Converting Solar Panels From Parallel to Series

[SteveJ]

The following scenario is for the Safari Trek that we just bought but the topic relates to any solar install and there are some pretty smart folks around here relating to solar.

I just purchased two sets of these:



https://amazon.com/gp/product/B07G19...?ie=UTF8&psc=1

Each set has two panels wired in parallel. My intention is to not use the included controller(s), instead using a 40 amp MPPT controller already installed. I also intend to change each set to series wiring but hook up the two sets in parallel when both are deployed. I think this will help with shading issues when camped in a semi forested camping area. If I were to leave each set in parallel and wire the sets in series as soon as one set had shading issues the whole system would be affected.

Am I on the right path here?

TIA.

[booster]

The series will gain you less voltage drop but will make shading worse, not better. 4 parallel would be best for shading issues.

[jakegw2]

As Booster says - you are picking the tradeoff you want to make. Series will result in less power lost to the cabling but will be very sensitive to shade. Parallel has the opposite tradeoffs.

That being said, unless you do almost all desert camping with no shading of the panels you should worry about shading (which causes a huge drop in output) far more than cable loss (small drop). Your relatively small cable run lengths are a very minor concern. Shading will be the biggest challenge you face, so parallel is the right choice here.

[SteveJ]

Thanks folks. With me knowing best on how I camp I'm thinking the individual sets in series and the sets in turn hooked up parallel will work the best. I was just a bit intimidated by opening up the junction boxes and seeing the diodes tied in. If I kept them as is and put them in parallel I would lose a lot of the benefit of the MPPT controller on hazy/cloudy days. If I keep them as is and hook the sets in series, one set getting in the shade would be ugly. With each set in series by itself then wired in parallel, if one goes in the shade the other set still has full output. Basically a trade off between shade issues and cloudy/hazy performance.

When done the configuration will be 2S2P.

I plan on using 12 gauge wiring to the panels, up to 50' in two sections. I found that even only using 150 watts of 12 volt panels in parallel at that distance there was a significant voltage drop, about 8% IIRC. Having 240 watts now with possibly a bit more if I wind up with a 12 volt fridge would necessitate using significantly heavier wiring or multiple conductors. It is all about the trade offs.

Like I mentioned, I think this setup will be the best for our use but there is nothing irreversible being done if it doesn't work out.

Off to play with the soldering gun...

Thanks all.

[GeorgeRa]

I considered using a multipolar rotary switch to select serial or parallel connection for my 3 x 100W Solar Grape panels. Switch would be accessible from inside, but located on the roof in a junction box. If I have to do some solar modification, I will still consider doing it.

[SteveJ]

Originally Posted by GeorgeRa

I considered using a multipolar rotary switch to select serial or parallel connection for my 3 x 100W Solar Grape panels. Switch would be accessible from inside, but located on the roof in a junction box. If I have to do some solar modification, I will still consider doing it.

My brain would really hurt trying to figure out the schematic on that, no doubt.

Making progress. I just found out that they shipped the wrong controller, no Bluetooth on the unit though the box says it is BT, and that's what I ordered and paid for. Thankfully, there is a Kohl's near me for "effortless" returns to the big A.

[booster]

Those are 17.8v power point panels so you would not lose much with all parallel on an MPPT, but at 50' line loss gets to be a big issue. Even if you do series parallel, you really should have bigger than 12ga. Most panels come with 10ga on them even for a few feet in length. You may want to go to one of the voltage drop calculators online and see what you would have at that length of wire.



Long wires is a big downside with portables put out of the shade on long cables.

[SteveJ]

Originally Posted by booster

Those are 17.8v power point panels so you would not lose much with all parallel on an MPPT, but at 50' line loss gets to be a big issue. Even if you do series parallel, you really should have bigger than 12ga. Most panels come with 10ga on them even for a few feet in length. You may want to go to one of the voltage drop calculators online and see what you would have at that length of wire.



Long wires is a big downside with portables put out of the shade on long cables.

Just did a drop calc using 50' of 12 gauge using a voltage of 35.6 as the maximum power point and 6.6 amps and it comes out to 1.47%. I can live with that.

Just for grins I did a calc using all parallel. I would have to use 6 gauge wire to get to 1.46% drop. It was almost 6% with 12 gauge. My earlier guess with 150 watts was obviously off, it calculates to about 4.4% using 14.4 volts.

I had thought of making the cable in two pieces as the 25' section does work in the majority of cases but with that small of a drop I'm not going to bother introducing another junction point into the system.

It will be interesting to see the final performance, though it will have to wait a bit. The charge controller that I received was supposed to have Bluetooth connectivity for setting parameters, it didn't. Apparently wrong controller with the right model number on it. A Bluetooth sticker shows on the Amazon description on the front of the unit but mine didn't have that sticker and Bluetooth finder on my phone could not find anything coming from the controller. Replacement arrives on 4Dec. I will post up with the performance once I get things up and running.

Oh well, maybe tomorrow I will proceed with domestic water pump replacement and relocation for a change of pace. I have an apparent air infusion issue and the pump is behind a 10(?) valve manifold. I going to relocate the pump for a lot easier access in case I have to deal with it while travelling.

Just for background, the battery bank consists of two group 31 flooded, total AHs are 210.

Thanks Jim. You and others(you know who you are ) on this forum are a wealth of info and expertise.

[@Michael]

Originally Posted by booster

What calculator did you use?
But....remember you have two wires, one each way, so you really have 100' of resistance and voltage drop that the controller sees.

That calculator uses one-way distance, so using 50' is correct.

* Please use one-way distance to the load. Not round trip distance.

One way of looking at this is to compare the wattage you'd get from the panels if they are in the sun at the end of 50' of 12ga to what you'd get if they were in the shade because you didn't have 50' of 12ga.

[SteveJ]

Originally Posted by booster

What calculator did you use?


Using this one


https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&vol tage=17.7&phase=dc&noofconductor=1&distance=50&dis tanceunit=feet&eres=6.7&x=56&y=18


I got a bit over 3% using 6.6amps and 35.4v at 50' of 12ga.


At 17.7v it was as expected double at a bit over 6%.


But....remember you have two wires, one each way, so you really have 100' of resistance and voltage drop that the controller sees.


Then on the calculator I got 6% drop at 35.5v with 12ga.


3.8% at the same parameters on 10ga.

I used the same calculator but mistakenly used the "2 sets of conductors per phase instead of a single set of connectors. I'm showing just over 3% also.

https://www.calculator.net/voltage-d...=6.7&x=41&y=22

I will be splitting that 50' cable into two 25" sections to get under 2% in most of my camping scenarios.

I also went a different direction on the charge controller. Ditched the no name Amazon unit. Ordered up a Victron 100/30 Bluetooth SmartSolar unit along with the Smart Sense wireless remote temperature and voltage unit as my install is already wired up in the interior. Ordered from Northern Arizona Wind and Sun. Nice dealing with a real business instead of the Big A. Their customer service chat worked well for a question that I had. Immediate and clear answer.

[SteveJ]

Originally Posted by @Michael

That calculator uses one-way distance, so using 50' is correct.




One way of looking at this is to compare the wattage you'd get from the panels if they are in the sun at the end of 50' of 12ga to what you'd get if they were in the shade because you didn't have 50' of 12ga.

These extra characters are typed in to meet the 10 character minimum.

[booster]

Originally Posted by SteveJ

I used the same calculator but mistakenly used the "2 sets of conductors per phase instead of a single set of connectors. I'm showing just over 3% also.

https://www.calculator.net/voltage-d...=6.7&x=41&y=22

I will be splitting that 50' cable into two 25" sections to get under 2% in most of my camping scenarios.

I also went a different direction on the charge controller. Ditched the no name Amazon unit. Ordered up a Victron 100/30 Bluetooth SmartSolar unit along with the Smart Sense wireless remote temperature and voltage unit as my install is already wired up in the interior. Ordered from Northern Arizona Wind and Sun. Nice dealing with a real business instead of the Big A. Their customer service chat worked well for a question that I had. Immediate and clear answer.

You are correct, not used to that so didn't even look for it. We always used calculators at work, but always had to enter both ways because they often weren't equal length in and out when we had common busses on lots of stuff. The one I grabbed must be primarily for cable run calcs.

[SteveJ]

Originally Posted by booster

You are correct, not used to that so didn't even look for it. We always used calculators at work, but always had to enter both ways because they often weren't equal length in and out when we had common busses on lots of stuff. The one I grabbed must be primarily for cable run calcs.

That's scary.

[Calson]

When evaluating solar panel output I look at how long it will take to get the batteries back to 100% SOC. If it takes 4 hours or 5 hours or even 6 hours, this is seldom of any consequence.

I would expect that a single MPPT controller processing 12 volt and 24 volt inputs would be reducing the output from the 24V segment to a far greater extent than any gauge or length of wire from the panels.

[SteveJ]

Follow up time. The Victron is all installed. Thus far I've not split the 50" cable from the panels in two, pretty much not worth dealing with. The phone app works flawlessly, good kit.

With an ambient temp in the low 80's(it is FLA) I was showing about 13-14 amps into the batteries with the batteries drawn down to 60% according to the monitor. I was showing +or- 200 watts on 240 watts of Mono panels, IIRC. I would ASS U ME the panel output will be better in summer sun.

On to planned maintenance things. Lotsa work moving into a "new" RV.

[Nic7320]

This is probably stuff you already know, but it's worth double checking:

Your MPPT controller has two numbers embedded in the part number.

For example, if you have a Victron SmartSolar MPPT 100/30, it can handle up to 100 Volts on the solar inputs, and up to 30 Amps.

Your panels have a Voc and an Isc rating. Voc is rated in laboratory conditions at a temperature of 25C, and increases with lower temperatures. Therefore, you should keep your series panel voltages below about 80% of the MPPT voltage limit (80% of Voc x number of panels in series), so it can handle the higher voltage the panels produce on colder days. On warmer days, the solar voltage goes down so there's no problem there. Also, with the panels at Maximum Power Point, the voltage drops a bit from Voc, but it's still pretty close to the Voc rating. With a parallel solar panel configuration, the input voltage is usually well below the controller limit, but in series configurations using four panels (with Voc around 20V) in series, it's about at the limit.

Isc multiplied by the number of panels in parallel should be below the current rating on the controller. In reality, the panels run at maximum power point, which is slightly less than Isc, so there's a little extra margin built in there.

Now if you're going to run two sets of two panels in series and then parallel the two sets, then you will have approximately 2 x Voc on your MPPT controller inputs as well as 2 x Isc.

This is all pretty simple math, but one concept does needs stated. For reliability sake, do not stress the controller close to its limits. Derating (reducing stresses to lower than the specified limits) is what gives you less heat stress and a longer life.