Solar Panels in Series with MPPT Controller

ChanW

Oh dear.... well, @Sharon_is_SAM, I have this 'stubborn' steak...


(I doubt that I remember how to use one!)

Sharon_is_SAM

I knew it!  My husband John isn’t the only one whi still has his slide rule!

gande

I now have in my Renogy Rover Li 20 controller with Bluetooth module and will be mounting it this weekend (hopefully).  I have a 2018 Tab 400 and would like some feedback as to where others are mounting the controller.  I have a single 12v battery and inverter already mounted in the storage area under the bed.  Is there room to fit the controller as well?  My trailer is stored offsite and I'll be picking it up tomorrow to bring home for this installation.  Does anyone have their controller mounted inside in the living space?

DigitalSorceress

@gande if your battery is inside, then absolutely put the controller inside and near the battery.. it's better for the cable from the panel to be longer and the cable run from controller to battery to be shorter.


I've toyed with the idea of putting batteries inside, and if I take the plunge and go LiFePO4, I likely will if for no other reason than to keep them safer from theft...

Verna

My solar charger is under the closet door—wonderful place, plus close enough to the 400 batteries. Easy to read from bed or from the kitchen.,

gande

Thanks @DigitalSorceress and @Verna.  I was thinking of mounting it on the panel below the bed.  @Verna, do you have a picture of your installation? 

Trailpixie

My controller is mounted above the thermostat on my T@B S

Jax0913

We connected 2 -100 watt panels in series with a 20amp rich solar MPPT controller (basically the rover) with the Bluetooth BT Bluetooth module. Just put myself on solar in full sun, and here are my readings. Not sure if I like those watts. Thoughts? Anyone else familiar with this Bluetooth controller? 

Edited for photo size - Sharon

DigitalSorceress

@Jax0913 if you think the series is causing an issue, try switching to parallel and compare .. see if the power goes up at all..

If one of the panels is in shadow, it can really mess with the total  output...

If you haven't got the needed branch connectors, try just connecting one panel and see what its watt output is.. then switch to the other..

if there's a marked difference in the output of the panels given the same light then you may find you get much better total output by going parallel

Series is only good when you have as close a match as possible... since the whole output will be limited by the weakest panel.

Trailpixie

your battery capacity is showing 100% at 13.7V. That is fully charged.  I think it is throttling back the power to a trickle-charge so it doesn't overcharge your battery. As you can see, it says "Float Charge mode." That means it is maintaining an already fully charged battery.

Turn some fans and lights and other stuff on and see if the wattage goes up.

pthomas745

Agree with @Trailpixie .  With that many watts coming in, you will have to turn on something that uses a lot of power, or disconnect your setup and let the trailer use some battery and see what you get.

dCliffhanger

Voltage drop is not dependent upon amperage. It is proportional to the resistance of the wire (gauge and length). There is a fixed drop in voltage as the wire gets longer. Therefore, if you start with a higher voltage by wiring your panels in series, the Percentage drop in voltage will be less, which when applied to the amperage will deliver more watts to your controller and battery.  You can take advantage of this by delivering more power to your battery, or by going to smaller gauge wire, or a longer wire and get the same delivered charge as running in parallel. Your option.

DougH

ChanW said:

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.

Part 2 is up on Slim's Aliner wind turbine experiment....

https://youtu.be/qhstBD0w2hg

Hard to believe he's averaging 3kW of continuous power for just a $100 investment.  Nice of the turbine manufacturer to toss in the LiFePO4 800Ah bank as well. 

Jax0913

@DigitalSorceress I am gonna give it a go. We camped 4 nights 5 days and by the 4th day I needed to add some juice to the camper. We were up in the Adirondacks and had a site that got good sunlight in the morning, but a couple of days with clouds didn't really help juice us back up. I parked it on our driveway when we got home and in full 80 degree sun the last two days she is still running close to 12.1 volts on the battery and I needed to connect to shore power. Gonna play around with a parallel connection and see how that looks. 

Can I just turn off my battery switch and swap my cables or should I disconnect from Charge controller?

Cbusguy

@Jax0913 looking at your screen shot your charger is in float mode,  it will not give you high current outputs in that mode as your battery is fully charged.     

The thing to remember is there are basically three main charge modes  bulk which dumps max current into the battery,    Absorption  to top up the battery after it is charged,  it uses a fixed voltage and variable current and float which is just a small current to keep the battery topped up.  

So what I see in your screen shot is fine.   Run the battery down a bit,  run your fridge on the battery and then take a look at it,   I bet it changes.

Jax0913

Thanks @Cbusguy. I have left it on battery since Sunday, we have had nothing but sunshine this week and checked my monitor this morning and was at 12V on the battery. Though I am not sure how reliable the battery percent is on the Bluetooth module. Seems like I am not generating enough power with two 100 watt panels in series. I would have thought I would be full after yesterdays sun, certainly not 50%. I plugged into shore power to try to gain some more juice. 

Cbusguy

without being able to see it real time it is tough to guess.   Sun angle and cloud density play into it.   my guess from your screen shot is that the sun is at a very low angle or you are in the shade.

The thing to remember when you go with a series or 24 volt config with solar panels is you are sacrificing current for voltage.     Most of the distances from the panels to the trailers is small enough that the benefit of 24 volt config is negligible.    And if you have one weak panel it impacts both of them.

Run the numbers,   an average 100 watt panel brand new typically produces 18 volts which is about 5.5 amps.     Best case  you get 11 amps in a parallel 12 volt config.    so now you put them in series,   so instead of getting 11 amps at 18 volts you are only getting 5.5 amps at 36 volts.   

The mppt controller,  will convert the voltage back to amperage but with a loss....     so realistically you may see 7-9 amps depending on the efficiency of your controller.

Troubleshooting your issue,  I would plug one panel in at a time and see if one is week or bad.....but I think you are better off putting your panels in parallel.

How is your controller hooked to the battery?  I would move it to the battery side of the disconnect and turn the disconnect off.

 I wonder if you have some phantom current drains in your trailer,  disconnect the battery and see what happens. 

Jax0913

Thanks @Cbusguy. I think I am gonna do some swapping and testing of my configuration and try them in parallel. I wired my controller directly to the battery, which now looking at that, doesn't really help my case when we turn our battery disconnect off. 

The controller is wired to the battery and the battery to the disconnect switch. 

 

Cbusguy

I have been using solar for a lot of years on a work shop,  with controller you really do get what you pay for........I switched from a renogy to a victron earlier this year and it made a world of difference.   The victron finishes bulk charging before lunch where the renogy would still be bulk charging when I would get home from work around 4pm.     some what subjective, sure.

The victron tracks faster so under cloud cover or overcast day it switchs charge currents quicker and oddly the renogy reported higher charge amperage's than the victron,  but yet it charged slower,   not sure whats up with that. 

rfuss928

dCliffhanger said:

Voltage drop is not dependent upon amperage.

Absolutely incorrect !!!

V = I x R   =>   Voltage drop (V) equals the current in the wire (I) multiplied by the resistance (R) of the wire.

DigitalSorceress

@Jax0913

""'Can I just turn off my battery switch and swap my cables or should I disconnect from Charge controller?"""

Always remember: never leave your panels connected to the controller unless you're connected to a battery.. if you do, you can damage the controller

always disconnect the panels THEN disconnect from the battery.. reconnect to battery first THEN to panels.

This is why any run from Charge controller to battery should be a "home run" and not go through the shutoff

DigitalSorceress

@Cbusguy  Down converting voltage to current.. My understanding is that the loss from this is far less than the loss from voltage drop over any non-trivial distance..

converting 40v at 5A to 12v at 16.666 A   (optimal theoretical) what kind of drop are you seeing?  From everything I'm told, MPPT controllers should be using switching converters and be 98% to 99% efficient.. am I wrong?

Trailpixie

All electronic theory aside...

You definitely have a problem with your system. For reference:

i took my my camper out last weekend. It has the 200 watt Renogy system wired in series with the rover controller and the same Bluetooth module. On Friday night I ran my battery down to 70%. Saturday was partly to mostly cloudy. I was parked in the trees with a very narrow window to the sky. By noon my two AGM batteries were recharged to 100%. 

My system is nearly identical to yours and running much better, so I think you have a failure somewhere. Have you done these things:
* used a multimeter to check that the voltage reported at the solar controller matches the actual voltage at the battery?
* confirmed that the LCD on the controller base unit is reporting the same info as the app on your phone?
* Confirmed that the controller is charging the right kind of battery? It should be set to “sld.” Make sure it isn’t “gel” or “li.” 
* taken the battery to a battery store and asked them to put it on a load tester to check if the battery is defective and unable to accept a charge?
* gotten on the roof and checked the connectors?
* traced the wiring to the battery and checked that the connectors securely attached. 
* checked the solar ANL fuse

you have installed enough solar power to keep that battery charged in everyday conditions. There is a hardware failure. 

Trailpixie

I love me some real world tests. This guy does some MPPT versus PWM and parallel versus series tests in full sun and shade. Very interesting results.


Series MPPT versus Parallel PWM
https://www.youtube.com/watch?v=XzuikyLpp1U 


MPPT parallel versus PWM parallel 
https://www.youtube.com/watch?v=cBWqnVpwtRo

Cbusguy

I doubt you will ever see those numbers.  

I would like to discuss further but the mods will smack me with the rolled up news paper again

ChanW

@Jax0913, note that the MPPT controller needs to be set to 12V when running your panels in parallel, and needs to be set to 24V when running them in series.

The controller auto-senses the voltage the first time the panels are connected, but after that you must manually set it.

Trailpixie

I think if I had it to do over, I would get four 50 watt panels and set them up in series parallel.

rfuss928

DigitalSorceress said:

@Cbusguy  Down converting voltage to current.. My understanding is that the loss from this is far less than the loss from voltage drop over any non-trivial distance..

converting 40v at 5A to 12v at 16.666 A   (optimal theoretical) what kind of drop are you seeing?  From everything I'm told, MPPT controllers should be using switching converters and be 98% to 99% efficient.. am I wrong?

Looking at the two screen shots provided:

Watts (power) = Volts multiplied by Amps

First case:

Solar Watts =>   35.90 V x 2.14 A = 76.826 Watts

Battery Watts =>  13.70 V x 5.53 A = 75.761 Watts

Efficiency = Watts In / Watts Out  =>  75.761 / 76.826 = 98.6%

Second case:

Solar Watts =>   34.10 V x .38 A = 12.958 Watts

Battery Watts =>  12.40 V x 1.01 A = 12.542 Watts

Efficiency = Watts In / Watts Out  =>  12.542 / 12.958 = 96.8%

The MPPT controller is operating in the typical 95-99% efficiency range of DC to DC converters.

It is odd in the first case the display indicates the battery is full at the same time it is delivering over 5 amps charge current - typical conditions during BULK charging.  Possibly a simple case of the battery voltage being meaningless while being charged or under load.  Seems misleading?

In the second case the solar output is very low and the charger is only able to deliver about a 1 amp charge.  Again, probably in BULK charging and the battery state of charge is probably much lower than the 65% the display indicates if 12.4v is able to drive any charge current.

JonW

For comparison, here's a real-world example:

I have (2) 120w, 6.0A, 20V flexible panels on the roof of my T@B wired in parallel connected to a Renology Voyager PWM controller in the tongue box, practically on top of the battery.

Doing the math, in parallel the panels are sending 12.0A and 20V to the controller, which is a 20A 12V controller (capable of handling a peak/max. of 26V).

There may be more effective or efficient setups and/or controllers out there but I'm happy with this setup.  I rarely use shore power, and do not disconnect the battery as the 320 U without a TV or AC has minimal parasitic drain.  Usually my battery is fully charged by mid to late morning.  I just set it and forget it...

The main reason I chose the Voyager controller is because it is waterproof and it can handle multiple types of batteries.

jkjenn

Trailpixie said:

I think if I had it to do over, I would get four 50 watt panels and set them up in series parallel.

That is what I did when I had my portable set up. I am hoping the permanent mount plus the golf cart batteries will work out for the shorter stays I am doing  these days.