question

tylera avatar image
tylera asked

Difference in Solar wattage between 24V and 48V system

Background: I'm on a boat with a 48V system and a 24V system. I have the means of charging the 24V system from the 48V system (not covered in this post). I have one set of solar panels which are connected to two MPPTS (I only activate one MPPT at a time), one is on the 48V system, one is on the 24V system. *



What I noticed was odd. When I switched from the 48V to the 24V, I noticed a doubling of wattage. Here are some graphs:


screen-shot-2021-04-21-at-90151-am.png


ID 256 is the 48V MPPT, ID 258 is the 24V MPPT. During this time, the MPPTs were in bulk. The lithium batteries are configured to push up to 56.8V/28.4V.**


Here's the voltage on the solar side:


screen-shot-2021-04-21-at-90157-am.png



I'm not sure why the voltage is held ~constant on the 48V (but solar) side. However at just above 60V it's still above the 56.8V needed to push to the battery, so I don't expect this is why the wattage is half. But I'm not sure. I do suspect the solar panels could be arranged differently, better (and probably arranged differently to take advantage of whether I'm pushing to 48V or 24V), but I suspect that might not be related to why the wattage is half.


Help?



[* Further background if you're curious: I'm on a boat with 48V propulsion. When I'm underway, I want the solar feeding into the 48V directly. When I'm not, I want it feeding into the 24V house battery bank instead of feeding it first into the 48V then charging the 24V off the 48V. Additionally, this lets me charge the battery over more time during the day.]


[** The 48V battery is a Valence which could be configured to charge up to 58.4V, but since we're not even to 56.8V, I suspect that isn't the issue here. The 24V is a Victron Li and 28.4V is the correct number here.]

mppt charging
screen-shot-2021-04-21-at-90151-am.png (291.3 KiB)
screen-shot-2021-04-21-at-90157-am.png (220.6 KiB)
2 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

tylera avatar image tylera commented ·

I believe (I use the word "believe" because the boat builder assembled the panels, the wiring is rather hidden) that there are two banks in parallel, each bank is (140W+140W+120W) in series.

0 Likes 0 ·
tylera avatar image tylera commented ·

The MPPT for the 24V is Victron 100V/30A, and the MPPT for the 48V is victron 150V/35A.

0 Likes 0 ·
2 Answers
wkirby avatar image
wkirby answered ·

So, MPPT instance 258 is the MPPT for 24V system and 256 is for the 48V system.

Your PV Voltage and current graphs hold the answer to your question.
Of these graphs, if you study 258 (24V) you can see that the PV Voltage is down at around about 48V. The product of this Voltage and the resulting PV current yield the most power from the PV array. Pick a spot at around 14:00 47V x 9A and you get 423W. Your power graph agrees with this too.

Let's head over to the other MPPT 256 (48V system).
Here you can see a rather flat line at around 60V instead of a curve in relation to sun light power during the course of the day.
The MPPT cannot operate below battery Voltage, so it can only search for a maximum power point somewhere between just above battery Voltage and the maximum PV open circuit Voltagae at a given time. Because the battery Voltage is higher, the window of search for maximum power is much narrower. Indeed, as shown on the 24V system, the Voltage of the maximum power point is at around about 48V. The MPPT cannot search down to this low a Voltage so it never finds the optimum power point for the PV array. The optimum power point that it does find is limited so it only gives what it has to work with which is a lower output power.

The way to remedy this is to present a higher Voltage to the MPPT controller.
You didn't mention which model of MPPT you have nor the current arrangement of your PV array. If you rearrange your PV array with more panels in series, you'll get a higher PV Voltage and so the Voltage at maximum power point will also be higher and hopefully above the 48V battery Voltage.

Of course, your PV Voltage must not exceed the maximum input Voltage of your MPPT.

Let us know some more of these details and we can advise further if you so require.


2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Elimac avatar image
Elimac answered ·

Are you sure the MPPTs ID are not in reverse?


You missed also to say the most important things:

1. What is actually your true PV Peak Watts installed;

2. What are the MPPT specifications.


I will assume for now you have ~>400W installed and that your MPPTs are rated with 10A output. Or something like this. If so, and assuming the MPPTs are in reverse ID, your observations are completely normal. Your wattage is limited by the MPPT output current. So, for example:

In 24V systems, you can extract tipically 27V x 10A = 270W max (aprox, depends on actually voltage)

In 48V systems, with the same MPPT - *and provided you have enough PV installed* - you can extract double the wattage: 54V x 10A = 540W max. But if your PV does not provide 540W, you won't reach it.


EDIT: If you are sure the IDs are really correct, then you must tell us more details about the PV. Number of pannels, voltage and current, and the way they are connected. Are you sharing them to 2 MPPTs ??

It could be that you don't have enough voltage to drive correctly the 48V MPPT input and it is barely working... add more pannels in series in this case, provided you have voltage room on the MPPT -> need to know MPPT specs.

1 comment
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

tylera avatar image tylera commented ·

IDs are correct. MPPTs are 30A (24V) and 35A (48V). I'll post the solar arrangement in the question.

0 Likes 0 ·

Related Resources

Additional resources still need to be added for this topic