My onan MCCK generator is overcharging the start battery

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Jul 31, 2012.

  1. sdowney717
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    sdowney717 Senior Member

    it is a MCCK 6500 watt.
    I discovered that the charge voltage goes up to 17V after it runs a while.
    This onan uses a 2 step charge regulator, where when the voltage hits a certain set value, this charge relay is energized and opens the high charge circuit leaving only the low charge circuit on.

    The hi and low setting for the voltage output is controlled by a center tap power resistor of 6 ohms and 225 watts and is adjustable for both hi and low voltage charging. Meaning there are 2 adjustable rings that can slide and power comes in at the center tap and goes out at each adjustable ring.

    I have traced the circuit and determined the resistor is set on both ends for 2.5 ohms from the center tap to each wire. Center tap is taking the output from S1 series field of generator. Power then flows thru the center tap adjustable power resistor and one side is always on, the other side is disconnected by the two step charge relay when the voltage goes above maybe 12.5 volts. The pull in coil ohms on the charge relay is 131 ohms.

    I took the cover off the two step regulator and when I manually hold it closed, it immediately responds to 16.5 volts from 13 volts after a crank start of the motor. When let go at 13.06 V it was already opened the points. So that is working fine.

    It is the low charge circuit that is putting out too much voltage. This is regulated by the other side of the 6 ohm 225 watt wire wound power resistor.

    I was thinking I need to try adjusting for a higher ohms but it already fairly close to the end, or simply add another power resistor into the low charge circuit.

    Are high wattage low ohm resistors easy to find?
    What ohm should it be, like 1, 2, 3 ohms?
    Anyone have a link to a source part to think about?

    this pic shows K5 charge regulator and R1 power resistor
    'C' is the low charge side, and 'A' the high power side which is opened and closed by the points in the K5 charge regulator.

    [​IMG]
     
  2. BertKu
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    BertKu Senior Member

    I personnaly don't think a different resistor is going to solve your problem. The charge voltage of 17 Volt, is that with a battery is connected or is it floating and you measure this with an high impedance electronic meter? You must measure it with a battery connected and then see what happens if the battery has reached 13.8 Volt. If the voltage creeps up to 17 Volt with a fully charged battery, indeed you have a problem. But a different resistor is most likely not going to help you. I had a friend who was highly upset when he had to buy too often new batteries at $ 300 for his golfcart. I took a 7815, a 100 uF elco, 4 x 2N3055, 4 resistors of 10 Ohm, 1 x 4007 and 2 x 40 Amp stud diodes, and he never complained anymore about having to buy new batteries anymore and that was 5 years ago.
     
  3. sdowney717
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    sdowney717 Senior Member

    that voltage is reached WITH the start battery connected.

    The power resistor simply lowers the output voltage and gets hot. It is very large about 1 inch wide and 6 inches long and is spiral wound with a wavy metal resistor on a center ceramic core.

    I dont see why adding more ohms to the output would not lower the output voltage?

    How would those diodes cope? The voltage is steadily rising as the battery is being forced to accept more volts and perhaps that 17V is near the max the gen can output or perhaps the 17V cant go any higher because the internal resistance of the battery wont let take any more volts.

    I think another idea is to just disconnect the low charge wire from the 2 step charge regulator. So that when the charge regulator points open, all charging goes off. Ok perhaps as long as the battery stays in the other circuits. I tried disconnecting the ammeter but then the gen wont start.
     
  4. BertKu
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    BertKu Senior Member

    Well than you have a problem. If your batteries are full and you carryon charging at such high voltage and current, your battery starts to exceed the gassing voltage of 13.8 Volt and start boiling. Goodbay batteries.
    the balance of the current will still exceed the battery leakage current, also the gassing voltage and the life time of the battery is at stake.

    Which diodes?
    I would consider to make a constant voltage circuit with 2N3055 and some components. 6 x 2N3055 gives you ample current (+/- 120 Ampere) . The same as I made for my friend. It takes a little longer to reach saturation, but your batteries will last. I also had a relay, which as soon the battery was nearly charged, changed over to the circuit with the 2N3055's.
     
  5. sdowney717
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    sdowney717 Senior Member

    sounds interesting, It already has the 2 step regulator charging relay, just need to regulate the low charge output in a better manor.

    so where does the excess voltage go in that circuit? Amp output for the gen is low for battery charging. It is around 10 amps only.
     
  6. BertKu
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    BertKu Senior Member

    Sorry for the late reply, watching the Olympics when I have a chance. Here is the circuit diagram attached. It depends whether you will have 17 Volt or more, you could use the 7815 regulator, if the Voltage is just over the 15 Volt, you may have to use 2 zener-diodes to make it to 14,4 Volt zener, which gives you an output of approx 13,5 Volt. Bear in mind, a lead acid battery charges already at 12,5 Volt. If the output voltage is below 14 Volt, it just take a lot longer. But you have in any way a 2 stage charging possibility.

    The excess voltage drop is absorbed in the 115 Watt each 2N3055. I promise you, you will never reach that power, if you use a couple of cheap 2N3055 's parallel.
    You may have to experiment with the 1 Ohm resistor in the base, should the base current/voltage drop be too great, you may have to drop it 0.33 Ohm.
    This is the best I can advise. Maybe we will argue again and you get a better solution from somebody else.
     

    Attached Files:

  7. BertKu
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    BertKu Senior Member

    Just to elaborate on the working. The output of the parallel switched 2N3055 is connected to your battery. The, too high 17 Volt or whatever is absorbed in the 2N3055 acting as variable resistors. The zenerdiodes keeps the input voltage on the bases of the 2N3055's constant at the voltage of the zener. There is a drop of 0.5 Volt between emittor and base, thus if your 2 in serial placed zeners are at approx 14 Volt, your output is 13,5 Volt and is below your gassing voltage of your battery. Should you use a 7815, you need some extra voltage to make the internal electronic circuit working, i.e. 17 Volt. The output of the 7815 is kept very stable from 0 - 1 Ampere. and is 15 Volt. The diode drops the voltage further by 0,5 Volt and the base voltage on the 2N3055 is thus 14,5 Volt. Also thereafter dropped by another 0,5 Volt between base and emitter. Thus the output is between 13,8 and 14 Volt depending on ambient temperature and base current. If the battery is charged it squeezes the 2N3055 to a close, as the base voltage is kept constant.
    With the zenerdiodes , one has to be carefull, as the voltage varies a little depending on the current flow through the zener. But in general an 0,2 Volt with very low currents to your battery is not damaging your battery. It even may compensate for your battery leakage current.
     
  8. Frosty

    Frosty Previous Member


    You sure it 13.8?
     
  9. BertKu
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    BertKu Senior Member

    Yes, I am dead sure. At 12,5 Volt a lead acid battery starts already to charge chemically or better said at 2.05 Volt per cell at 2.3 Volt you start with the gassing voltage if a battery is fully charged. We all charge at 14,0 Volt to 14,4 Volt, to charge the battery faster, but if you keep it at 14,0 Volt or higher after the battery is full, you will start seeing those bubbles in a battery. That is the gassing voltage of 13.8 Volt or 2.3 Volt per cell.


    charging at 2.30V to 2.35V/cell
    Advantages
    Maximum service life; battery stays cool; charge temperature can exceed 30°C (86°F).
    Disadvantages
    Slow charge time; capacity readings may be inconsistent and declining with each cycle. Sulfation may occur without equalizing charge.

    charging at 2.40V to 2.45V/cell
    Advantage
    Faster charge times; higher and more consistent capacity readings; less sulfation.
    Disadvantage
    Subject to corrosion and gassing. Needs constant water. Not suitable for charging at high room temperatures, causing severe overcharge.
     
  10. Lt. Kludge
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    Lt. Kludge Junior Member

    If I am reading the correct service manual that is a 1.56A charger on the low charge rate. What sized battery do you have that is reaching 17V? Have you confirmed the actual charge current?
     
  11. BertKu
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    BertKu Senior Member

    But even if the battery is small and the voltage creeps up to 17 Volt, it is not advisable, also not for a big battery. The system should charge first a good strong charge of 10 - 20 Ampere or whatever and then trottle back to maximum 13.8 Volt and shut down after a few hours automatically. He may measure a leakage current from an LED, but still, the voltage should not creep up, that is never good for a Lead acid battery, whether a SLAB, liquid, deep charging etc. But the chargers I have been making in the last 15 years seem to work well. I haven't received any back and never had complains.
     
  12. pistnbroke
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    pistnbroke I try

    Frosty you are right ( again) gassing starts at just over 14.4v .....
    Rather than rush in with electronics a better diagram of the original would help to get it adjusted to give 14.4v ....
    Lots of stuff on the web on these generators /diagrams/manuals/fault finding ....need to know the model...
     
  13. BertKu
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    BertKu Senior Member

    That is indeed correct for the absolute maximum threshold circumstances, like 10 - 15 degrees celcius ambient temperature, but if the temperature creeps up, the gassing voltage is reducing to 13.8 Volt. One can now argue to keep the battery in the deepfreezer, but that is impractical. I always have played it safe, as I don't know at what ambient temperature my customers are using the batteries.

    just visit:

    http://www.powerstream.com/SLA.htm

    and check the gassing voltages for the various types. You maybe sitting at the South pole, 14,4 Volt is then not a problem, but the other person has the battery under the bonnett of a car and the ambient temperature is then 50 degrees Celsius. It totally depends on the user.
     
  14. pistnbroke
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    pistnbroke I try

    We ar here to help this guy not argue but are you telling me its 10 deg C under a car bonnet ( hood) ..

    fix the generator dont add more crap.
     

  15. BertKu
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    BertKu Senior Member

    You are a bad looser
     
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