charging a 24v starter bank with a 12v alternator

Discussion in 'Electrical Systems' started by gdufus, Sep 5, 2011.

  1. Injuneer
    Joined: Sep 2011
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    Injuneer Junior Member

    It depends on the MPPT controller. Most of them are in fact buck converters, i.e., they will step down the solar panel voltage, but there are some that will also boost the lower solar panel voltage up to the needed battery voltage.

    Sorry, I wasn't suggesting that you do this battery charging with solar panels, I was only using the MPPT design as an example of a DC/DC converter that is capable of delivering enough current to charge batteries.

    I understand. However, in your situation I might investigate getting a couple of 12 volt chargers and connecting those in series for 24 volts. I would expect you could get the two standard 12 volt chargers for substantially less than the $450 24 volt commercial unit.
     
  2. powerabout
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    powerabout Senior Member

  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    As long as the manual switch you use is 'break before make' you need not worry about fireworks. If you're a bit absentminded like I am sometimes, a relay solution is better. I once made a motorized rotary switch for a similar application; an extra contact ensured that the starter button didn't operate until the battery switch was in the proper position. But the owner was confused when he pushed the button too early and nothing happened, so I had to add a flashing warning light!
     
  4. BertKu
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    BertKu Senior Member

    Assuming that you have a 24 Volt battery and a 12 Volt generator.

    To solve the problem is very simple. All what you need is A BEER, a toggle switch, 2 dual 40 Ampere Schottky diodes, ($2 each) , 2 capacitors 10000 uF/35 Volt and lots of pain in your fingers from switching the toggle switch up and down.

    What will happen.

    The capacitor No 1 will via the diode will be charged to 14,4 Volt. Assuming the alternator is 14.4 Volt. By switching the toggle switch now over, the capacitor negative will be connected to the + of the alternator and the 14.4 Volt charged capacitor, discharges into the second capacitor, which was allready charged to 14,4 Volt. The two capacitors in serial will give you 28,8 Volt minus the losses. i.e. losses over in the diodes.

    After you have laughed your head off, about this stupid, but very good working solution, the beer comes into the picture.

    You find a friend who is a fundi in electronics and you give him the beer.
    You just ask him to replace the toggle switch from making your fingers tired, for 2 Mosfets transistors and a 555 integrated circuit.
    there you have it, you have for a couple of dollars a voltage doubler and you charge (take slightly longer, as the voltage will only be 28 Volt and not 28.8 Volt. But what the hick, it works.
    Bert

    P.S. One has to understand that batteries will be charged as soon the chemical needed voltage is reached and should be approx 12,5 Volt for a lead acid battery. (12 Volt) and 25 Volt for a 24 Volt battery
     
  5. gdufus
    Joined: Sep 2011
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    gdufus Junior Member

    allright, I went with enjuneer, and got the second exact same model 12v charger (80 bucks) and it actually works, it looks a bit silly, and I used a lot of ducktape to isolate the connection between the chargers, but we're happy campers, and the hatch in the kitchen floor basically obscures it from visitors eyes :)

    the beer solution sounds very good, but I'm by no means an electrician, and I can imagine having a hard time finding the transistors, diodes, mosfet thinghy's etc here in Panama. But thanks for the laughs, anyway :)
     
  6. Injuneer
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    Injuneer Junior Member

    Glad that worked out.
     
  7. BertKu
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    BertKu Senior Member

    As long we keep our sense of humor, we live longer and happier.
    Glad you found a solution.
    Bert
     
  8. goboatingnow
    Joined: Oct 2008
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    goboatingnow Junior Member

    if your bank is made up of 12v sets then the cheapest is to isolated the batteries from the DC network, parallel them and charge with 12 v.

    Note that its entirely possible to get high current Dc DC convertors and change the voltage to 24v, however high current units are expensive

    It also may be possible to use a MPPT solar controller , some have buck/boost capability and could take 12 and output 24 and charge the battery.

    Dave
     
  9. Steve H
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    Steve H Senior Member


  10. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    If I were in an emergency situation, I would do the following.

    a) Disconnect the earth from the 12 Volt alternator, and isolate the alternator from the engine.
    b) find the middle point of the 24 Volt battery. i.e. 12 Volt
    c) Start charging the upper 12 Volt of the 24 Volt bank ( I would have disconnected the negative of the 24 Volt bank)
    d) Then re-connect the 24 Volt bank and the alternator
    e) charge the lower 12 Volt part. Between negative and middle point

    This is off course, when one is able to get hold of the middle point.
    Otherwise one has to open the battery and get hold of the plate.
    Bert
     
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