Parallel rather than Series?

[Nobadays]

I'm trying to help a neighbor with a small 12v solar system for his cabin. He ended up running his batteries down after blowing a fuse on the cable between the SCC and the batteries. We think he may have blown the fuse by using a battery charger ran off a generator while the batteries were also being charged by the SCC, but really not sure why it blew.... might have been a third panel he added... more about that below.

He bought his panels, SCC, batteries and inverter through a local Amish business. The SCC came with 10' of PV cable/MC4 connectors attached and about 5' battery out cable with alligator clips attached, all wire 10awg (2/0 from batteries to inverter). The Amish supplier drew him a picture of how to wire everything.... and he chose to have him parallel the 2 panels. The neighbor went back a week later and the guy sold him one more panel, it too to be wired in parallel, said he though that would be ok.

Specifics:

Panels: 240watt VOC 37.0, VMP 29.9, ISC 8.59, IMP 8.03
SCC: Temank 40a, 100v, 550w max @12v MPPT
Inverter: Aims 2000/4000 12vdc
Batteries: 2 x 420ah Trojan 6v wired series for 12v

So with only a 10' wire run from the panels to the SCC line loss would be minimal. But, I guess my inclination would to be to series the two panels and let the MPPT SCC convert the extra voltage to amps to charge the batteries. QUESTION: Does it make that much difference in this case? With these panels in series he is looking at around 16amps input/output for charging... correct?

Now for that 3rd panel...he had added it in parallel but took it off when problems began. After looking at his energy usage I calculated that he would use roughly 12% each day (if he shuts the satellite internet off at night, 20%+ if he runs that 24/7.... I think he will be shutting it off now.) Where we are in the mountains he get ~5 - 6 hours a day of charging sunlight. (For the following I based my figures on seriesed panels so I'm guessing paralleled might be different? Showing my solar math ignorance! See below. ) Figuring an average of about 19 amps charging current (conservative I hope) I'm coming up with about ~5.7hrs for charge/absorption. If he added that 3rd panel in series - it would definitely require a new 60/150 SCC - he could charge back up in about ~4.7hrs. PLEASE CHECK MY MATH. In a parallel configuration 2 panels should provide ~16amps for charging so, ~6.15hrs. With 3 in parallel, ~21amps or, ~5.4hrs. Really uncertain on these numbers.

With the 3 in parallel he would be coming really close or at 100% charge each day the sun shines, if my math is correct. The SCC is 40a so good there at ~21amps, in parallel the voltage would stay ~30.0 (his SCC today was reading 34.2v coming in) so not exceeding the 100v in the specs, BUT.... with 3 240w panels in parallel this comes to 720w, far exceeding the specified 550w max. Does this matter if the amps and voltage are well within specs?

I really don't want to mess up his equipment, if it were mine I'd go for it! Is there a better way this system could be set up.

Thanks for the help!

 

[jasonhc73]

Just configure the panels to whatever the inverter is capable of.

The question to answer is, is the area subject to cold weather? The PV voltage is much higher in the early day in the cold, and that could ruin the inverter. If you are within the specs of volts and amps, max it out, and let it work.

 

[GMB]

This video does a good job of explaining the math behind hooking up solar panels in series and parallel. Should answer a few of your questions.

 

[Nobadays]

Jason, yes the area is subject to cold weather.... we are at 9,500' in southern Colorado. Aware of spiking voltages in very cold temps, but if using the 3 x 240watt panels wired in parallel he would see voltage at only 1/3 of the 100v the SCC is capable of handling (vmp 29.9) and amps at roughly 1/2 what the SCC is capable of handling (imp 8.03).

But.... I guess I can't wrap my head around the SCC stated max wattage of 550w. Can we connect 720watts...If, the voltage and amperage inputs are within safe ranges do the watts matter?

GMB... good video with straight forward explanations of the math. I knew that running series increased the voltage allowing longer wire runs with smaller wire... we have done this on our cabin and workshop systems. Our runs are in the 30' - 40' for those systems. But I was also under the impression that an MPPT charge controller will take that excess voltage and increase the output amps for charging the batteries. I guess I thought that was superior to parallel wiring. In the video he indicated parallel was preferred over series. Interesting!

Because my neighbor's wire runs are short, parallel will work fine. Now to answer the question above.....and still wondering why the fuse blew between the SCC and batteries.

Thank you!

 

[chrisski]

I think the charge controller has a limit of 550 watts for a reason. I’m pretty big of sticking it specs, so I’d say no. That spec isn’t just for the day, but for the year ater year of use you can get off it.

Some charge controllers will allow double the wattage if you go to 24 volt, so perhaps that’s an option, Which would come out to 1100 watts at 24 volts

There the “myth” of needing to connect the battery before the SCC and disconnecting in vice versa. I say :”myth” because every single SCC mahal I read said it, but there’s plenty of people who say it doesn’t matter. I follow the manual and disconnect panels first. If something happens like I forget to turn oN the battery first, I’m not worried about My SCC blowing.

I would not go against what the manual says.

 

[Nobadays]

Chris... thanks, and likely good advice. He really needs a bit more power to get his batteries topped up each day.... hoping there is a way to use that 3rd panel! Yes this SCC will allow 1100w @ 24v. However my neighbor already has a 2000/4000watt, 12v inverter so he is pretty well stuck with a 12v battery bank.

In my workshop system I run 24v but use a buck convert to drop the voltage to 12v for my 600watt inverter.... just for lights, diesel heater and internet extender. A step down converter for his 2000watt inverter would cost more than an inverter itself.

 

[Hedges]

Can't put 3 panels in series because Voc > 100V even at 25 degrees C.
All in parallel should be good. Current approaches 30A so wire ought to be 10 awg.
Individual fuses per panel could be 15A or whatever label says.
Technically, 10 awg supposed to have 30A fuse but I'm not worried because I know it can handle 40A and panels will rarely reach 30A.
If a single fuse, better be at least 35A or panels could blow it.

I would orient the three panels in various direction, so not all peak current at the same time. That might drop current to 21A, and then 30A fuse would be fine. Also drops peak watts to within what the SCC can do.

Alternatively, buy one more panel and wire 2s2p. Aim one pair in series about 9:00 AM and other 3:00 PM (non-daylight savings time), so peak power might bit 700W not 1000W. That's STC ratings, pretty close to 550W PTC.

If he needs more watts winter or summer, tilt panels according to the season.

 

[jasonhc73]

An MPPT basically just turns off the solar panels, if there is too much for too long, or the battery is fully charged, or no load.

Do like Hedges said, Have one face East, one South, and one West. There, problem solved. No way to have all three amping out the charge controller.

 

[Nobadays]

Hedges, Jason.... thanks! These panels are mounted on the roof of a medium sized well house... 4 panels max across the roof. It is south facing, in a forest. Just not sure how easy it would be to aim them different directions. I guess I'm not seeing 30 amps in parallel happening. Even using the ISC max should 25.77... maybe at very cold temps it could approach 30 amps, but with a 40 amp SCC he would still have some head room. I'm not familiar with STC and PTC ratings but if the suggested 1000wats of panels aimed as indicated would yield around 700w STC... and 550w PTC, it would seem 720w720w- all 3 aimed south - would also be just over the 550w PTC.... correct?

 

[Hedges]

STC is standard test conditions: 1 full sun of 1000W/m^2, panel at 25C, test with a brief flash before it warms up.
PTC is something like 25C and 1m/second wind over it. Typically 10% or 15% lower power.

MPPT charge controller delivers more current than panels produce. Similar to a transformer, takes in volts x amps = watts and delivers (lower volts) x (more amps) for same watts. Three panels in parallel are 720 W STC, maybe 620W PTC, would deliver 50A into a battery that is 12V at the moment (less current as voltage rises to 15V)

Yes, 720W all south might be close to 550W, not sure that was the problem especially late in the year with sun lower.

South facing sloped roof is good, but can you tilt one partially West and one partially East without shading the others?

Instead of:

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Or maybe as a trapezoid like:

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As for hours of sun, look up an "Insolation" map or calculator. In San Jose I figure average over the year 5.5 effective hours of full sun. Some places get down to 2 or fewer in the winter.

 

[HRTKD]

Are the alligator clips still being used? If so, get rid of those and use a ring terminal instead.

 

[Nobadays]

Well there you go! "....Three panels in parallel are 720 W STC, maybe 620W PTC, would deliver 50A into a battery..." this likely explains why he burned out a 40amp fuse. Thanks.

Well, I'll talk with him and let him know that the only way to use all 3 is to aim them in different directions.

Thank you for helping me understand! I am the "guy" everyone up here comes to with solar questions, I know a lot, but smart enough to know the more I learn the more there is to learn... I love it!

 

[Nobadays]

HRTKD.... no, that was one of the first things I changed. Ring terminals.

 

[wade0000]

I'm in the same boat with you with maxing out on the CC versus the panels.

4 Panels top power combined: 1400W
CC top power 520@12V battery system and 1040W at 24V battery system... 1040W in my case.

So, I have maxed out at 1040W several times in Sept.. so come summer, I expect it will be common.

 

[Nobadays]

Talked to my neighbor today and he has decided to go with a bigger charge controller. I'm glad to hear this! I think he is going with a 60 or 80 amp SCC as he really wants to build a good sized system that will really meet his needs instead of sitting in the dark... or just turning on 1 light bulb because they want to stream some TV of the satellite internet. I will help him design something that will meet their needs.... but I did tell him we will set down and start with an energy audit to determine what he really needs rather than just through money at panels and equipment!

 

[chrisski]

There’s a few things in my RV that would be hard to do an energy audit without having a shunt on the battery.

One is the Blower for the Propane room heater, which I can’t acceSs the plug or connection, so I need to let that be the only thing on and watch the battery monitor. There’s other things like the fridge is installed behind a panel with no documentation to say wattage. Other things like that.

Watching the draw on the battery helped me find a few incandescent bulbs I did not know I had.

Turned out very few items I actually use I could measure with a kilowatt meter.

 

[schmism]

There’s a few things in my RV that would be hard to do an energy audit without having a shunt on the battery.

One is the Blower for the Propane room heater, which I can’t acceSs the plug or connection, so I need to let that be the only thing on and watch the battery monitor. There’s other things like the fridge is installed behind a panel with no documentation to say wattage. Other things like that.

Watching the draw on the battery helped me find a few incandescent bulbs I did not know I had.

Turned out very few items I actually use I could measure with a kilowatt meter.

a simple clamp meter will give you all the information you need about how much different circuits draw (in amps)

 

[chrisski]

a simple clamp meter will give you all the information you need about how much different circuits draw (in amps)

I have one hanging on my garage wall I forgot I bought. Should have got that out of the box!!

 

[HRTKD]

I found placards on all my RV appliances that provided power consumption.

 

[Mark44]

These days, even if you have nothing on placards, the Interwebs can be very helpful! Look up the model and possible year, and poof!