Solar Panels in Series with MPPT Controller

Trailpixie

I recently spec'd a solar setup for my dealer to install, and I think it's pretty spiffy. I got two 100 watt flexible Renogy solar panels and I wired them in SERIES. This puts out about 38 volts that is passed into the controller. higher voltages run more efficiently over wire runs, so I have less loss in the run from the roof to my controller. Then at the controller, it transforms the voltage down the the more appropriate 14.5ish volts, depending on the exact state of the battery at any particular time. 

There is another benefit of this configuration. The wiring is much simpler. By putting the panels in series, you don't have to run all the wires from each panel back to a single connection point. They can be daisy-chained with one another. It is also trivial to add future solar panels. I have 200 watts, and I could add another 100 watts and not  create a nightmare of wiring, and more importantly, it won't diminish performance like you will with panels in parallel. 

I am using Renogy panels with the Rover Li 20 controller.

How good is it? When I woke up in the morning, my battery gauge read 70%.  It was overcast and rainy. I was camped under trees in a canyon in the woods (seriously). By the end of the day. my batteries were restored to 100%. I don't know if that is typical performance, but I'm pretty happy. 

--side note--
I am using two Optima Blue Top AGM batteries in parralel. I'm still not sure to trust the onboard battery gauge calibration because AGM batteries are designed to run down to lower voltages without negative effects, So I'm taking that with a grain of salt. AH on an AGM batteries seem to not be directly comparable to those on a traditional flooded RV batteries.  ¯\_(ツ)_/¯
--end side note--

morey000

Yup.  exactly my thoughts when I designed my system.  Higher voltage = lower voltage drop through the wires.  I only have 1 75Ahr battery and 1 100W Renogy panel, but the same Rover 20A MPPT.  figured I could always add battery and panel if needed.  

jkjenn

Yup, when I was FT for 3 months I recharged every single day with 2, 100w panels run in Series to the MPPT controller.

DigitalSorceress

This is excellent info and it absolutely agrees with the research I've been doing.. 2 panels in series at 24v actually ends up around 40v
 and an MPPT controller will work.. but it MUST be MPPT and not PWM. PWM will waste the extra voltage.

Jax0913

Great info! We hit the Renogy Memorial Day Sale and picked up a 200Watt flexible kit with wires and adapters for $430. I upgraded the PWM for a 20a MPPT from Rich Solar that I read is compatible with the BT module. Looking forward to getting this baby all set up this week! 

DougH

I saw on YouTube tonight that Slim Potatohead is planning an experiment with a $150 400W micro turbine wind generator to complement the solar on his Aliner.  Will be interesting to see if he runs them in series into a 1000-1200W MPPT controller, a hybrid wind/solar controller, or two MPPTs.  Intruiging idea.

DigitalSorceress

I just ordered a pair of 100w Renogy panels and an MPPT controller for my own setup... I plan on wiring them in series so I can use a bit of a longer run .. the spot where my camper goes at a particular event is going to be in pretty deep shade but there's an open area about 25' away which I can place the panels..

Now to build a frame

ChanW

That's interesting! 

He's a good vlogger, too. Comfortable to listen to!

DougH said:

I saw on YouTube tonight that Slim Potatohead is planning an experiment with a $150 400W micro turbine wind generator to complement the solar on his Aliner.  Will be interesting to see if he runs them in series into a 1000-1200W MPPT controller, a hybrid wind/solar controller, or two MPPTs.  Intruiging idea.

gande

Trailpixie, I'm fairly handy, but my knowledge of electricity and solar wiring is minimal.  Would you (or anyone else in this thread) be able to post a wiring diagram of your set-up?  I've purchased a Renogy 200w suitcase and the Rover Li 20 controller for my Tab 400.  The controller should be arriving this week and I'd like to get this set up ASAP.  I'm especially interested in the SERIES WIRING between the two panels. Thanks for your help!!

ChanW

@gande, you need not fret, it's pretty basic wiring. The Renogy docs are pretty good too. Here: https://www.renogy.com/template/files/Manuals/RNG-CTRL-RVR203040%20V2.0.pdf

For series wiring, you connect the negative wire on one panel to the positive wire on the next panel. Then you take the positive wire from the first panel and the negative wire from the last panel and connect those as inputs to your controller. 

Your controller will need to be set for 24V input but the specs for your controller say "auto recognition" so that probably is automatic.

ColoradoJon

I'm curious why you want to do this?  Wiring in series doubles the voltage, but the amperage stays the same, so your second panel is doing nothing but increasing the voltage.

In a 12V system, each 100W panel produces up to 8.3 amps @ 12V (theoretical optimal).  Two panels connected in parallel would produce 16.6 amps @ 12V (theoretical optimal).  In series you only get 8.3 amps, but at 24V.

I would think that you would benefit more from the additional amperage, not the voltage?  If you are losing voltage due to length of your cable run wouldn't it be better to increase the wire gauge to account for the loss rather than losing a whole panel worth of amps?

Sorry, I've never researched wiring panels in series so this is new to me.  What is the advantage?

DigitalSorceress

@ColoradoJon  Power (watts) = Current (Amps) x Voltage (Volts) .. or p=i*e

so when you bring up the voltage at the same given power, the amps decreases.. since the carrying capacity of wire.. and voltage drop is current-based. .by going 24v instead of 12v you get less voltage drop/ loss over a given length of the same gauge wire

Basically, you do this to allow you to use higher gauge (smaller.. the higher the number the smaller the wire)  wire (saving money and minimizing loss due to resistance)

An MPPT controller will properly bring the output for the battery back to 12v .. the shorter run from controller to battery will be at 12 v and need heavier wire, but

That's the reason.. you can trade out current for voltage and vice versa for the same wattage.. the characteristics of the wire to carry them and the loss due to resistance.. favors higher voltage  (It also actually favors AC instead of DC for really long runs which is why Tesla and Westinghouse beat out Edison on that fight... in the great current wars.

Those big power lines you see between cities? 500,000 volts because there's much less current needed to carry the same amount of power.. it's the current flow that heats up wire and feels resistance.

You can think of electricity and wiring like pipes and plumbing.. if you increase the pressure (voltage) you can get the same total flow (current) out of a smaller pipe....


DC is fairly safe at lower voltages. .you don't really want to go much over 48 v though ...

Also Also, note that most 12v solar panels are actually pushing like 18v and a 24v setup might be pushing 40v.. just due to their nature

ScottG

I'm curious about this, too, but I think the math can get a little more complicated than it appears at face value.

My panels actually put out >18V in optimal lighting conditions--I think this if fairly typical. This means the theoretical maximum amperage is closer to 5.5A than 8.3A.

However, what I notice anecdotally is that under less than optimal conditions I may be lucky to get 13V and mere fraction of an amp. I.e., the charging amperage falls off at a much greater rate than the voltage. The magic of how all that works is out of my pay grade.

I suspect the primary advantage of the series wiring is that if the panels are used with an MPPT controller the additive excess voltage can be used to boost the charging amperage to (presumably) more that what you would get with a similar parallel set-up--particularly under those less than ideal conditions.

There's probably some advantage to combating voltage drop as well, but off the cuff I agree that the better approach to that would be to use cables of sufficient size.

My knowledge is admittedly pretty thin here--if anyone can confirm or refute my assumptions please do so!

DigitalSorceress

@ScottG  TL;DR: primary advantage of series wiring is that it is the current that determines how big a wire you need .. since P = I*E  if you increase E (voltage) then you decrease the I (Current) while maintaining the constant P (wattage)

Higher voltage travels better down longer wires

So, if you have solar panels right on your roof and want to do in parallel, you can but you need heavier gauge wire to do it than you would need were you to do 24v.. if you want to take your panels off the RV and run them a bit of a distance so you can park in shade and put panels in the sun (some camp sites setup would block too much sun where the camper is) then going 24v means you can run longer wires with the same gauge ..

Wire gets heavy and expensive fast.. that's the big benefit of 24v  but the MPPT will convert it down to 12v for the actual battery charging so your RV battery /electric system stays 12v.. only the panels to Controller is higher voltage

ColoradoJon

@DigitalSorceress - I understand the Watts = Amps X Volts formula.  What I don't understand is why would you cut your amps in half just to get a higher voltage to the controller?  This is like buying two 100 watt panels but only getting the use out of one.  As @ScottG says the panels can produce well over 12V, up to 18V or more.  I suppose if your wire run was so darned long that you cannot get the voltage that you need to the controller this would be a solution.  Connecting panels in series is typically done for 24V and 48V battery systems, not to overcome voltage drop in long cable runs.

By connecting your panels in series you are getting twice the voltage to the controller, but only HALF the Amps.  Even if a good MPPT controller can squeeze more amps out of the excess voltage you have to make up for more than half the loss to make it worth it.

DigitalSorceress

Because 24v at 5 amps converts back to 12v at 10 amps in the mppt at no loss.. you generate at 24 v and benefit from low amps on the long wire from the pannel to the controller.. then the mppt gives you back 12v at the original amps.. no electricity is lost or destroyed save the resistence from the wire.. resistence which is less for lower current higher voltage

ColoradoJon

DigitalSorceress said:

Because 24v at 5 amps converts back to 12v at 10 amps in the mppt at no loss.. you generate at 24 v and benefit from low amps on the long wire from the pannel to the controller.. then the mppt gives you back 12v at the original amps.. no electricity is lost or destroyed save the resistence from the wire.. resistence which is less for lower current higher voltage

No loss?  There is always loss when you convert.

I guess the problem I have with this is that our RV systems are so small that the amount of efficiency that we are talking about here is so small that I'm not even sure that it is a problem.  Put the charge controller close to the batteries if voltage loss is a concern.  The distance from the panel to the controller is less of an issue.

Anyway, I digress.  Interesting discussion for sure!  I'll need to keep this in mind when we install our home solar setup next year

rh5555

The disadvantage of wiring in series is that the overall current is limited to the worst performing panel.  If one panel is in the shade, the array will produce the same (radically lower) output as if all the panels are in the shade.  If one panel points away from the sun, the array will behave as if all panels are pointed away from the sun.  If panels are wired in parallel, then each panel produces what it can, and the outputs simply add.  Shading one panel will not impact the output from other panels.

On a related subject, here are some numbers for how much voltage you're going to lose on your cables to/from your panels:

8 Gauge Copper wire: 1.3mV/Amp/foot

10 Gauge Copper wire: 2.0mV/Amp/foot

12 Gauge Copper wire: 3.2mV/Amp/foot

14 Gauge Copper wire: 5.1mV/Amp/foot

(remember to double the distance from your trailer to your remote panels, as there are 2 wires effectively in series).  So if you're using 12G copper wire of length 20' and your charging current is 10A, then your voltage loss will be 3.2x20x2x10 = 1280mV or 1.28 volts.  That would seem to be bad, but, interestingly, your cable losses are greatest when your panels are producing the most power (and maybe you can afford a little loss).  When the panels are producing less (cloudy day) and you can little afford any extra losses, then the cable losses will be lower anyway because your charging current is lower.

DigitalSorceress

ColoradoJon said:

DigitalSorceress said:

Because 24v at 5 amps converts back to 12v at 10 amps in the mppt at no loss.. you generate at 24 v and benefit from low amps on the long wire from the pannel to the controller.. then the mppt gives you back 12v at the original amps.. no electricity is lost or destroyed save the resistence from the wire.. resistence which is less for lower current higher voltage

No loss?  There is always loss when you convert.

I guess the problem I have with this is that our RV systems are so small that the amount of efficiency that we are talking about here is so small that I'm not even sure that it is a problem.  Put the charge controller close to the batteries if voltage loss is a concern.  The distance from the panel to the controller is less of an issue.

Anyway, I digress.  Interesting discussion for sure!  I'll need to keep this in mind when we install our home solar setup next year

I need to run my panels 40' from my camper at one event.. I'm putting the panels in series so that they'll be OK with 10 AWG .. less loss that way.. The benefits of higher voltage over a run that long outweigh the miniscule penalty for the MPPT controller converting back to 12v..


2x 100 WATT solar panels

PARALLEL
200w = I*12v  16.666A
40' 10 AWG
Voltage drop: 0.80
Voltage drop percentage: 6.66%
Voltage at the end: 11.2

SERIES
200w = I*24v  8.3333A
40' 10 AWG
Voltage drop: 0.80
Voltage drop percentage: 3.33%

Voltage at the end: 23.2

So by simply putting my panels in series instead of parallel I halve the voltage drop..

To me, that's a win.. if I want to have 12v run that length with the same drop I need to use 7 AWG (https://www.calculator.net/voltage-drop-calculator.html) but since nobody carries 7, we'd need to go with 6 AWG wire

Lowes:
10AWG is 50 cents a foot:2 wires = 80'  ($40 for the wire)

6 AWG is 93 cents a foot:2 wires = 80' ($74 for the wire)

SO to have the same electrical carrying capacity/loss I need to spend $34 more on wire alone .. or run series instead of parallel

and 6 gauge wire is way heavier and bulkier to deal with ..

ChanW

Thanks for putting some numbers to the question @rh5555. That explains why I went with the setup I did, at 24V! 

I have ours set up to do 12V when it's on the road, with a relatively shorter cable. Now I wonder if I should keep it 24V, even with the shorter cable...

ScottG

Inspired to do a bit more reading reading (yeah, just enough to be dangerous...) I may well have been over thinking my previous assumptions regarding the benefits of series vs parallel wiring. Like DigitalSorceress originally said, most sources indicate that the primary advantage of series wiring is the ability to utilize smaller gauge wires. There are two elements to this:

• For safety reasons, wires must be sized to accommodate the maximum amperage they are intended to carry. In a large array with multiple panels wired in series this number can get large pretty quickly as the amps are additive.
• For practical reasons, the effect of voltage drop between the panels and the controller is mitigated by the lower fixed amperage and higher initial voltage delivered by panels wired in series.
  

However, the biggest apparent disadvantage to series wiring is that partial shading of any of the panels in the series will have a more dramatic effect on power output than with a similarly sized parallel array.

So, for my low wattage/limited amperage system that is frequently battling partial shade conditions (even with almost 40' of lead) I'm not quite sold on the advantages of a series set-up. I say this even though I am using teeny 12g wires on that lead (it's what I had lying around so I thought I'd give it a try...). Although I'm sure that gives the armchair engineers fits, I've still observed 18V at the controller end in optimal conditions. In short--with what little I've observed and what littler I know--I worry a lot less about voltage drop than I do about getting my panels into the sun.  :-)

YMMV. I think the point here is that are many variables at play and what works best in one situation may be sub-par in another. 

Cbusguy

For the average tabber,  the gains from 12 volt to 24 volt will be negligible as the wire run from the panel to the controller will never be more than about 10-15 feet from the trailer.     

Trailpixie

Here are a couple of thoughts about what I was thinking when I got my Renogy system and put it in series with the MPPT controller.

Solar panels maximum output is something around 18 volts. Batteries charge at a voltage around 14 volts. With PWM controllers, the last 4 volts or so are basically cut off and wasted.  That voltage isn’t transformed down into additional amperage.  With an MPPT controller, the whole 18 volts is transformed into additional amperage. In the case of two panels in series, the whole 36 volts are used.  With the extra voltage of two panels in series operating in the shade you are more  likely to hit that 14 volt threshold to start charging (even if it is at a low ah level).

 Covering only half of your solar panels is unimportant to me.  I think it will generally be an all or nothing kind of thing, so worrying about one solar panel being shaded and dragging down the performance of the other doesn’t bother me too much. (See above regarding lower ah charging is better than none.)

wiring in series is tidier.  This was actually more important to me than maybe it should be. With parallel wiring you need more wire runs and you need bulky connectors on the roof to combine the panels.  The series wiring only one wire to connect the panels needed two wires to go into the camper. Those two single wire strands could easily run from the roof array all the way to the controller without any connectors or splices. This tidiness made me happy. 

ColoradoJon

You can wire in parallel at the panels, and then have one run back to the controller at your battery tub - just like your serial connection.  There is no need for multiple full length runs back to the battery.

Like @Cbusguy said, the losses are negligible unless you have ridiculously long runs, and even then as long as the charge controller itself is close to the battery you are getting plenty of voltage.  The panels produce more than enough voltage for typical runs (up to 18V in some panels).  However this only occurs in optimal conditions.  I doubt you will see this in the field very often.

@DigitalSorceress posted some nice numbers but did not include the loss generated by the transformer conversion (transforming volts back to amps).  This loss reduces the effectiveness even further.  Like everything else in the world, you cannot make something out of nothing.  On a larger system, like one for a house, it might be enough to consider in design.

It's just a case of connecting the wires differently so it's just a matter of preference.

Thanks for the discussion!  Insightful...

rfuss928

DigitalSorceress said:

2x 100 WATT solar panels

PARALLEL
200w = I*12v  16.666A
40' 10 AWG
Voltage drop: 0.80
Voltage drop percentage: 6.66%
Voltage at the end: 11.2

SERIES
200w = I*24v  8.3333A
40' 10 AWG
Voltage drop: 0.80
Voltage drop percentage: 3.33%

Voltage at the end: 23.2

So by simply putting my panels in series instead of parallel I halve the voltage drop..

These calculations are erroneous.

The Voltage Drop should not be the same in both examples.

If you are using the same wire for both the SERIES and PARALLEL examples, then the Voltage Drop should be half for the half current as described in the SERIES example.  The lower current and lower voltage loss is the whole point.  An even greater advantage will result than your example indicates

Possible correction:

 SERIES

200w = I*24v  8.3333A
40' 10 AWG
Voltage drop: 0.40
Voltage drop percentage: 1.67%

Voltage at the end: 23.6

ScottG

If I had the equipment and the inclination, I'd like set up an experiment to monitor panel input and charging output under different lighting conditions using the following configurations:

• Parallel panels with PWM controller
• Parallel panels with MPPT controller
• Series panels with MPPT controller

Connecting wires would be adequately sized for each configuration to handle the amperage and minimize voltage drop. Other variables such as the number and size of the panels and the length of the run would be held constant.

While I understand and agree with the arguments for saving a buck (or perhaps a couple pounds) on wiring, I guess I'm ultimately interested in which set-up would best deliver the goods. That may even differ depending on conditions.

I agree with @ColoradoJon that it is an interesting and insightful discussion. Thanks for your thoughts (and keep 'em coming)!

DigitalSorceress

If I were doing 4 panels, I'd do two parallel pairs in series.  If I am doing 2 panels (my setup plans currently) I will likely do series but I'll get a couple branch connectors so parallel is an option if I want to play with the numbers a bit

Still I feel strongly that in my situation when I need to run 40', I'll be much better off with series if the two panels are at the same spot and not in danger of partial shade.. That is the one big drawback to series I hadn't considered.. the whole partial shade thing...   the campground where I need to do this is basically heavily wooded and there's a big clearing (about 50 yard diameter).. it means you get no usable sun (due to the ring of trees blocking early morning/late evening sun) even fairly far into the clearing until about 10 am and the usable sun ends maybe 5 pm and if the shadow is creeping along I could see it making more shade on one panel than another at some point.. )

With my 2 100w panels, I'm not in any real danger of overtaxing my 10AWG wires even in parallel at 40' so it's a bit academic... but still I prefer the idea of higher Voltage for the longer haul and letting the MPPT step it down. I have the option to do either and will see how that works out..

Cbusguy

@rfuss928 I was thinking the same thing,   but it wasn't worth trying to get into a long winded explanation.  the result of my one line response.   Gotta love non engineering types trying to get technical.    I especially enjoy when they use DC formulas to calculate AC Power.   

For all the bravado,   All we are doing is charging a 75-100 amp  hour battery,  ideally in one day.   2 100 watt panels through a cheapo PWM controler should be able to do it easily in a day.   Total cost should be about $150 bucks.

The rest is just one upsmanship. 

ColoradoJon

I dunno, some of us have 230 amp hour capacity and a few have even more than that.  It would take almost 7 hours of optimum solar production @ 200 watts to recharge half of my battery capacity.  I would not besmirch anyone for trying to squeeze extra charging efficiency from their systems.  It requires nothing but an alternate method of connection, so why not?  What we are talking about here does not require any great knowledge of electrical systems, just a general understanding and some good common sense.

@DigitalSorceress has a 40' run which is a lot, and it appears she made a copy/paste error in her calculations, but the process she was illustrating is sound.  My calculations resulted in much higher voltage drop than she portrayed, so her numbers are a bit conservative, but they illustrate the point just fine.

ChanW

That's 'zactly how I see it.

And I'm an 'engineering type'! (And just as socially awkward...   )

@DigitalSorceress said:

I need to run my panels 40' from my camper at one event.. I'm putting the panels in series so that they'll be OK with 10 AWG .. less loss that way.. The benefits of higher voltage over a run that long outweigh the miniscule penalty for the MPPT controller converting back to 12v.. 


2x 100 WATT solar panels

PARALLEL
200w = I*12v  16.666A
40' 10 AWG
Voltage drop: 0.80
Voltage drop percentage: 6.66%
Voltage at the end: 11.2

SERIES
200w = I*24v  8.3333A
40' 10 AWG
Voltage drop: 0.80
Voltage drop percentage: 3.33%

Voltage at the end: 23.2

So by simply putting my panels in series instead of parallel I halve the voltage drop..

To me, that's a win.. if I want to have 12v run that length with the same drop I need to use 7 AWG (https://www.calculator.net/voltage-drop-calculator.html) but since nobody carries 7, we'd need to go with 6 AWG wire

Lowes:
10AWG is 50 cents a foot:2 wires = 80'  ($40 for the wire)

6 AWG is 93 cents a foot:2 wires = 80' ($74 for the wire)

SO to have the same electrical carrying capacity/loss I need to spend $34 more on wire alone .. or run series instead of parallel

and 6 gauge wire is way heavier and bulkier to deal with .. 

Sharon_is_SAM

@ChanW, do you still have a slide rule like my husband?  😂