solar panels, series or parallel?

Tundra57

So when you have two solar panels(the same type), you have the option to connect them in series (like our two 6v batteries) or in parallel (negative to negative, positive tp positive). There is a lot of opinion out there as to which is best.

If the panels are in series, the voltages for each panel add be t the current is limited by the lowest current panel.

If they are in parallel, the voltage will be the highest of either panel, but thr current will be the sum of each panels current. Diodes are privided the nside each panel to prevent a higher voltage panel driving current into the lower voltage panel snd wasting energy.

I am sitting in the sun with my 200watt external panels wired in parallel, in good sun. The volage is  15.7v 6.3 solar amps (controller in trailer) 13.54 volts 6.9amps to battery.

My tab solar is in shade, the panels are in series. 19.92v 0.5Amps solar battery  13.36v 0.6Amps.

So heres my thoughts:

If you have a long run of wire to your solar panels it would be best to have the controller near the battery in any case to reduce current and hence cable energy losses which are proportional to current.

In this case the panels should be in series.

If the panels are in shade so that the voltage from them is too low to get the charger to function, must me 4-5v higher than required battery voltage, then in series the voltage could be enough to use what little energy there is.

However, since in series the current is limited by the panel with the smallest output, this is not the best solution when one of the panels is in the dark.

My panels seem to be more efficient in parallel. If they are in enough sun to wake up the charger.

So it seems that if you will not have access to the panels after installation, then series is a good compromise. But if they are external and you can chose then if its dull, or the panels are far away chose series, but if its a really bright day with both in sun, chose parallel.

So anyone got any other thoughts?

xdr

We have been thinking through this as well, reading and learning from this site and other YouTube vids. 

Most likely we would only use the panels if we were parked under the trees to keep the Tab cool. Our rooftop solar keeps the batteries charged nicely and provide the power we need, so usually the external panels would not be necessary. 

When they are necessary, it sounds like it would be best in parallel, is this what you have found? 

Xdr 

MarkAl

Since I've chimed in similarly to other threads, these are my thoughts: Good discussion on situations which is always the problem in real life non-perfect solar setups. I agree if the trailer can get enough sun I wouldn't bother with the remote panels (assume these remotes have a single, separate controller from the T@Bs). When the trailer is in the shade, the remotes can be your BEST solution. Lets also assume you will locate them to both get very similar lighting (the analysis won't work otherwise). Now on a cloudy day the two panels need to be wired in series to, as you've said above, create enough voltage to turn ON the controller (the currents should be similar for similar lighting and panel type). This has the additional benefit of keeping the current low, which reduces the power transmission losses (it's not just portional but a squared function).  But what if you get them in a  sunny area? Then you really only need 1 of the 2 panels but lets say you use both (why not?). You can leave them in a series setup assuming your controller can handle the large voltage AND you continue to have less current so less power loss*! There is no advantage to having them in parallel.  This analysis does not work if the two panels have different light levels. Then it gets more complicated...

*This is why power transmission lines run at very high voltages to minimize power losses.

CrabTab

@Tundra57

What gauge wire are you using on the run from the external panels to the controller?

I'm using 10GA with a 30ft run.

ChanW

We actually have two separate sets of cables - a shorter set of smaller gauge for short distance runs at low voltage (parallel), and a longer set of heavier gauge for a long distance run at high voltage (series). (And if things got really desperate, we could put them together for a really long run, and lots of voltage drop!)
That gives us options for setting up in the shade.

Tundra57

@CrabTab 10 gauge too. And 30 feet. Was standard length from ACOPOWER.

Tundra57

@MarkAl i am aware its a square function thanks. I didnt want to get overly mathamatical

I think you are wrong about the series only.

I swapped my  panels from series to parallel connection in both full sun and part shade. With the same constant load in my tab.

The parallel connection yielded more charge current to the battery both times.

That is why in my opinion series is only good for very long lengths (more than 30 feet) and small gauge (less than 10) of solar panel wire (if the controller is in the panel then it makes no difference), or when the total solar panel voltage is too small to allow the charger to push current to the battery.

Series connection will limit the current to the output of the darkest panel.

In parallel the brightest panel can still function if its voltage is enough. Also each panel, in my case has a protection series diode in the panel. 

So in parallel you get one voltsge drop, but in series you get two. If they are nornal diodes this could be any thing from 0.4 to 0.7 volts x two, which could be significant.

Anyway the results I gave were from my panels and controllers, others might be different.

In the end its either speculative data, or in my case empirical.

MarkAl

Good information! I will test it out when I get the opportunity, at this point I'm still setting up the system and it's speculative. My setup is switchable as I expect more situations will develop (with low and variable light where I need to reconfigue it and I have plenty of voltmeters. I'll instrument both panels and see what happens. I agree with your example of one dark panel is when series doesn't work and why I required both panels to have similar sunlight. Parallel in sun & shade can be more efficient when using a single controller as you stated. I would not think there was any advantage at similar light levels if the panels are matched in parallel. In low light series is the only hope of getting the controller to turn ON.  I frequently see my panel at 10-12 volts but if both were similar then in series you should have 20 to 24 volts (until the loads kick in and then it will go down to whatever the panel can deliver.

But under load and different or low outputs the solution can be different. Each configuration strongly effects the results.

CrabTab

Thanks @ChanW and @Tundra57. Sorry for the off-topic question. 

Tundra57

@CrabTab no problem. Questions always yield much useful discussion.

jkjenn

@Tundra57 putnof curiosity,, when you say yielded more charge current, what reading are you basing that off of?

Tundra57

I had lights, fantastic fan and fridge running, no shore power. I read the current and voltage output from the victron app going into the battery. I have 440mAh worth of AGM batteries.

I took multiple readings and changed between series and parallel 3 times.

Not a flawless comparison, but it did agree with what Renogy expected.

jkjenn

I meant, which metrics are using to compare?

Tundra57

@jkjenn same load, same setup. External panels in full sun at the end of 30 feet of cable #10 gauge solar panel 1000 Volt outdoor wire with waterproof connectors. Tab in part shade.

Just moved cable connections from series to parallel on the external panels. The internal solar tab controller is turned off. It takes maybe 5 mins to swap modes on the external panels.

I monitored the controller smart solar app.

Switched back and forth a few times. Charge current output into the battery was always higher in parallel mode. Effective watts from the panels was increased. Not a great deal more, maybe 15% max.

Nothing changed but the panel configuration and how full my teacup was at the time.

I have done this comparison on different days with similar results.

I have no hard data to say why there is a difference and Renogy did not provide an answer.

There are so many variables, in my case parallel is better for most circumstances.

I have not checked this out with heavily discharged batteries and with full 200W from the panels. In my trials i have not had more than 8 Amps or so in the solar panel wires. Controller is in the Tab.

If you have any comparison methods you think would be more consistent with your tests, i can give it a shot.

RCB

Duh, ya I got a solar panel on my 400, or maybe it is two. At times I think I know something about them, but this discussion shows I know little. Thank you all for this insight. 

Does my 2019, 400 have one or two panels ? Are they wired in series or parallel ?
Thanks.

rfuss928

IMO the obsession with large gauge wire or the performance difference for series vs. parallel connection is not justified for these small portable systems when the overall system performance and efficiency are properly considered. 
Caution - lots of numbers and math follows.
For example:
Wire Gauge

100 feet of #10 copper wire is 0.100ohm,  #8 is 0.063

If you have 100 watt panels at full power (very rare occurance) the maximum possible current is about 6 amps at close to 17 volts.

The power lost in the 100 feet of wire (50 foot pair) is 3.6 watts in the #10 wire and 2.3 watts in the #8 wire.  (Watts = amps squared times resistance)

About 1 watt difference in losses at 100 watts or ~1% of full power.

When bulk charging is complete, the current at absorption and float stages will be about 1 amp or less and losses in power and voltage will become insignificant.
Series vs Parallel:

When panels are shaded and only making low voltage - generally less than about 14 v - they are not producing any significant power no matter how many are placed in series to boost the voltage.  In most cases, significant power production does not begin until OCV reaches ~16 volts.  So, series connecting two panels that are only producing 10 volts each to achieve 20 volts will still not deliver any meaningful power.
  
At full power, using the 100w panel described above separated into two 50 watt panels in series,  the current would become 3 amps at ~34 volts.  The power loss in the #10 wire would be reduced to 0.9 watts - one quarter of the parallel connection.  That sounds like a big reduction but the effect on charging will only be an additional 2.7 watts out of 100 watts.  At a more typical 50% power, the difference would be only ~ 0.7 watt out of 50 watts.

So yes, there are very small gains to be had but nothing that will make any difference in whether or not your solar charging system will be capable of maintaining your batteries while off grid.  On the other hand, careful panel placement and tracking throughout the day can easily double the power harvested.
Be careful of obsessing over the details and get out there and have some fun camping.  

Tundra57

@rfuss928. Interesing insight. However the conversation was whether series or parallel made any difference and why you might chose either.

Not obsessing, just discussing. Yes placement is most important, but that is rather obvious.

Tracking is why i purchased external panels and two not one so its easier to keep the whole surface facing the sun.

I am enyoying camping thanks.

Tundra57

@rfuss928 and to your very valid point I need to get out more. Boondocking with solar will teach me to be more frugal with my batteries.

MarkAl

@rufus928 To your point on series connecting still won't add power if the voltage sags too much when a real load is attached. I'm hoping I can get 10-15 (out of 200) watts. My new panels are due in next week and I can start experimenting. I'm hoping I can get data on the low light power ability of the panels. So much depends on how the internal connections are designed and specific shading. Sadly the 100 watt panels don't have a lot of options for internal connections. Fortunately I've an engineering friend who lives off grid in a solar powered house who's also eager to work this problem.

Tundra57

@MarkAl be interesting to see what further insite you have. @rfuss928 had a great writeup.

rfuss928

My two 68w roof mounted panels are 8 years old and on their second controller and set of batteries.  The controller is central between the two panels and it is a simple choice to wire series or parallel.  The primary reason I switched controllers was to gain the option of wiring the panels in series with the same hopes as MarkAl - that series would get some more power output in marginal conditions.  It also forced the change from a pwm control to mppt to tolerate and utilize the high input voltage from series connection in normal operating ranges.
My subjective observations are that I see some very low charge currents (<0.5A or <6w) in marginal conditions that probably would not have yielded any output with the // and pwm.  Nothing that would make any difference in the systems ability to support off grid camping.  I haven't tried the mppt with the // connection.  The engineer in me is curious but the pragmatist is too lazy to chase the milliamps.
It will be interesting to see what MarkAl discovers.  
Have fun!

MarkAl

This weekend (hopefully) will be my first weekend w/ the dual remote system installed. Regarding series or parallel, I'm installing a DPDT switch to select either. I'm very interested in charge rates as I can move the array around and check various lighting situations in real time. Yes I'll report back. My thought is it'll make little difference except in special cases.

BrianZ

Here's how we solved the series vs parallel dilemma..  We got only a single 100W panel. 
Problem solved!  

Really though, with our limited boondocking and modest power needs of only about 20AH per day, when paired with our upgraded 150AH 12V golf cart battery, it seems to be all we need.  Our panel is not permanently mounted, so can be used on the car roof while driving or on the ground while camping.  We have a 40-ft 8G cable for optimal ground placement and a 15 ft 10G cable for car roof or extension.  A charge controller box with connector pigtails is temporarily mounted in the tub to a battery pigtail.

MarkAl

We just got back from a 3 day trip with the proto-type 2 panel remote system with a switched series or parallel ability. I found:
- Early morning or late afternoon with no direct sunlight the series would activate and produce a very low amount of power 6-15 watts while the parallel setup up would not activate the controller so no power was sent to the battery. It was interesting to watch in series as the voltage climbed and sagged based on the controller attempting to get maximum power from the panels. This is similar to my home experiment on a drizzly day. Also found with a diffused light source placing the panels just facing up to the clouds produced more power than directing them at an angle.
- Any real sunlight on the panels did not have a great deal of difference in the power generated if they were parallel or serially connected. I was a bit surprised that it didn't matter much if one panel was more shaded than the other (reasonable shade not towel over it). It seemed like there was some load leveling by the panels themselves in both cases to produce a similar output.
- Cable resistance loss was not measurable as the currents were not high and the power varied too much during the switching. So the higher current penalty of the parallel was not observed.
- Also my camp setup (again) sucked for the factory panel, I got about 40 watt-hrs while my remote while not particularly well placed but facing the correct way with some opening through the trees did over 160 watt-hrs while we were hiking during the day. During this time I left the panels in parallel as I was sure the panels would not get full sun at the same time.
- Still more to do especially as I don't have stands made for the panels yet and only a 20 foot cable but this was the first trip were at the start of each evening we were at 100% charge - so radio, lights and laptops were used in the evening.
- Is it worth it? Probably not:
   - I enjoyed the stimulation of setting this experiment up and am pretty convinced that the 2-100 watt panels with the 100 watt on the roof is overkill for anybody who camps in a less tree'd location but for my projected late season / winter use it'll probably be okay - no generators.
   - Is the series / parallel switch a good idea - only if you are anal retentive or expect a lot of shade / clouds. Then series will trickle charge your batteries. Otherwise I'd stay w/ parallel but with two controllers - factory roof and remote panel - also start with one remote 100 watt. With the flexibilty of the two I believe in most situations it will be very adaquate - except for that week of wet weather in the trees when the refridge get's turned off and headlamps are on to save the battery. But then I'm probably using the gravity water filter (don't want to put my wife through a bad water episode) and pumping it into my fresh water tank and digging holes for the cassette toilet dump. Kidding there's usually an outhouse somewhere.
- VE.Smart Networking seemed to work well at integrating the two controllers with the battery's needs.

Next up: moving the shunt with all the other electronics.

Tundra57

@MarkAl. Awesome writeup. Since i installed the BMV712 and set up the network, my two controllers share nicely now too. :-)