Lowest OCV An MPPT Is "Worth It"?

Hi Mex,

Blue Sky 3024 derates the amperage when using higher voltage. I would be unable to increase my wattage by much, but I am operating pretty much in the "sweet spot" for MPPT conversion of volts to amps.

Some folks have suggested that the charging day is effectively longer with higher voltages. I don't know if this is so or not.

Mex, you have to be careful with percentage gain and absolute gain as to what is worth it.

So if you got 20amps at high noon with PWM you would get 22amps with MPPT. Earlier and later when amps are lower it is still 2 more amps but the percentage gain goes UP. 2 over 10 is 20% (battery voltage is also lower earlier in the day which gets you more for MPPT so it is still 2 amps)

The percentages sound terrific, but it is still only 2 amps more for the day of say 8 hours--16AH. Do you even notice 16AH out of your day? I don't.


It makes no difference in MPPT if you have 12v panels in series to get 24v or one 24v panel except when you start looking at things like controller Voc input limits. A BIG DEAL!

There are all sorts of other factors to do with physical panel size and weight, controller read-outs and adjustable set points etc.

A factor often ignored is that you can mix and match by having more than one controller on a bank. I can have PWM 12v and 24v MPPT on the same bank and that is no different in daily AH from if I had matched panels and a very expensive bigger controller.

The limitation there is my PWM controller cannot do the 24 panel and I can't put the 12 and 24 on the same controller. I could if that one 24 panel were two 12s though. 12s give you more options for set-ups.

In one example, I am looking at panels with 21 VOC and optimum 17.5 WV

Head-to-head, how much or how little will MPPT offer over PWM?

Discharged every day, at first the battery bank has zero chance of 100% replenishment with the first batch of panels.

There is a massive tradeoff comparing high voltage high wattage panels at low cost with an expensive MPPT -VERSUS- higher cost lower OCV panels using PWM. My interest is knowing the parameters of the "Value Bell Curve". How "much" is minimum? At what WATTAGE point of value does high OCV panels with MPPT offer a clear advantage?

Pardon my lameness. We engineers like to have things explained to us minus buzzwords, and emotional bias. I hope you understand. Thank you.

Mex enjoys tinkering with batteries and charging them, as much as he enjoys tinkering with the finest minds here in the tech section, and making people, newbies included, actually think, and analyzing their actual needs, with an emphasis on fine tuning the equipment to the specific application.

I'm fine with making people think for them selves. Plant some seeds, sit back, and watch them grow, just like this thread.

Good points made above. There are many variables so each person needs to find his own answer depending on his situation where and when he is using solar for his RV, and especially what he expects the solar to do for him.

Some want it to get the batts "full" every day or at least often enough, while staying above 50% SOC. Some just want to go longer before needing a recharge so they can wait till they get shore power and not have to haul out the generator during a long weekend.

I have posted some direct comparisons using PWM and MPPT which can be found in the archives so I won't repost all that again here. You do get a small amount of AH more with MPPT, but this may not help you have a better day. Eg, you might get to "full" either way that day, just a little sooner with the MPPT. Do you care if you get there by 1pm instead of by 2pm? "It depends."

Mr Wiz has it right about the individual charging profiles of various charge controllers. It makes a difference only if you manage to get the batts to mid 14s early enough in the day so what the controller does next matters. IE, when there is enough daytime left to still get something you want to do done.

Some controllers just stay at the 14.4 until dark. (many of the $13 PWMs are like that--they work great BTW) This can be good if you need all that time till dark to continue the recharge or it can be bad if you want to drop to Float sooner so you don't spend "too much time in the 14s" for battery life if that is a concern.

The LandStar PWM (cost $40) I had last year was not adjustable,. But it did go to 14.6, stayed there for two hours, and then dropped to 13.8 if it was still daylight by then. That was good profile for our situation.

It can be useful to have some adjustable voltages in the controller you can play with to suit your own situation, but not always. Here is where the dollars go buying controllers, where you can pay vast sums for something that may not do you any real good.

On solar now with 130w PWM on a 270AH bank and separated from that, 230w MPPT on a 458AH bank. It varies depending on AH draws which bank gets closer to "full" each day. When one gets "done" and there is enough daylight left, I can use one to help out the other. This is where the different controller voltages and profiles come into play.

The PWM Solar30 (cost $34) has an adjustable high set point and once the batts reach that voltage it stays there till dark.

I have it set to 14.8v. If I want to do an "overcharge" to get the battery SG right up to "true full" then I can raise that set-point to 15v. It happens by a fluke, that this adjustment is out of calibration by 0.5v so that works out perfectly for this job--it goes to 15.5v and stays there. ( I set it at 14.3 to get the 14.8)

It also has an ammeter reading but this one is calibrated right so that is very useful. The Trimetric is set on the other bank and without the ammeter in the Solar30, I would not see what the 130w panel is putting out. Meanwhile, the other bank is on the Eco-Worthy MPPT (cost $102).

This controller has adjustable voltages too, except now you can set the Float voltage as well as the high set-point. I have that at 14.8 too, but as soon as it gets there it drops to Float. This is not what I want, same as Mr Wiz mentioned in his case. The darn adjustment for Float only goes to 14.4. not 14.8 so I have it there. I find that 14.4 if on long enough daylight after being to 14.8 is high enough to get the batts near full SG, so that works out ok.

What to do with the extra amps of one if the other is behind that afternoon? I jumper the banks temporarily. I can get more amps into the other bank and also if desired I can run the 458AH bank to 15.5v on the PWM controller to do an overcharge.

If I spent a ton of money, I could get an MPPT controller that did go to 15.5 if set there. Or for free, I could do panel-direct. :)

Mex would need to say what he wants to achieve and what his equipment options are before anybody could say what might be "worth it." Even then only Mex would really know what would be "worth it" for him in particular.

Hi,

I chose to go with 33 volts in a series/parallel configuration. At the time the panels cost about the same as an MPPT charge controller, so MPPT was the way to go. The choice allowed me to use the #10 prewire from the factory and have low transmission losses.

Now, with cheap panels, if there is room on the roof a pwm offers a slightly better "bang for the buck". Why only slightly better? Because 17 volt panels are generally more expensive per watt than higher voltage ones.

I do regularly see 17 amps from 256 watts of panels. In winter I've seen 15.4 volts and that has sometimes caused the inverter to shut down from over voltage.

My only mistake was maxing out the charge controller. That means any upgrade in wattage will require a new controller.

I'm running 48 volts with a 10 gauge wire, MPPT is the only way to go.

I did comparison tests between my Xantrex PWM and my blue sky 2000e solar boost mppt , the 2000e mppt won out
If you are looking at high voltage panels, you have no choice, use mppt or waste a lot of watts
If your looking at panels 17.5 to 19v , it's going to depend on the particular controllers being compared
My Xantrex basically goes four hours at bulk then shifts to 13.2 volts , no good to me because my batteries are not recharged in only for hours of solar
And it does not restart/reset unless it's night or I disconnect the batteries aka manual reset, but this is a quirk in the Xantrex programming and might not be in the charge profile/program of the PWM you are looking at

Hi,

I used to have a PWM 15 amp controller, and to that I connected a 3 way switch. With it I can turn off the solar, or switch from PWM to directly to the battery. I could watch the amp meter both ways, and got a minimal increase in amperage with it "On", VS going through the PWM controller. The controller triac does get warm, so there is some voltage loss there.

With my SolarBoost 50 controller (MPPT) they report that I should get 20% more power under certain conditions. They measure the amperage going into the controller and the amperage going to the battery, and claim that their controller magically increased the amperage. And it does! So I might have 22 amps going into the controller and 25 amps going out. This is because the controller it letting the solar panels put out nearly 16 or 17 volts, while the controller is converting it's 22 amps X 17 volts to 25 amps X 13 volts to recharge the depleted battery.

The math works too, as there is more wattage going into the controller than coming out. 374W input and 325 W output.

What they do not tell you is that the solar panel will also put out more amperage when it is not restricted in producing higher voltage. So if a bypass switch is installed, the 21 volt open circuit panel can put out 13 or 14 volts at a much higher amperage than while putting out the 17 volts that the MPPT controller restricts them to.

So back to the original question. I probably will not be buying another MPPT controller (well the SP50 will handle my needs for now) because the cost of solar panels has dropped significantly! Even tilting and tracking panel racks have become a much longer time to pay for themselves with the extra wattage they allow you to collect. Why? The cost of solar panels themselves has dropped by 1/3 to 1/2, so installing more panels and not tilting them has become cost effective. So is the lower cost PWM controllers. At $13 for a 10 amp controller, you can afford several of them for the system, and not spend $300 or more for the MPPT controller.

If you are working with high voltage panels and 12 volt batteries, then the MPPT controller is sort of a requirement. Yet you can also order 12 volt nominal panels and connect them with the lower cost PWM controllers, and install the whole system fairly low cost.


So to have 1,000 watts input to the battery each hour, you can install a 2 axis 1,100 watts worth of panels, and a MPPT controller, and hope for the best. This would be the most expensive system. $1,000 in panels, $600 for the two controllers, and $600 -$800 depending on the cost of the panel mounts and the base they need to sit in.

You can install 1,200 watts worth of panels and fixed mounts, still collect about 1,000 watts with a MPPT controller. $1,200 in panels and $600 for two controller rated at about 50 amps each.

You can install about 1,400 worth of solar panels, 4 each PWM controllers, and fixed mounts, and have the lowest cost system. It will still put more than 1,000 watts into the system, using only about $1,400 in panel cost, $80 in controller cost, and wiring.

Fred.

KD4UPL wrote:
I don't understand the question.
Every installation we do uses an MPPT charge controller so I'll try to pass on what I know about them.

"Open-circuit voltage (abbreviated as OCV or VOC ) is the difference of electrical potential between two terminals of a device when disconnected from any circuit. There is no external load connected. No external electric current flows between the terminals. It is sometimes given the symbol Voc.

The open-circuit voltages of batteries and solar cells are often quoted under particular conditions (state-of-charge, illumination, temperature, etc.)."

17.5v, 18v, 20v, 22v??