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Author Topic: Solar - 24vdc Panel with 12vdc Charge Controller  (Read 5295 times)

veggie

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Solar - 24vdc Panel with 12vdc Charge Controller
« on: August 17, 2019, 01:54:57 AM »
Hi All,

I came across this video where a fellow uses a small buck converter to match a 24 volt (28 -34 vdc) solar panel
to a 12 volt charge controller to charge a small gel cell battery.
https://www.youtube.com/watch?v=AD7g5BNy5fM

This interested me because I have a few 50 watt (24 volt) panels and I would like to use them on some protects involving 12 volt gel cell batteries and a microprocessor. I also have a few cheap Ebay buck converters kicking around.
Now granted, this may not be the best accepted practice, but if one has the components sitting around collecting dust, maybe it's ok.
So my question is...
Does anyone see a problem with this setup when used for small batteries and low amperage?
I'm not really concerned with efficiency. I just want to use a surplus 24 volt panel to charge a small 12 volt battery using a 12 volt charge controller.
The microcontroller would run 24/7 so it will draw from the battery constantly.
Battery charges in the day and draws down at night.
I am aware of the necessary depth of discharge limitation that I must consider with the microcontroller draw.
I have multiple 50 watt, 24 volt panels if needed.


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BruceM

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #1 on: August 17, 2019, 04:25:34 AM »
As long as the solar charge controller can manage with the high EMI and ripple of the buck converter, I see no problem.  A PWM type charge controller will be likely to do fine since it was already coping it's own rather substantial EMI. 


veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #2 on: August 17, 2019, 02:31:09 PM »

Thanks for the guidance fellas. I'll give it a try.  :)
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BruceM

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #3 on: August 17, 2019, 02:50:26 PM »
Glort's got it right and I missed the obvious. No need for two stages of pulse regulation when one will do.  Set the buck regulator output voltage depending on the type of battery and typical daily depth of discharge and time to charge. 

 You might be able to find a 24/12V PWM controller that can do directly what you want, and is smart enough to go to float charge voltage when charge current tapers off.  That will increase battery life somewhat, perhaps adding a year.  You can either do a seasonal average temperature adjustment on the charge voltage yourself or better select the charge controller for one that does it automatically for you.   That will add a year or two to battery life.  Few (lead acid) batteries die, most are murdered before their time.





mike90045

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #4 on: August 17, 2019, 04:07:56 PM »
Watch out for the simple Buck converter not being able to start or be stable with the variable output from the PV panel. if it hangs up, you won't get charge

veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #5 on: August 17, 2019, 10:31:43 PM »
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BruceM

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #6 on: August 17, 2019, 11:07:18 PM »
Nice find Glort. That does look like it will take a 24V panel input to a 12V battery as Veggie desires, though I couldn't find specs or manual that specifically elaborates that. 

The only concern with a fixed voltage converter of either PWM or MPPT is that never going to float or never going to bulk charge voltages in some cases can limit lead acid battery life.  You can deal with it by lowering the fixed voltage if it's mostly in float, or raising it if it's going to be discharged more over night.  I do it myself on my PV linear regulator for the car, even though it does shave a year off the battery life for overcharging and drying out the AGM battery.  I didn't have room for more circuitry, was trying to squeeze it on a nice surplus heat sink I could slip in next to the battery.  So I just do a seasonal fixed voltage adjustment, lowering it for summer, raising for winter.

I think PWM vs MPPT is irrelevant.  MPPT is mostly hype and I certainly don't give a hoot  about the last 15% of PV power under certain low light situations.  PV is so cheap now that it's nuts not to design for plenty of extra PV for winter/cloudy days. 

While I was sitting here, my 1 hp air compressor is topping off my 500 gallon tank.  I know it's still pumping as there's a bug in my battery charge controller which causes it to go to bulk charge voltage when there's a big load on the inverter.  It's a handy bug- I can tell when the well pump or air pump is finished when it goes back to float, but I'm trying to fix it anyway.  I just added a resistor and capacitor on one critical signal that had some ripple, but alas, it apparently wasn't the one that was causing it to trip out of float.  The other is a stinker, the trace that must be cut is completely under the IC.  Grrr.  I'm trying to get it finalized so I can have new clean PCBs made; it's my original prototype and it's got lots of rework on it. 






 



« Last Edit: August 17, 2019, 11:22:50 PM by BruceM »

BruceM

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #7 on: August 18, 2019, 05:27:13 AM »
You may be right that some cheap PWM units aren't true buck converters.  In that case, the PV array voltage must match the battery well or true, it won't work worth a damn.  Probably MPPT is the best way to insure that you're getting at least a decent DC-DC Buck coverter even if not true MPPT.  MPPT assures that someone might have at least thought about the panel voltage/amps curve, and will do a proper Buck (downward DC to DC) conversion (with inductor as you mentioned) with decent efficiency so is probably best for most consumers, who unlike me don't give to shits about conducted or radiated EMI.

For my 12V and 6V batteries, I have a cheap adjustable 5A DC power supply (switcher) for equalization charging.   My 120V bank controller does equalization on demand, with a 5 hr timeout. Each 12V battery in series is held at the proper equalization voltage by it's individual battery shunt regulator.  I do it the end of the month on a clear day.   I avoid electric cooking that day as 153V is a bit much, though it does heat a 115V rated soldering iron quite fast!

The bug in my 120VDC charge controller is that it should stay at float voltage (136- ish volts depending on temperature) as there's ample PV power to run the inverter and maintain other minor loads. While I have found the bug useful it's still a bug.  When I filter the netcharge signal tomorrow, that should fix it.









« Last Edit: August 18, 2019, 05:42:15 AM by BruceM »

veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #8 on: August 18, 2019, 04:40:34 PM »
 I couldn't find any specs either, however there are a few videos (if your Russian is good)
Here the fellow looks to be feeding 24 VDC and tuning the output to 12 VDC
https://www.youtube.com/watch?v=3btBG0DtSEs

Here a fellow is testing the unit with instuments.
No dialog, but maybe BruceM and Glort will understand what the heck is going on.
Part 1:
https://www.youtube.com/watch?v=h0G2XQceHCM
Part 2:
https://www.youtube.com/watch?v=ROOU8c9TgM0
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BruceM

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #9 on: August 18, 2019, 05:15:05 PM »
Interpreting the videos without any diagram or verbal explanation of his test setup requires psychic abilities I lack.   Youtube at it's lowest.

veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #10 on: August 19, 2019, 05:38:38 PM »

Thank you for the translation "Mr Glort Yakimov"   ;D
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veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #11 on: August 28, 2019, 02:23:04 AM »
Glort, did you receive your eBay MPPT converters yet?
I'm curious to see how they work.
Mine are still 4 weeks away.
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veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #12 on: September 05, 2019, 02:46:26 PM »
Thanks Glort,
I assume you are talking about the module shown in the picture below...
From what you have seen, will it work as a crude charge controller with my 24-34vdc panels charging a small 12vdc battery?
I have a 50W (24vdc) panel and wish to solar charge a small gel-cell battery which powers a micro-controller at a remote site.

Mine have still not arrived  :-[
« Last Edit: September 05, 2019, 07:30:46 PM by veggie »
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veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #13 on: September 05, 2019, 11:52:26 PM »

Thanks Glort,

I might order a couple of them myself actually, I found a good small Panel I was given a while back and I  thought about putting it up the back in the machinery shed and putting it on the  tractor as the regulator is not working on that and I am just recharging the battery every so often which isn't great for it.
  In any case sometimes it's a couple of weeks before I use the thing so having a charger on it would be much better.


That's the exact reason I bought two.
My tractor can sit for months before I use it. I want to use my second 50 watt panel to keep the battery charged.
However, I'm not sure that this little controller is smart enough to scale back when the battery is full.
Will have it investigate.
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veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #14 on: September 06, 2019, 02:54:02 PM »
Well ... if that's the case then I would say that a price of $10 for these little units is the best bargain out there.
An adjustable MPPT charge controller for small batteries (or maintaining a vehicle battery) for this price is a great deal.
Having said that, I am mindful of where it is made. I will report back once my charging system is active.
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