MPPT sizing and solar panel suitability

[dutchvanlife]

Im looking for advice for my off-grid van build. In the van I already have a nice 12V 560Ah DIY lithium battery with a Victron Multiplus and now I want to add some solar. Due to size constraints I found the highest theoretical WP I can get is with one big panel. I am looking at a 495WP JA solar panel(click for specsheet). For the MPPT I wanted to go with the Victron MPPT 100/50. But when I fill in the specs from the sheet into the Victron MPPT calculator, it tells me to get a 150/35 instead of a 100/50. I don't get why it says this since the 100/50 can take the voltage of the panel and the 150/35 will not be able to charge the full capacity of the panel to the battery. Can anyone tell me if I have missed something? Also, would a panel like this even be suitable for a my use? Thanks!

 

[rmaddy]

I just entered the numbers for that panel in both their online calculator and their Excel spreadsheet. The online calculator is giving bad results in this case. I'm seeing the same bad recommendation. In the Excel version I selected the 100/50 and it is a good fit for the panel. You might want to send a note to Victron support with the details so they can fix the online calculator.

One downside to one big panel like that is that it weighs 26kg (almost 60lbs). Installing it will be a pain. There may be some advantages to two smaller panels over one if you can find the right size that gives the same wattage. Smaller panels are lighter and easier to install and if one panel goes bad you can still use the other in the meantime. But if the one big one fits, it should work.

Do keep in mind that a 495W panel will require close to 15 hours of good sun to fully replenish a 12V 560Ah battery from 0% to 100%. Obviously you won't need that level of recharge but even 50% will take close to 8 hours of good sun which could take a couple of days.

 

[HarryN]

I help people put solar panels on conversion vans. The single large panel vs several modest size ones is a routine discussion on the forums.

A good test is to take a piece of 3/4 ply wood the size of the panel that you have in mind and see if you can physically put it on top of the van without scratching it up. Just a slight breeze can play havoc with a big glass kite.

My preference are the ones that are ~ 26 x 58 to 60 inches long and to mount them cross wise on front to back rails. Not cheaper to buy but a lot less risky.

 

[mikefitz]

I don't get why it says this since the 100/50 can take the voltage of the panel and the 150/35 will not be able to charge the full capacity of the panel to the battery

As regards to the controller, I would select the 100/50. Its highly unlikely you will get more than 35 amps from the controller, so the 150/35 would work, but be operating nearer its current maximum.

Mike

 

[dutchvanlife]

@rmaddy thank you for taking the time to look it up for me in the calculator and the excelsheet, very kind of you. The weight and handling of the panel is something I hadn't really taken into consideration, I'll have to see if I'll be able to get a big panel up there. @HarryN and @mikefitz, thanks for your additional thoughts, again very helpful.

One more question, would a single panel like this not struggle in lowlight conditions? The open circuit voltage is 'only' about 45 volts. If I would put some panels in series and get closer to 100 volts then I think the solar array might be more effective in the winter for example. I'm just not sure how big the difference would really be...

 

[HarryN]

In order to be effective in tough lighting conditions, an MPPT controller really needs the panel Vmp to be:

(highest battery charge voltage expected) + ( 5 volts for the controller to turn on reliably) + (at least 5 volts to make MPPT usefully better than PWM) + (5 volts to deal with the lower Vmp in overcast conditions)

So roughly (15 volts) + ( 5) + (5) + (5) ~ panel official rating Vmp of ~ 30 volts in a 12 volt system.

It can operate with a lower Vmp, but will not work as well as it could in tough conditions.

The output of a panel various considerably due to the region, season, time of day (sun angle), weather, and small amounts of dirt (like a windshield has on it unless it is very clean)

The 300 watts on top of my minivan van vary from 25 watts during fire season to ~ 220 watts at that moment of perfection moment of just cleaned the panels and the sun directly overhead. Usually it is in the 150 - 200 watt range in the afternoon.

The more common solar panel Vmp range is called nominal "12 volt type" with a Vmp ~ 18 volts and "24 volt type" with a Vmp label of ~ 36 volts.

Published Vmp numbers are a common industry method of measuring and comparing panels so people can understand how to compare, but it is not a "constant" in real world use. In real world use, the Vmp that the controller will see is often closer to 16 volts in a 12 volt system.

You can wire 2 of the Vmp 18 volt panels in series to obtain an effective Vmp of 36 or continue to add them up as you like.
___________

 

[HarryN]

This is where the PWM vs MPPT controller discussion gets interesting:

A PWM controller does not need such a high voltage to turn on, so it is more like:

(15 volts) + ( 1 volts) = Vmp 16 volts needed to operate.

The Vmp level is reached very early in the day and sustains until fairly late - almost sundown, so while the MPPT controller might be optimal for that perfect moment in time, under real conditions, the PWM controllers actually can outperform them on average through the day and year.

Depending on what product I am building, I use a few different controllers:

For the 24 volt systems, I often pair them with a panel array that is similar to a "24 volt panel" so Vmp ~ 36 volts and a Bogart PWM controller setup. That combination is extremely effective in really terrible lighting conditions and can easily keep up with the wattage that you are planning. It even works in downtown San Francisco which is pretty awful lighting and heavy overcast.

If you have not yet purchased your inverter, perhaps consider to run it as a 24 volt system?

 

[mikefitz]

would a single panel like this not struggle in lowlight conditions?

By the time the voltage falls below the switch off voltage of the controller, V batt +1 V, the current will be very low so there is little to be gained.
Any slight gain in power is perhaps cancelled out by the lower efficiency of the conversion process at higher voltages. Best conversion efficiency is at 1.5 to 2 times V battery ( from the data issued by Epever).

Mike

 

[carboncontext]

In order to be effective in tough lighting conditions, an MPPT controller really needs the panel Vmp to be:

(highest battery charge voltage expected) + ( 5 volts for the controller to turn on reliably) + (at least 5 volts to make MPPT usefully better than PWM) + (5 volts to deal with the lower Vmp in overcast conditions)

So roughly (15 volts) + ( 5) + (5) + (5) ~ panel official rating Vmp of ~ 30 volts in a 12 volt system.

It can operate with a lower Vmp, but will not work as well as it could in tough conditions.

The output of a panel various considerably due to the region, season, time of day (sun angle), weather, and small amounts of dirt (like a windshield has on it unless it is very clean)

The 300 watts on top of my minivan van vary from 25 watts during fire season to ~ 220 watts at that moment of perfection moment of just cleaned the panels and the sun directly overhead. Usually it is in the 150 - 200 watt range in the afternoon.

The more common solar panel Vmp range is called nominal "12 volt type" with a Vmp ~ 18 volts and "24 volt type" with a Vmp label of ~ 36 volts.

Published Vmp numbers are a common industry method of measuring and comparing panels so people can understand how to compare, but it is not a "constant" in real world use. In real world use, the Vmp that the controller will see is often closer to 16 volts in a 12 volt system.

You can wire 2 of the Vmp 18 volt panels in series to obtain an effective Vmp of 36 or continue to add them up as you like.
___________

I'm not sure I see the logic in your calculations for the minimum solar panel array Vmpp.

Firstly, I would say that it would be better to consider the actual MPPT controller output when it is providing absorption voltage to the battery, rather than the battery voltage itself. For a 12V battery, the 12V 4S LiFePO4 Battery w/ BMS this is 14.5V.

Secondly, the 5 volts gap for the controller to turn on properly should be surely be in reference to the Open circuit voltage (Voc), not the Vmpp. If the MPPT is off, then no current can flow from the panels, and so they can only produce the Voc.

Thirdly, every source I've seen suggests that the voltage produced by solar panels remains more or less constant regardless of irradiance levels. This is true until only very low irrandiance is applied, at which point the panels are producing next to no power anyway. You can actually see a visualization of this in the charts at the bottom of the spec sheet posted by the OP.

Finally, I would say that its important to consider heating of the solar panel when looking at voltage. Most panels will have a voltage temperature coefficient of around -0.25%/C. The panel posted by OP, has (at STP) Vmpp = 38V, Voc temp coeff = -0.275% rated at a cell temperature of 25C. Lets suppose the panel heated up to 85C, the Vmpp should now fall to 38 * (1+(0.00275*60))=31.73. It would also be a good idea to consider voltage drop as the power flows from panel to SCC, lets assume a high value of 5% of this, reducing the minimum Vmpp the SCC would see from the panels to 30.13. This is more than enough for our 12V battery, and is actually sufficient for even a 24V setup. Victron MPPT controllers only require a 1V difference between the panels and the output power once they are turned on.

 

[HarryN]

I'm not sure I see the logic in your calculations for the minimum solar panel array Vmpp.

Firstly, I would say that it would be better to consider the actual MPPT controller output when it is providing absorption voltage to the battery, rather than the battery voltage itself. For a 12V battery, the 12V 4S LiFePO4 Battery w/ BMS this is 14.5V.

Secondly, the 5 volts gap for the controller to turn on properly should be surely be in reference to the Open circuit voltage (Voc), not the Vmpp. If the MPPT is off, then no current can flow from the panels, and so they can only produce the Voc.

Thirdly, every source I've seen suggests that the voltage produced by solar panels remains more or less constant regardless of irradiance levels. This is true until only very low irrandiance is applied, at which point the panels are producing next to no power anyway. You can actually see a visualization of this in the charts at the bottom of the spec sheet posted by the OP.

Finally, I would say that its important to consider heating of the solar panel when looking at voltage. Most panels will have a voltage temperature coefficient of around -0.25%/C. The panel posted by OP, has (at STP) Vmpp = 38V, Voc temp coeff = -0.275% rated at a cell temperature of 25C. Lets suppose the panel heated up to 85C, the Vmpp should now fall to 38 * (1+(0.00275*60))=31.73. It would also be a good idea to consider voltage drop as the power flows from panel to SCC, lets assume a high value of 5% of this, reducing the minimum Vmpp the SCC would see from the panels to 30.13. This is more than enough for our 12V battery, and is actually sufficient for even a 24V setup. Victron MPPT controllers only require a 1V difference between the panels and the output power once they are turned on.

The Vmp of a panel is not a constant, in spite of what some claim. It is a constant under some specific lighting conditions (for instance a test stand with a constant angle and spectrum / varying intensity ).

In the real world, the panel Vmp does shift substantially, and it is at least in part due to the spectrum changing over the course of a day, overcast conditions, etc. I have 2 solar test stands at my shop and routinely watch this happen, and it happens on both mono and poly panels.

You are correct that the solar controller turn on is in fact a Voc effect, not Vmp as I incorrectly wrote.

I have not been able to achieve anything close to a reasonable working setup with any MPPT controller with only a 1 volt (Vmp - Vbat). In fact the exact arrangement of nominal 24 volt panels into a 24 volt battery pack failed to work (it was an AGM setup) in overcast conditions. It just would not stay on. I am absolutely convinced that to make MPPT work well, the (Vmp - Vbat max) needs to be at least 10 volts in order to be useful.

I swapped them out for bogart PWM controllers and the problem went away, because those will in fact work with a very low (Vmp - Vbat) differential and in overcast conditions.

 

[eXodus]

Do keep in mind that a 495W panel will require close to 15 hours of good sun to fully replenish a 12V 560Ah battery from 0% to 100%. Obviously you won't need that level of recharge but even 50% will take close to 8 hours of good sun which could take a couple of days.

12V systems usually include alternator charging as backup., and hardly anybody every charges from 0%

I have not been able to achieve anything close to a reasonable working setup with any MPPT controller with only a 1 volt (Vmp - Vbat). In fact the exact arrangement of nominal 24 volt panels into a 24 volt battery pack failed to work (it was an AGM setup) in overcast conditions. It just would not stay on. I am absolutely convinced that to make MPPT work well, the (Vmp - Vbat max) needs to be at least 10 volts in order to be useful.

One of the reasons I run 80V from the panel to the MPPT for a 24V system. It turns on very early in the morning and stays on till it's almost dark.
I can confirm your observation, most of the time the MPPT turns on around 35-36V (LFP batteries are resting at 26-27V)