I’m currently having an issue with a MTTP solar charge controller that has a max PV voltage input limit of 32V.
The panel I have is meant to have a max output of 29.9V but the controller is reading sometimes 32.8V and going into overvoltage fault mode and not charging the battery.
I need to put some sort of diode or regulator on the + line to the MTTP charge controller to drop the voltage by 1 to1.5V so it reamins below the 32V threshold when the panel is outputting its max voltage without loosing much charge efficiency from the panel when its not at its max.
Here are the specs of the solar panel:
Max Peak Power
Over 300W
MPP Current
13.73
MPP Voltage
29.9
Module Efficiency
20.9%
Open Circuit Voltage
35.9
Short Circuit Current
14.63
Any assistance and direction to products or parts would be greatly appreciated .
Hi Grant
Welcome
An interesting problem. First thoughts would be a couple of diodes in series to drop 1.2 - 1.4V. They would have to be pretty big to handle the 14A of current.
Problem here is that drop would not be selective. This drop would be there across the output range and you would lose out lower down.
the simplest way out would be to get a MPPT controller that will handle the voltage. You don’t have much in the way of “Fudge Factor” anyway. A 32V controller with an expected input of 30V is not a lot of headroom. Most electronic bits have some sort of tolerance range including your Solar so you might have just got everything going the wrong way but all the bits could be still within spec.
The alternative as I see it and keep things simple is a relay and comparator arrangement which will switch into circuit a couple of diodes when the solar voltage hits say 30V. This is still going to require some external electronics.
I did get involved with something similar on a larger scale a lot of years ago. We had a tropposcatter radio system powered by 2 battery banks of each 24V @ 2000Ahr total 24V at 4000Ahr, continuously float charged. Requirement was for 14 days operation with no mains power. Periodically these batteries required a “Boost” charge where the charging voltage increased. When this happened there were a bank of diodes switched into circuit to the load (radio system) to reduce the voltage to the load back to the required 24V. This switching done by contractors driven from the charger system when in “Boost” mode.
That was a similar situation to yours when you think about it.
So short of getting a controller that will handle the volts that is the simplest thing I can think of, comparator, heavy duty relay and large diodes.
Cheers Bob
PS: Google “MPPT controller” and there are heaps of commercial products that will do this job with plenty to spare.
Thanks so much for replying.
I think for the moment you are right I will just put a couple diodes inline to solve the issue for now.
As you said I just need to find some that can handle the current.
Hoping someone might be able point me in the right direction for those.
Although long term I’d like to look into and make something as you have suggested that will switch when the panel voltage drops so we still get the benefit of the larger panel.
The design of the system changed midway in the project which is what caught me out. The original solar controller had a 100V limit so we were well and truly covered.
Most of the commercial controllers I looked up yesterday have a max solar voltage of between 50 and 100V. Prices vary depending on features etc.
If you are going down the diode path the diodes have to handle the largest likely current which could be 15A or more if you add to your system.
The easiest way would be use a high current bridge rectifier where you will finish up with 2 diodes in series in parallel with another 2 diodes in series. Connect to + and -, + being cathode and - being anode. Ignore the other 2 (AC) connections.
One which will give you plenty of margin would be MB354 rated at 35A and easily mountable with a hole in the centre for a small screw to attach to a heatsink if required. This is readily available at Jaycar, cat no ZR1324.
Bear in mind that if using the diode/relay method you are going to have to spec your relay to support this 15A which is beyond the rating of the relays found in the common relay modules available. Then you are going to have to drive it. The common comparators will not drive such a relay directly unless you get lucky. Then you have the comparator circuitry. Then of course you have to put all this together. All up you could find this costing just as much as a new controller that will do the job. Just connect Panel, Battery and load. Done.
Cheers Bob
PS: I am assuming a 24V system here. You have forgotten to tell us that little bit.
Yeh its a long story but the short version is I had a full Victron system designed (its for a local club I volunteer for) then someone said they could get some thing cheaper but not all the components and then we ended up with a mix of the originally designed system and the other gear.
So here we are having this chat
Thanks so much again for your suggestions Im going to give the rectifier a go as its not a big outlay and I can test it pretty easily.
Cheers
Grant
ps. No we are running a 12V DC system it was just easier for the setup we have.
Yes thanks for the suggestion. I did do a rough test on that but even with bypass diodes you sacrifice too much panel capacity when shading the panel and its give pretty inconsistent results.
I really only need to limit and regulate the panel voltage by around 0.8V and I think doing this with Bob’s suggestion give me a more reliable and consistent result.
It all tells the story doesn’t it. There is an old story about paying peanuts and getting monkeys which I am sure you have heard before.
Most of the stuff “on the cheap” is just that. Reliability could be pretty questionable and support and information almost nil. If you wanted longevity and reliability and all importantly support I think you should have stayed with your Victron system. I think by now you are probably aware of this. This Victron stuff seems to be pretty versatile. There is another contributor on this Forum who has such a solar system and it even has the ability built in to monitor and display his water tank level.
Perhaps the “someone” who suggested to go cheap might have a solution for your present problems. Ask them, I certainly would.
Cheers Bob
Hi Grant
How did you go with the diodes. Should work OK but you will have that voltage reduction across the board.
If you want to go down the diode path and switch the diodes here is a circuit that I am confident will work.
It looks a bit complicated but is the simplest I can think of.
Disregard the vertical black line at the left cutting R2 and R5, this the cursor which got into the picture somehow.
You need a comparator to compare the solar voltage against a reference. In this case I chose a reference of 4.1V as it happens Core have a board mounted device all ready to go. SKU ADA2200. As this is basically to work at 3.3 or 5V I have added R1 to help keep the current down at 12V.
1/2 LM393 comparator is configured as a Schmidt Trigger by R2 and R6 to provide a bit of hysteresis and prevent chattering at the switch point thus a clean switch.
The LM393 is an open collector device with a grounded Emitter. As the actual switch Core have a convenient high side Mosfet switch which I have used here. Core SKU DFR0457. I have chosen to use the comparator to switch the low side of the Opto Coupler as I think for this application it works better that way.
As this is meant to operate at 3.3 or 5V cI have included external resistor R7 to keep the LED current to a safe value at 12V. As the LED is basically a diode R8 is required to help the hysteresis happy and working properly.
This switch simply switches 12V to a suitable relay. This relay should be able to handle the maximum expected current. I think about 15A so I suggest 30A to allow some headroom. You may find a suitable automotive type easier to find. Diode D5 is required to soak up the back EMF of the relay at switch off. The type suggested is a tried and proven Schottky unit widely used in motor speed controllers with motors up to about golf buggy size.
In operation the diode bridge discussed earlier or other voltage dropping diodes are connected in series between the solar panel and the MPPT controller. The relay NC contacts are connected across these diodes and effectively short them out. When the solar panel gets to a predetermined voltage the relay will operate allowing the diodes into circuit thus lowering the volts applied to the controller. When the solar voltage drops the relay releases removing the diodes from circuit.
With this configuration the relay only operates for the lesser part of the day if and when the solar voltages approach maximum level.
The relay should operate with a solar voltage of 29.26V and release at 27.6V. These are theoretical values and due to component tolerances may not be exact in practise. The switching point can be trimmed by adjusting R3 and / or R4 but should be pretty close. You should not have to make radical changes here as a small change here will have a much larger effect at the solar voltage point by a factor equal to the divider ratio. The hysteresis can be changed by changing the value of R6 but the 1.7V (calculated) as it currently stands should be OK. Increasing value gets the switching points closer and decreasing spreads the points further apart. There is a lower practical limit here which should be about 100k. That is don’t go below that.
I stress this circuit has not been tested. But I have used many similar circuits in the past so am very confident. As I say it might need some fine tuning as the figures quoted are calculated but should be very close.
Cheers Bob
That looks great and thank you so much for the detailed lay out.
For the moment I will stick with the basic solution you suggested while I gather the parts and have a go at building the circit you have so kindly shared with me.
I have plenty of older equipment at home to test it on before hooking it up to the main system.
the only thing I’d like to be able to add to the circuit is a way to have a slight delay when the voltage drops before the relay trigger just so it wasn’t constantly switching back and forth if there is fast moving cloud and the sun is coming out and then going behind cloud constantly although maybe that wont be an issue.
tanks again Bob I really appreciate your advice and time.
Hey Bob
Sorry. Clearly this is just a little beyond my skill set but I do I have someone I can lean on to help me give this a try again I really do appreciate all your responses and assistance with this it has been fantastic.
I’ll be sure to let you know how it goes.
Kindest Regards,
