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Author Topic: Solar - 24vdc Panel with 12vdc Charge Controller  (Read 420 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. 


glort

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #2 on: August 17, 2019, 05:34:16 AM »

I would ask why even bother with the charge controller in the first place?

The buck converters I have seen all have output ajustment  over a fairly wide range. If you set them to a max limit of 14.5/ 14.8, whatever you like your batteries to flat at, then a charge controller is unnecessary because they will never go above that.
If you load them up the voltage will drop and the Buck converter will flow more power up to it's and the panels limit ( no reason you can't series the panels if the Buck will handle it) and get more output. If you have multiple panels, I'd series them in an east, west and flat configuration. that way you will get the earliest charge into your batteries, the latests and the best possible in  crappy weather.
Angling the east/ west panels at 60O will also allow you the most CONSTANT and even charge into the batteries which is what you want over the highest output especially if the batteries are small.

As you have the Buck set to 15V in the diagram, turn it down a fraction more and that's it.  Once the battery hits that voltage the buck will only supply the MA it needs to float and anything else the load requires.

The PWM controllers will pull the panel voltage down to the battery and are no different to direct connection EXCEPT for the cut off. I have a panel hooked to the exact charger in the vid which aren't bad for the cheap thing they are but My Buck controller is a High amp Fixed output. You would have to be sure the Buck converter would cope with the potential draw of the Charge controller  but again, I don't see the need for one.

If you measured a lot of ripple from the Buck, just add in a nice fat capacitor or bank thereof.

I have a battery charger I use as a power supply and it's low output works on the half wave from the rectifier which gives terrible ripple and some devices wouldn't work because of it.  A 370UF 330V Cap ( was what I had)  Smoothed it out very nicely and still gives lower power but a lot cleaner DC than before.  When using it as intended as a battery charger, the batteries themselves too care of the ripple.

I see no problem, I just wouldn't worry about the charge controller to start with and maybe throw in a cap on the output side but I don't see it will be needed unless you are running  like a 2Ah battery or similarly tiny.

veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #3 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 #4 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 #5 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

glort

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #6 on: August 17, 2019, 04:41:28 PM »

 You might be able to find a 24/12V PWM controller that can do directly what you want,

Why do you think the PWM controller would not give max battery Life Bruce.?  If the max voltage was set to the point you want the battery to Float, why would it go over? I would think it would be just a matter of hooking everything up and setting the controller with a Multimeter to keep the battery where you wanted it.  I would assume that the "charge" current would taper off as the voltage got higher which would be exactly what one wanted.

In any case, I got a couple of these on order that should do the Job:

https://www.ebay.com.au/itm/5A-MPPT-9V-12V-24V-Solar-Panel-Regulator-Controller-Battery-Charging-Auto-Switch/282624044890?_trkparms=aid%3D555021%26algo%3DPL.SIMRVI%26ao%3D1%26asc%3D20190711100440%26meid%3Df9c476a752e84f6382173eb02ddf7d28%26pid%3D100752%26rk%3D2%26rkt%3D16%26mehot%3Dag%26sd%3D233167705792%26itm%3D282624044890%26pg%3D2047675&_trksid=p2047675.c100752.m1982

Being MPPT they should handle the 24 to 12 Down rating and are also adjustable for the desired end voltage.

There are also these Boards  https://www.ebay.com.au/itm/DC-DC-Buck-Constant-Voltage-Current-Converter-DIY-Lithium-Battery-Charger-Board/112546564848?hash=item1a344c4af0:g:fB4AAOSwuq9ZY04k&frcectupt=true

And the same with a readout: https://www.ebay.com/itm/Solar-Panels-MPPT-Controller-5A-DC-buck-Step-down-Constant-Voltage-Current-Modul-/191927867733

Plenty of cheap solutions in any event.

veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #7 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 #8 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 »

glort

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #9 on: August 18, 2019, 01:39:33 AM »
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. 

No, I couldn't either but as it does not specifically say it won't do the 24/12 and knowing other MPPT controllers can, just have to give it a go. If the magic smoke comes out, I'll claim a refund.
I have seen descriptions of  things like this that one either ends up having no clue whatsoever as to what the thing actually is for , or, The impression given to everyone who reads said description is completely and utterly opposite to the actual function of the component!

Funny enough, when I was looking at these different boards the other week, I was specifically after some that were NOT MPPT.  I came across several that were described as PWM  but the picture clearly showed the inductor that told me they were MPPT which wasn't what I wanted.
Of course the number of PWM controllers that are described ( and Priced) as MPPT  is well known.
The Chinese are anything BUT ethical or honest Business people.

Quote
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.

Hmm, equalising charge.  I guess one could always have a second Controller set higher and switch that one in now and then to give the battery a boost. Matter of fact a Timer board could be put on said controller so it kicked it in automatically every so often for a given time to raise the voltage.  Plenty of the little timer boards that will do week intervals now.

Quote
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. 

I think it depends on the application.
PWM will normally  pull the panels down to battery voltage rather than keep them at their Pmax which can be important is some applications. If the array is dragged down, it will certainly kill the efficiency not just at the beginning and end but right through the day.  Of course depends if you can use all the array's output and for how long.  On big systems and in bad weather, that might be important to some people.

In playing with the idea of a solar only direct control water heater, PWM is fine but it does need to take into account the sweet spot in the panels.  It's a real annoyance to have 2KW of panels hooked to an element and get 500W output if you are lucky.  Resistance matching leaves a lot to be desired as well.  No matter how many panels one has, it's nice to get some semblance of correct operation out of them.
I can have an array hooked directly to an element and get Diddly out of it. I then take that same connection and put it into a GTI and hook the element to that and get 5X more power output.
Not so much a matter of cost of panels for me, it's now a matter of space to put them all!  :0)

What I want to do is use the simple PWM but have control parameters on it so it will run in the panels happy place.  With that accomplished, I agree, MPPT is largely a waste of time in most applications.  My axiom with solar  from early on is inefficient is efficient when the cost of getting efficiency out of something is weighed against just throwing more brute force at it.
In the case of PWM with things like an element, that may not be the case always.

I certainly agree with the idea of over designing for the low radiation periods. All my arrays are pretty much double the inverters capacity or more.  I realised over winter that the output from the solar wasn't that much down through the main 4 Hours, it was just the things were only at the good end of production 4 Hours instead of more like 8 in summer. It wasn't so much an output thing as a time problem.

I also noticed on cloudy days the output from a 2KW inverter could be virtually the same or even better than a much larger one.  More dependent on what it was hooked to rather than the size of the inverter itself.

The system I put up at my fathers place is a design disaster in effect.  Wrong direction, terrible shading, wrong tilt..... But the Little inverter it's on is overclocked to hell.  The output is pretty useless when the the panels NEVER see direct light but the other 9 Months of the year, It's halved his bill and the output is better than I would have expected by a long margin.  The extra panels make a BIG difference to what the thing would do if set up as our regs allow with only 30% over drive. 


Quote
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.

Ummm, how is that a Bug?
Load comes on, battery voltage will drop presumably enough to trigger charge into bulk state, Charger kicks in hard and basically supplies load direct at least to the max it can so when load kicks off, battery's recharge to float voltage maybe in minutes or certainly a lot quicker than they would have if all the load was borne by them alone.

What do you see as the problem and what would you like to happen?


BruceM

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #10 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 #11 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 #12 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.

glort

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #13 on: August 19, 2019, 06:52:03 AM »

My Russian is more Corroded rather than rusty however in the first vid the guy is definitely feeding the board 36Volts so it will handle that at least. He says he first feeds it 73 V from what I can make out but that seems kind of high although maybe it's just momentarily as a stress test. The light bulb he is testing it with however is 24V so that does not tell us if it will do the 24>12 conversion but adds hope it should if it will pull down from 36.

 He shows the first board and compares the heat sinks to the blue one he calls grey?? and says he fried the first one running it flat out and the heat sink is too small. He compares the heat sink on the second one and comments how much larger and capable it is. oh, he may be referring to the heat sink being grey on the first one. It's definitely too small for his liking. He says he fried the thing at 12V because the amps were too high and running it at 19V is better and will charge a 18V battery down to a minimum of 21V input.
He tests the various settings on the pots and concludes the thing does do  true MPPT and will start up down to 11V but one of the pots adjusts that kick in voltage.  He says after setting the pots, in practice with a load they are about 20% out to what he set them to with the 24V bulb as a load.


The second guy is looking at the MPPT function and determines the board does do true MPPT and will Buck and boost. He tests the ripple which is low, under .5V 
He tests  the startup function as with a dead battery and determines it will trickle charge the battery first then kick in the full charging and then taper off the last 15% of the charge and gradually decreases from there. He demonstrates it will do full MPPT and hole the panels at voltage while varying the output.

In the second Vid he tests the ripple at high power and it is still good but cooling the IC makes the thing over shoot a little.
Driving the thing at full load makes it overheat after a while and it would be better at 3a unless a cooling fan is used on it.

Seems the board for once works as advertised and does a good job.  Someone in the China factory making these obviously slipped up and did something right!

What I would like to do is use this board to control a Bigger Mosfet that will hold the panels at the set voltage while allowing the board to be the base controller to drive a higher voltage and amperage load. The buck and boost would be irrelevant, just use it to hold the panels in a 10V range where they will still be outputting 95% of their power.

veggie

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Re: Solar - 24vdc Panel with 12vdc Charge Controller
« Reply #14 on: August 19, 2019, 05:38:38 PM »

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