Adapting solar panel for tractor battery tender

Discussion in 'The Projects Forum' started by mtonge, May 31, 2016.

  1. mtonge

    Thread Starter New Member

    Apr 19, 2016
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    I have a small solar panel that I salvaged from a fence charger that outputs 10.45v in full sunlight. I want to mount it on my tractor (6v, positive ground system) to keep the battery charged. I would like to wire it permanently into the electrical system without it effecting or being affected by the (generator) charging system. I found this schematic in an older thread:
    [​IMG]
    I ordered the parts from Mouser and bread-boarded the circuit. With these component values (and my solar panel) I can dial the output of the regulator from 1.2v through 7.9v max. The completed circuit outputs 6.79v max. The original description of the circuit is for 12v input and 9v out from the regulator. Can anyone suggest better resistor values for the voltage divider so that the regulator will output 9v with the input of 10.45v from my panel? Also, I added a diode between the charger output to the battery to isolate the circuit from the generator when the tractor is running. Will this prevent the zener diode from reading the charge across the battery? Any comments or suggestions are appreciated. Thanks, MT.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Do you mean in addition to the D1 and D2 already shown? D2 already prevents reverse current from the charger into your circuit above.

    Your problem is that each diode, and the regulator, have voltage drops associated with them. It's about 0.7V for each diode and about 3V for the regulator. There is such a thing as a low dropout regulator, which might be useful here. There are also schottky diodes, which reduce the diode voltage drop to about 0.4V.

    I see no purpose for D1 as long as you hook up the panel properly.

    Backing up farther, it's possible that your panel is small compared to your battery, in which case you don't need anything more than a single blocking diode. The panel may not be capable of seriously overcharging your battery. Do you know its ratings?
     
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Seems overly complicated.

    If it was me I would just put a simple zener diode based voltage shut on the solar panel to keep the peak voltage around 7.5 volts and put a diode after that to keep the tractors generator from feeding back when the engine is running.

    5 watt 7.5 volt Zener diode and a 2 amp standard diode and you're good to go with plenty of reserve capacity built in.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Your battery capacity is far greater that what the panel can put out, so it is safe to just connect them directly.

    Keep D1, forget the rest.

    I've been running some auxiliary 12 volt power on a lighthouse for years with that set up.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!
    As already mentioned, you only need D1. The capacity of your solar cells will be much much less than the battery capacity, so it can't charge the battery hard enough to cause damage. The battery loading will decrease solar cell output voltage, but that increases the output current (IIRC solar cell current capacity is measured into a short).
     
  6. mtonge

    Thread Starter New Member

    Apr 19, 2016
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    Thank you so much for the replies, I have been waiting weeks for a reply in another forum. Wayneh, I don't know the panel specs, I scrounged it from an old, 6v fence charger. The tractor battery is large (6 volt) but I don't know its rating in amp hours. Thanks for the component suggestions. I just thought that I could change the voltage divider values (R1 and VR) to get a higher output from the regulator. Tcmtech, a current shunt through a zener diode would have been much simpler. I could connect the panel as ErnieM and dl324 suggested, but I would worry about it (I'm just a worrier). Below is a picture of the circuit board.
    [​IMG]
    I tested it with the panel connected and got the max output reading on my volt meter.
    [​IMG]
    I have more Mouser orders in my future. I will order the parts that Tcmtech suggested and give it a try. In the meanwhile, I have ordered an enclosure and will complete this project soon. As is, it should work as a float charger to keep the battery up when I'm not running my 1946 Farmall model H tractor. I mounted the panel to the light bar with some angle brackets and rubber shock washers.
    [​IMG]

    [​IMG]
    I hope you enjoyed the pictures, and thanks again for all the replies. This is the right forum for my electronics questions. On another note, PhotoBucket is really bad with pop-ups and malware. I have to move my pics to another sharing site. Any suggestions?
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    You're burning daylight by being paranoid. Temporarily connect the solar cell in full sunlight across the battery and you'll see that the voltage drops to whatever the battery wants it to be.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Since a solar panel is a current producing device it is it's very nature to drop its output voltage to match the battery no problem. Little garden solar lights work like this. So does the 200amp-hour batteries connected to 120 watts of solar panels on a lighthouse I tend.

    It is a tried and true method as long as the panel can only put out a limited amount of current, say 1/10 to 1/20 of the battery rating. That panel you are using cant be putting out too much judging by its size.

    You must have some fence with that panel. The one I use (also on that light) is no where near as big.

    I love your workbench, and you did a very nice job on the build too.
     
    Last edited: Jun 1, 2016
  9. mtonge

    Thread Starter New Member

    Apr 19, 2016
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    Should I put a low ohm, high watt resistor in series with the panel, or am I just paranoid?
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    You're still being paranoid, but use a resistor if it makes you feel better.

    I doubt that the solar panel would output more than a quarter amp. I use one of similar size on several of my vehicles and output is around 100mA.
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Absolutely not. No positive reason to do so, and you will waste early morning and late afternoon power.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    That panel is smaller than some of the ones I see around here, although I've also seen some maybe just 4 inches square. I agree with the others that there are no worries with a panel the size of the one pictured. Just use a blocking diode and you're done. If it saw full sun 24hrs every day, I'd take a closer look but as it is, no worries.
     
  13. vhenuscam

    New Member

    Mar 9, 2016
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    a bit paranoid here. glad to have read this thread. helped a lot! thanks.
     
  14. tcmtech

    Well-Known Member

    Nov 4, 2013
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    There are some medications that can work wonders with that paranoia problem. :rolleyes:
     
  15. wayneh

    Expert

    Sep 9, 2010
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    My go-to meds are Bombay Sapphire or Buffalo Trace. Both very effective. ;)
     
  16. mtonge

    Thread Starter New Member

    Apr 19, 2016
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    3
    Apparently this same circuit diagram has been popping up in this forum from time to time. This has been an educational exercise for me, and I have learned a lot. If everything worked perfect the first time, I wouldn't learn anything. I'm glad that this post was of interest to others as well. Thanks again for all the comments and advice. I think I'll take wayneh's tonic (in moderation) and move on to the next project.
     
    wayneh and ErnieM like this.
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