the results that's not. :rolleyes:
     
  20. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Rather where I have went over the years when dealing with power conversion issues and other things now. Why build when you can likely buy something purpose built and reliable for way less.

    When I was in the oil field we had that 12 to 24 volt systems up conversion issues all the time with the fraccing equipment I worked with. Everything industrial we had used 24 volt systems but every truck, vehicle and like mobile power source was 12 volt and it was quite common to have a piece of fracc equipment sitting for hours with it's engine off but needing to keep it various electronics/electrical power systems (lighting, PLC's, unit monitoring/operational sub systems and network interface gear) up and going.

    The only solution we had was to charge one of the 24 volt system batteries at the time off of a 12 volt based power source for a hour then move the jumper cables to the other before it ran down to far and keep repeating that until the engine could be started again.

    Just a week before the oil fields shut down I had ordered four 9 - 18 VDC to 28 VDC 280 watt boost converters (~$15 each) so that I could make a pair of 500+ watt capable booster units to solve the problem. Never got to finish the converter projects though. :(
     
    Last edited: Sep 22, 2017
  21. kinarfi

    kinarfi Well-Known Member

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    I just about had this finished and it was time to go, should have finished it, the inverter idea failed miserably. my charger starts at an amp and ramps up to 7.5 amps and the inverter shuts down,so I had to run the generator.
    I have this running now and seems to work quite well, killed one UC3842BN, so I added the soft start, seems to work like I wanted, the higher the input voltage, the higher the amps in. WHEELCHAIR BATTERY CHARGER-SOFT-UC3842B.png
     

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