question

mark-volt avatar image

MPPT 75/15 charging voltage below 13v


My question is about something I do not understand. screenshot-2021-10-24-12-48-35-290-comvictronenerg.jpg My setup is an MPPT 75/15 connected to 2 Sunpower spr e flex solarpanels. It's feeding a bank of 2 x105Ah lead acid batteries. Charger settings are standard for lead acid. Everything worked fine for about 2 years, voltages and current correspond to sun position, strength and cloud conditions.

Now, since 2 weeks charging voltage does not exceed 13v. even with the app showing healthy PV conditions. I have to add that I have disconnected any load, and only 1 of the 105Ah batteries connected, to make the screenshot. I will add the csv from the last 30 days as well. (no I won't, the csv file is not an accpted filetype, other possibilities?)


On the screenshot there is a load drawing some current, switching between 0.1 and 0.3 amps. But... the load output of the MPPT 75/15 is disconnected... Before the low charging voltage problem, the load visible in the app was null as it is supposed to be without connected loads.


What do you think the problem can be? Is it reasonable to assume the readings on the app are absolute and should add up to a correct charging profile, or am I missing something?


Dear answerer, I'm hoping you can help..







MPPT Controllersbattery charging
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13 Answers
Mike Dorsett avatar image
Mike Dorsett answered ·

First, it would appear that your panels are connected to the MPPT in parallel. A series connection to the MPPT would give you a higher panel voltage, and the MPPT more 'headroom; to convert the solar power to charge the battery. The MPPT requires the Solar voltage to be 5V higher than the battery voltage to start - once started this can drop to ~ 1V, BUT in this low headroom range, power may be limited. It is MUCH better to operate with the Solar input at higher voltage.

Secondly, a screenshot of the actual voltage settings for the battery would help.

Do you have another source of charge for the batteries? Can you try an equalisation charge to see if this corrects the problem?

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mark-volt avatar image
mark-volt answered ·


Hi Mike, thank you for your reply. The system is in my camper and I am away from home without a seperate charger, but with some alternative charging from the alternator. I set the equalisation on in the app today, but with the weather conditions it may not have enough headroom to get active for long. I will report on any activity with results. I have connected the panels in parallel as they are different sizes 1 x 50 and 1 x 110 watts. Not very smart arrangement but that was part of my learning curve..

A question that sticks in my head is why, even with little headroom, is the MPPT behaving different from before with the same PV readings. In the history, just a week before, it was maintaining the batteries with a day cycle voltage mininimum of 12.9V

Here is the settings screenshot :

screenshot-2021-10-25-11-16-06-057-comvictronenerg.jpg


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I would be good for you to actually show a screenshot of your mppt's history. Eg....

screenshot-2021-04-18-10-53-13-263.jpeg

That's a good suggestion klim8, I don't get the landscape mode as in your example however. I have made a pdf of the csv file generated by the app, I will post it in the main thread.
kevgermany avatar image
kevgermany answered ·

I think you're going to damage your batteries with that equalisation setting.

You say charger settings are standard, but the screen shot shows User defined.

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That's true Kev, the daily equalisation setting you see is just for this day today. To see if it kicks in and does anything. I will switch the equalisation off later on today.

Also true, you see the user defined mode, but the values are the same as factory settings for lead acid.

Actually, I see now that the settings I had do deviate from standard setting choices. So your observation was correct. I do not think however, it is of influence to the problem I am having, would you agree?
mark-volt avatar image
mark-volt answered ·

Here is the PDF with data from the last 30 days:

SolarHistory.pdf


solarhistory.pdf (46.6 KiB)
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kevgermany avatar image
kevgermany answered ·

Let's see what the others say, but looks as if it's simply lack of sunlight. The charger has stayed in bulk mode for the last two weeks and power generation has dropped a lot. So it's not fully charging your batteries.

What I've noticed on my system is the PV voltage doesn't really drop, but max current does. MPPT will keep panel voltage at optimum and net effect is that as light intensity drops, current drops at similar voltage.

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Good observations, and it's a good balancing act of the MPPT. But it in the end it's sunpower that it needs and with the winter sun position and clouds circumstances are minimal. What I don't see the logic of, is the situation as in the screenshot. Input voltage steady around 19 volts, a proper charging current output, but the output voltage way below the set bulk charging voltage. So in effect almost no charging the batteries.

kevgermany avatar image
kevgermany answered ·

It's the voltage of the batteries plus a little extra for the very low charge possible. . And if they're undercharged..... Voltage rises as they charge. When there's a lot of charging available, the controller will be able to apply a higher voltage to charge the batteries at a higher current.. But under low light, it can only apply a voltage which matches the current it can supply from the panels.

Remember also that the panel voltage will drop if the MPPT takes too much out of them. So it optimises current drain to maximise power from the panels.

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mark-volt avatar image
mark-volt answered ·

Kev, I think I'm starting to see...

The MPPT calculates available PV panel power from volts and current measured, and shows the numbers in the Victron app. But those numbers change under load. The calculations made by the MPPT for optimal power point tracking and output to the battery are indicated by the relation between the PV voltage and current vs the output voltage and current.

What we don't see in numbers, is the conversion job done by the MPPT. But with proper understanding of all factors involved, sunpower and position, conversion , battery state, we can know what is happening.

Which means MPPT numbers should not be taken at face value, they should be properly interpreted.

Please correct or add. Did I get what you are saying?

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kevgermany avatar image
kevgermany answered ·

You got it.

Plus the T of tracking... The MPPT will self adjust as the light levels change. Very clever piece of kit.


One other thing to bear in mind is that panel voltage is affected by temperature. As the panel heats its voltage drops. And as temperature drops voltage increases, something to watch out for if you go for series connection in future.

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mark-volt avatar image
mark-volt answered ·

Thank you Kev for your support so far. I pulled the future forward. Yesterday I rewired and monitored the system in series whith plenty sunpower.

That magnified a missing part of my understanding. I calculated that the wattage in both configurations should enable the MPPT to give an output of 14.4 volts. However it chooses not to. Given that the information I find online says we need a current to be 2 volts above the battery voltage to charge across it's internal resistance, my question remains why the MPPT does not apply the set charging voltage of 14.4 volts. Or closer to it.

I have a feeling it hangs on the part of the story that involves load and resistance but I am not getting that yet. What do you think?

screenshot-2021-10-28-14-07-48-503-comvictronenerg.jpg



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I realise you explained before that it's low light intensity. But in the numbers I don't see that reflected. I mean, if it says for example 30 watts of combined volts and current available to work with, that should be hard fact no? And so how does that differ from a day in summer..
Nope. You're assuming what's available. You cannot go on voltage to calculate available power. As soon as the MPPT draws current from the panel, panel voltage drops. The less light there is, the more it drops for the same current. The MPPT Is balancing current from the panel against the voltage drop it sees. Calculates power and tried different settings to maximise power draw from the panel. As you're seeing, it doesn't let panel voltage drop much.


In summer with more sun, more current will come at about the same voltage. The panel is not pushing against a fixed resistance, it's being drained by a sophisticated consumer.


mark-volt avatar image
mark-volt answered ·

Everyone, especially Kev, thanks for your input to my learning curve. I hope to wrap up the answer to my question in a clear and concise order of words, to mark as answered. So that others may find it in condensed format. I will wait however, untill I can access the MPPT to review the latest data. That will then be 14 days only charging with PV, no interference.

"After some time of letting your last comment sit like a charge in my head, my brain lowered resistence and accepted the load" .

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julesspinner avatar image
julesspinner answered ·

9d0b6dfd-00ad-4213-a180-7873589f9e59.pngHi guys. Sorry to jump on this thread but i think i have the same issue. Despite no other load on the batterys my mppt seems to be drawing anywhere between 1.5a and higher.


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Stupidly i have run the battery down very low. And obviously theres zero sun here atm

That's showing that it's charging. Discharge shoes as negative current.

But I think you have posted the same thing in two threads.


mark-volt avatar image
mark-volt answered ·

Lets see if I got the answer to my question.

Question: why does my MPPT not deliver the set charging voltage, or rather, how can I understand the MPPT behaviour?


From top to bottom.

The sunpower that reaches a PV panel is variabel. Sun position relative to the panel and sky conditions dictate the amount of volts and current that can be produced, within the limits of the panel.

The MPPT measures the input power and starts negotiating the optimal power transfer between the panel and the battery. It does this by testing the load it can put on the panel against the resulting voltage drop. It also measures the state of the battery. The MPPT will calculate the power it has to work with, to maximize the voltage/current combination it can supply to the battery.

A battery needs voltage and current, according to it's state. When the state of the battery goes from 'empty' towards 'full', the internal resistence in the battery increases.

When the battery is low in charge it is also low in resistence. So now, the voltage supplied by the MPPT only needs to be a little higher than the voltage of the battery at that moment to charge the battery. In this battery state the MPPT will maximize current over a just high enough voltage.

As the batteries charged state increases, the voltage must also rise to overcome the increased resistence. The MPPT can then up the voltage and lower the current. It will track that regularly to maintain the maximum power point in energy transfer.

The MPPT has by design, complete freedom in transforming voltage and current from the panel, into the most appropriate charging feed to the battery.

It is guided by both the available sunpower, and the state of the battery.

It will present the battery with the optimal voltage/current balance possible, within the panel and battery conditions.

So MaximumPowerPointTracking in the case of charging a battery, is also tracking the batteries maximum charging curve.


That's it folks.

My 10 cents change.


Please add, correct and modify, and hopefully I can mark as answered ;)

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kevgermany avatar image
kevgermany answered ·

On the solar side great.

On the battery side, the charging current is partly inhibited by the rise in battery voltage as it charges. Internal resistance is close to negligible for lead acid and actually decreases as the battery becomes is charged. So can be ignored for this purpose.

What also affects lead acid charging (more than voltage change) is that charge acceptance reduces as the battery becomes more charged, particularly over 80% SOC. This is why good chargers have an absorption phase at constant voltage. If the voltage were increased to push the current up during this phase, the battery cannot accept the charge, so the water is converted to hydrogen and oxygen instead.



(The low internal resistance is why they're used for starter batteries where hundreds of amps are needed)

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