MPPT or PWM for older panels in parallel?

[markopolo]

There's a 40 watt panel from 1994 and an 85 watt panel from 2006 on my van. I assume they are the 12v type and are paralleled. The van has spent a lot of its life under cover in garages.

The controller dates back to 1994 also I think...... I think it is time to upgrade the controller. I don't have any specs on it and it seems to work intermittently. I'll see 5 amps coming in then 0 amps then 5 amps again etc.

Is there any reason to spend the extra bucks on an MPPT controller on a old 125 watt array? I have no plans to purchase more panels for the van.

[avanti]

I'm no expert on solar, but as understand it, you would get MORE benefit from a MPPT controller on a crappier array. Doesn't the benefit accrue when the output of the array is marginal?

[gerrym51]

unless there is a substantial price difference-which i think not-i would go with mppt

[mlts22]

I'd go MPPT. It not just helps with getting more energy from a panel, but they tend to be multistage, figuring out when to bulk charge the battery versus floating it. Some like Morningstar's even can equalize.

[booster]

That is a real good question. Not only do you have a year built mismatch, but also a size mismatch. You may also have design type (poly or mono) mismatch, and a number of cells mismatch. Yowser!

My best guess is that the best thing to do is use two controllers, one for each panel, with the output voltage set the same. MPPT would probably give you the best output, but another like you have might be the best solution.

You may want to give Blue Sky a call and ask them, once you have all the data on your panels. When I called Grape Solar about the added panel I did this year (which was generating a lower voltage than the ones I had, he said that if it was within a volt or two, and the same number and type of cells, it would be fine (it was the identical match to the others). If it was off of that, he recommended two controllers.

[markopolo]

Thanks for all the suggestions and info.

I bought and installed a really inexpensive PWM controller to use to test the setup and give me some baseline info. First problem I've run into is that there are only 3 wires coming into the old controller - batt +, solar + and a common negative or ground (battery neg & solar neg joined somewhere). There are two panels in parallel on the van. I think the common neg or ground is a problem for the new controller. The old controller was meant to be installed like that. I installed it (split the existing single ground to test) and drove the van out of the garage to check input. It was very overcast but voltage remained well over 13v even after removing all surface charge by running the fridge electric element so obviously there is input. However, the controller charge light never came on nor showed any amps flowing in. I don't know if it is a defective controller or the common ground causing the issue. My understanding is that current can only flow in one direction along a wire at a time. The controller could be using the neg wire to measure amp flow. It going to be fun figuring out this old wiring!

[booster]

Does the shared negative wire check to van and battery negative? I would imagine it does. They may have used the chassis for the negative of the solar to save a wire from the roof. At least on the controllers we have had, I think they need the battery ground separate to measure amps to the battery, and the solar negative to measure input amps, as both get displayed. Were you able to find a manual for the old controller?

[markopolo]

Yes, I found the install sheet & info on the old controller: http://specialtyconcepts.com/SPECIALTY_ ... _SHEET.pdf

Common neg, amps in were measured on the positive side.

[markopolo]

I should add that I think the on / off operation of the old controller was normal after reading the info on the spec sheet.

[booster]

Made sense as soon as I saw the picture of the controller. When I was messing around with getting an ammeter for alternator to coach, I learned that only analog meters (or inductive) will work on the positive line. Digital meters all want to be on the negative. Your original looks to have analog meters, so that would allow the common ground. New controller must be digital.

I just looked at the Blue Sky install instructions, and it says specifically to not tie the grounds together for battery and panels, but it is an mppt.

[markopolo]

I separated the ground wires today and that solved the issue with the charging light and ammeter. This controller also shows the panel voltage and the battery voltage so it is a useful tool for $28. delivered.

I used the info Booster posted here: http://www.classbforum.com/phpBB2/vi...php?f=9&t=3605 to see how the array output compares to what I could get. I'm seeing lower than expect wattage. It might be coincidence but I get the results expected from a single 85 watt panel not an 85 watt plus a 40 watt panel. The next test will be to separate the panels and test them individually to make sure both panels work. If they both work individually then I'll go with two controllers.

[booster]

You may be getting into an issue where the panels mismatch enough that one panel is actually consuming power from the other. Grape told me that the rule of thumb is to make sure they have the same number of cells per panel (or on a panel with multiple sections with diodes, the same amount per group), and that the cells are same type (so are same voltage). You may want to just check the open circuit voltage on each panel and see how far apart they are, as that eliminates all the counting and guessing and gives you the answer right away. You might just need a blocking diode on one of the panels.

[booster]

It is crazy how stuff doesn't like to play well together.

We had a couple of odd things happen with our solar on the last little trip, which was first with the 300 watts and running the coach off the third charger leg. The solar would sometimes be on like it should be, sometimes it would be off for no apparent reason. No rhyme or reason to it that was obvious. Checked it out today and found the the Blue Sea ACR control relays have some funny safety "rules" that the follow. What was getting us was that they won't close if they don't see a battery voltage on each side, before the charge voltage comes on. But, if they are already closed, and you remove the battery from one side, they will stay closed, and that is what was getting us. If we ran the van, with the engine separator on so it was charging the coach, it would close the ACRs, and then the solar would hold them closed when the van went off. If you just had the solar on, the ACRs wouldn't close because they couldn't see the engine battery. What got us was that if the the ACRs were closed, then held closed by the solar, the solar power could loop back to the third leg of the charger, shutting off solar like if it was on shore power. If the van wasn't run first, the solar would stay on. So the solar was had its own automatic, self generated shutoff!

I took the solar off the auto shutoff control and we will run it manually until I do the single bank upgrade when I should be able to address it.

Nice day today, so I took it out and tested the output of the 300 watts in parallel, particularly because of the #10 wiring from the panels. The solar calculator predicted about 252 watts out of the panels. We were getting right around 225 watts input to the controller, and 208 watts out to the batteries, so our percent under predicted is a bit lower than we were with the 200 watts, I think, but I will have to check. Being over 200 watts will more than take care of what we need, so we should be good, and it will get better with higher sun.

[markopolo]

Nice day here too.

Do I use battery voltage or panel voltage to calculate watts? 6.2 amps were coming in at 2pm ........... better than nothing

[booster]

Originally Posted by markopolo

Nice day here too.

Do I use battery voltage or panel voltage to calculate watts? 6.2 amps were coming in at 2pm ........... better than nothing

If you are trying to see what the panels are putting out, I would use the panel output/controller input voltage and amps, if you have both like I do. With a PWM controller your amps in and out probably will be the same, but the volts will drop. We lose about 15 watts through the controller.

You are getting fairly close to the same ratio as I did today, although I was at 11:00 sun time. You are about 80 watts from 145 watt panels for 55%. We were 208 watts from 300 watt panels for 69%. You definitely seem to be getting some power from both panels. Not bad for old mismatched panels

[markopolo]

Maybe my setup is performing OK

Solar noon is 1:16pm today here. This calculator - http://www.where-rv-now.com/Notes/Solar/#Calculator - indicates 6.4 amps would be expected from 125 watt panels at this latitude at solar noon today using a basic controller. I left the 16.9v default in the calculator. I plan on washing the van today so I should remember to check what's coming in at 1pm-ish.

The sun's altitude at 1:16pm will be 59.86 degrees. Sine is approx .86 so 125 watts x .86 = 107.5 watts.

[booster]

That is an easy to use calculator, for sure. I don't understand for sure why he gives the expected output in amps. When you do the conversions and compare it to the other calculator, it appears he is counting on you having the max power rating voltage (for you panel) at all times, and that is not what happens with ours.

I plugged in our data, and had his predicted amps, but it was at quite a bit lower voltage than the maximum panel, so I had less watts. The predicted were pretty high if you used the maximum output voltage. I wish he would have given the results in watts, as that is what really matters.

I need to do a few comparisons with the other calculator at different spots and see how it all compares.

It does sound like you are in pretty good shape-do you know what the voltage was when you got your 6.4 amps?

edit

Just read his blurb a bit more-he states non-current boosting controller, so his amps is going to be the same in and out of the controller, I think. The calculator is appears to be using the panel max output voltage to calculate the amps off of the solar predicted watts (like from the other calculator). His actual usable wattage, I think will be less than the amps time max output voltage, and will actually be the amps times his charge voltage.

I need to watch and see how much, if any, the MPPT charger improves over what you would get with the PWM, assuming it works like above.

[markopolo]

It was around 13.2v with a load I think. Amps in would jump around 6.2A, 6.3A & 6.4A.

Today's numbers were interesting and fun to watch.

I put a 120 watt load on the system to make sure I was getting the max input from the panels.

With the load:

6.3A coming in & 13.2V at panels with 12.5V at the batteries (under load).

No load: (as the batteries recharged)

14.2v Panel, 6.2A, 13.7V Battery
14.3v Panel, 6.2A, 13.7V Battery
14.6v Panel, 6.1A, 13.9V Battery
15.6v Panel, 5A, 13.9V Battery
16.1v Panel, 3.5A, 14V Battery
16.4v Panel, 3.2A, 14V Battery
17.4v Panel, 2.9A, 14.1V Battery
17.7v Panel, 2.3A, 14.1V Battery
18v Panel, 2.A, 14.1V Battery
18.3v Panel, 1.5A, 14.1V Battery
18.4v Panel, 1.2A, 14.2V Battery
18.8v Panel, 0.8A, 14.2V Battery
18.8v Panel, 0.7A, 14.2V Battery
18.9v Panel, 0.6A, 14.2V Battery

The batteries were fully charged when I started the testing so they recovered quickly.

[booster]

It is very interesting that your readings that are at the max output, like the ones with the load, and the early battery charging are all just about right at 85 watts at the output of the controller. Not bad, I would think for the system you have. I would think you will easily get 40ah on a good solar day.

[booster]

We had really good sun today, so I moved the van onto the most level area to check some more.

I would have expected to get the most output when the panel was at the max power voltage rated volts (17.7) but I didn't. I got maximum panel output at about 16.5 volts with the output to battery side loaded down to get to about 13.0 volts (charge setting at 14.7v).

Level van and panels, highest sun which was 59.89 degrees.

Highest output was at

16.7v and 14.1 amps from the panel or 237 watts

On the output side it maxed there also

12.9v and 17.4 amps or 224.5 watts

The solar predictor based on sun angle predicted maximum of 259 watts so we are at about 91.5% from the panel, and 86.7% at the controller output. Using the 86.7% on the predicted AH per day prediction on Marko's calculator, we should get a bit over 120AH a day under very good conditions and the same sun angle. That is a third of our battery capacity, and about two+ days of use.