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Help with PV panel hook-up?....

 
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A newbie here to PV panels so please bear with me....  Recently purchased (used) a 327W PV panel for placing atop an electric 36V golf cart....see thread and recent entries here for background information on the project and a link to specs of the panel:  https://permies.com/t/80/24710/Solar-Electric-UTV-Polaris-sucks

With panel awaiting installation, I'm confused about the leads that I thought would be marked + and - at the back of the panel.  Instead, there is some box on the back of the panel that has two leads running from it, neither of which clearly indicate a positive and negative orientation.  Photos are below.....can anyone help me determine how I would connect this panel to an MPPT controller?  I have not yet purchased the controller and am trying to do this as a newbie step-by-step.  I'm hoping first to use a multimeter to assess how close the panel is to the rated specs for DC voltage and current. Thanks!
PanelConnectors.jpg
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PanelBox1.jpg
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PanelBox2.jpg
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By usual convention in the picture where you are holding the two wires the top one is negative the bottom positive. The positive wire's mc4 connector jutts out the negatives is flat and accepts it.
 
John Weiland
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Excellent!.....Thank you David B.   Seems strange to me that they still wouldn't put the little plus and minus signs on the connectors, but as you say it has become convention to do it this way across the industry.  As the panel is rated at ~50+ volts and ~5 - 6 amps, I will cautiously attempt multimeter readings with our 'smoky' sunlight, knowing the power could still pack a punch under these conditions.  If all goes well, the next step will be finding an MPPT controller that can link up the ~48V panel to the 36V battery bank (lead acid) and hope the chosen controller can charge LiFePO4 replacement batteries in the future. Thanks again!....
 
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John Weiland wrote:Excellent!.....Thank you David B.   Seems strange to me that they still wouldn't put the little plus and minus signs on the connectors....



If you look closely at your picture John, you can see a black + & - symbols just above where the wires exit the box.
No big deal to test the panel with a multimeter in full sun either.
 
John Weiland
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Gerry Parent wrote:

John Weiland wrote:Excellent!.....Thank you David B.   Seems strange to me that they still wouldn't put the little plus and minus signs on the connectors....



If you look closely at your picture John, you can see a black + & - symbols just above where the wires exit the box.
.............



Doh!!..(palm to forehead)....Thanks, Gerry!  OMG my eyes are getting bad.....Fortunately with your comment and the power of high-res cell phone cameras, I could see exactly what you noted when zooming in on the above photo.  Thanks you guys...will be back soon with test results.
 
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Could you please post another pic of a close-up of the solar specifications on the back of the panel?   The one obsured by your hand is out of focus.  Most likely, the 50V specification you are referring to is the open-circuit voltage, or Voc.  That is the voltage a meter will read when the panel is completely disconnected, meaning there is no load on it.

The voltage at which the panel can make it's maximal power is documented as the Vmp.  That I think is more likely to be in the 38-42V range.  This is quite important, because a 36V battery is NOT charged at 36V, but maybe 41-42V.  We might need a bit more information to help you put together the best performing system.

BTW, amps is documented the same way.  open circuit amps is identified as "Isc", and maximal amps when power is flowing is "Imp".
 
John Weiland
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My newbie adventure continues!....... ;-)

Initially, using a multimeter, I had hope to just probe the poles within the box shown in the figure a few posts up.  I was confused to be getting no readings, then looked more carefully at the box and the lid that covers the box.  The lid appears to have some contacts that need to be pressed into some receiver posts within the box....a safety measure I assume?  So the cover was replaced on the box and all subsequent measures made from the leads.  The polarity worked out just as David B. had indicated.

Now, with the panel laying on a cart angled mostly straight up and given a slightly overcast day at high noon (sunny, but a mix of wildfire smoke and high cloud haze), the open circuit voltage measured at 61.8 and the open circuit amps measured 5.8-5.9.  More detailed specs are given in the screenshot below.  Michael Q., am I correct in seeing on that spec sheet that the Vmp is ~54.7?  Is there some way that I can set up my measuring approach to determine Vmp myself?  Again, thanks all so far.  If the panel looks acceptable, next steps will be to decide how to mount the panel on the cart (panel weight ~41 lbs/18.5 kg) which already has a plastic canopy and sturdy supports and settle on a charge controller.  More to come....

Edited to add rough coordinates of our region:  46.8772° N, 96.7898° W.
PanelSpecs.JPG
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Michael Qulek
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John Weiland wrote:

Now, with the panel laying on a cart angled mostly straight up and given a slightly overcast day at high noon (sunny, but a mix of wildfire smoke and high cloud haze), the open circuit voltage measured at 61.8 and the open circuit amps measured 5.8-5.9.  More detailed specs are given in the screenshot below.  Michael Q., am I correct in seeing on that spec sheet that the Vmp is ~54.7?  Is there some way that I can set up my measuring approach to determine Vmp myself?  



Yes, 54.7 looks like the correct number.  Actually measuring the Vmp is a bit trickier than measuring Voc.  With Voc, you simply place the probe tips against the bare panel connections and take a reading.  To measure Vmp, the panel needs to be under load.  And, the Vmp can change a bit depending on what the load is.  Usually, a large enough load will drag the panel voltage down to what the load wants.

One way would be to wire four 12V batteries in series to make a 48V bank, and connect the raw panel leads to the bank.  The bank is a big load, and will likely drag the Vmp down to ~50-51V, depending on the state of charge.  With a fully charged battery, the load will be small, and the Vmp might ratchet up to around 54V or so.  With a significantly drained battery, maybe it will be drawn down to ~49-50V.

Another way to create a load is to wire four 12V automotive bulbs in series to create a 48V load, and then connect the panel to that.  The bulbs should light up.  Again, how much the voltage drops will depend on how big the bulbs are.

Since this panel is to be used in the application of charging a 36V battery, you must be careful to select a real MPPT controller to manage it.  The MPPT will transform the raw 50 something voltage down to exactly what the battery wants, maybe ~40-42V.  Beware though, there are a lot of fake MPPT controllers out there.  One clue is a cheap price.  A 30$ MPPT is likely to be fake.  Another clue is the shipping weight.  A controller that weighs 6 oz is fake.  One that weighs 6lb is likely to be a true MPPT.  A third clue is the max DC voltage.  If the controller says it has a max voltage of 36V for a 12V battery, that is a red flag.  Real MPPT controllers start at ~100V as the max they can handle.
 
David Baillie
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Michael is correct you will not get good charging using that panel for a 36 volt battery unless you cheat a bit. You would need a buck boost charger made for golf carts like this one:
https://www.lensunsolar.com/index.php?route=product/product&product_id=620
I have not used it so won't vouch for the quality but there are a lot of different manufacturers making similar units it boosts the voltage to that required by your battery.
 
John Weiland
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Thanks for additional input Michael and David....  I'm sure learning a lot here.  As I may have noted earlier, part of my interest in this project is to just get my head around EVs and battery power storage.  I suppose like many, when it comes to drive systems like cars, trucks, and farm equipment, I tend to think in terms of gas, diesel, and internal combustion engines.  This project is giving me a lot of insights into not only EV's but eventually power for the home that is off-grid or reduced-grid.

So I'm hoping I guessed right in purchasing a charge controller, a photo of which is shown below.  It weighs 1.6 kg and seems to have the blessings of many of the reviewers, which I know is no guarantee, but may help.  The specs *seem* to allow for the type of set-up that I'm looking for, but again as a newbie I'm grateful for the advice being provided here and hope it may help others on similar projects.  The controller was a bit spendy, but if it's a good one it should pay for itself down the road.  Additionally I feel (?) I may have gotten a decent deal on the panel (used) at $100....but again, time will tell.

Charging the 36V bank:  Drawing on analogy here, when I'm concerned about my riding mower or tractor charging the starter battery properly, I often look at the voltage at the terminals when the engine is running vs. when the engine is off.  I think when the charging system is working property, the 12V battery being charged registers about 14+V at the poles (charging) and something in the 13V range when not charging.  Does this sound correct?  By analogy, in order to see what the 36V bank in the golf cart *normally* charges at using the grid-based charger that came with the cart (I think it puts out 18A during charging), could I not also use a multimeter to see what the voltage is at the two main connection points within the cart?  Would this allow me to see the necessary charging voltage and then, in some way, be able to see if the PV panel would meet that, even if the amps being pushed by the panel are lower than what the main (120V grid-based) charger normally provides?

Again, much thanks for the help being provided through this discussion!
ChargeController.JPG
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David Baillie
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John Weiland wrote:Thanks for additional input Michael and David....  I'm sure learning a lot here.  As I may have noted earlier, part of my interest in this project is to just get my head around EVs and battery power storage.  I suppose like many, when it comes to drive systems like cars, trucks, and farm equipment, I tend to think in terms of gas, diesel, and internal combustion engines.  This project is giving me a lot of insights into not only EV's but eventually power for the home that is off-grid or reduced-grid.

So I'm hoping I guessed right in purchasing a charge controller, a photo of which is shown below.  It weighs 1.6 kg and seems to have the blessings of many of the reviewers, which I know is no guarantee, but may help.  The specs *seem* to allow for the type of set-up that I'm looking for, but again as a newbie I'm grateful for the advice being provided here and hope it may help others on similar projects.  The controller was a bit spendy, but if it's a good one it should pay for itself down the road.  Additionally I feel (?) I may have gotten a decent deal on the panel (used) at $100....but again, time will tell.

Charging the 36V bank:  Drawing on analogy here, when I'm concerned about my riding mower or tractor charging the starter battery properly, I often look at the voltage at the terminals when the engine is running vs. when the engine is off.  I think when the charging system is working property, the 12V battery being charged registers about 14+V at the poles (charging) and something in the 13V range when not charging.  Does this sound correct?  By analogy, in order to see what the 36V bank in the golf cart *normally* charges at using the grid-based charger that came with the cart (I think it puts out 18A during charging), could I not also use a multimeter to see what the voltage is at the two main connection points within the cart?  Would this allow me to see the necessary charging voltage and then, in some way, be able to see if the PV panel would meet that, even if the amps being pushed by the panel are lower than what the main (120V grid-based) charger normally provides?

Again, much thanks for the help being provided through this discussion!


Victrons are really good controllers. The problem lies in the working voltage of your solar panel. It's working voltage is lower than the absorption charging voltage of a lead acid battery. To get the best use out of that charge controller you would need say two panels in series to give you closer to 72volts coming from the array.
 
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David Baillie wrote:.... The problem lies in the working voltage of your solar panel. It's working voltage is lower than the absorption charging voltage of a lead acid battery. To get the best use out of that charge controller you would need say two panels in series to give you closer to 72volts coming from the array.



Wow....I'm really swimming in uncharted waters for my knowledge base here.  So David, would that Buck Boost charger that you linked rectify my problem and allow me still to use the single panel that I currently have?  If so, I would just leave the Victron in the original packaging and return it, using the savings to get the proper charging module.  

Also, am I thinking correctly that the absorption voltage is different if the batteries are, say, in a 50-60% charge state  vs a 90 - 95% charge state?  Because for our use pattern which is mostly very short distance/time and low load conditions and only used a few days per week, I'm wondering if it would still charge okay in good sunlight when the bank is near full, but require the wall charger if it gets to a lower state of charge.  I realize much of this conceptualization is quite vague and subject to many variables.  As I have not installed the panel yet, it's still possible that I could get a second panel and have them mounted in a manner in the yard to just provide a stationary charging station as an alternative.  Thanks for this and any additional advice you can provide.
 
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John Weiland wrote:

David Baillie wrote:.... The problem lies in the working voltage of your solar panel. It's working voltage is lower than the absorption charging voltage of a lead acid battery. To get the best use out of that charge controller you would need say two panels in series to give you closer to 72volts coming from the array.



Wow....I'm really swimming in uncharted waters for my knowledge base here.  So David, would that Buck Boost charger that you linked rectify my problem and allow me still to use the single panel that I currently have?  If so, I would just leave the Victron in the original packaging and return it, using the savings to get the proper charging module.  

Also, am I thinking correctly that the absorption voltage is different if the batteries are, say, in a 50-60% charge state  vs a 90 - 95% charge state?  Because for our use pattern which is mostly very short distance/time and low load conditions and only used a few days per week, I'm wondering if it would still charge okay in good sunlight when the bank is near full, but require the wall charger if it gets to a lower state of charge.  I realize much of this conceptualization is quite vague and subject to many variables.  As I have not installed the panel yet, it's still possible that I could get a second panel and have them mounted in a manner in the yard to just provide a stationary charging station as an alternative.  Thanks for this and any additional advice you can provide.


So... if you are using a lead acid battery the absorb charge voltage is higher than the rated voltage of the battery. roughly speaking if its a sealed lead acid absorb voltage for a 36 volt bank would be 42.6. Your victron would do bulk charging but without absorb you would sulphate your batteries and they would have a shorter life. the charge controller I listed is a type that is being used in a lot of carts. I do not have first hand experience with that particular unit but it would be your best bet for a single panel cart mounted setup.
 
Michael Qulek
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Could you please post the specs/pics of the batteries in this golf-cart?  Really, the exact battery is what will dictate exactly how you should charge them.  Let's take the example of a common golf-cart, the Trojan T-105.  Used a LOT in the US.  It's a 6V 225Ah battery.  Six would be wired in series to make a 36V battery bank.  It will want to be charged at ~44.5V (max).

Flooded lead-acid likes to be charged best at ~ 1/8th of C.  Since C is 225Ah, then it wants 225Ah/8 = 28.1A of charging current.  Your panel is 327W, so it's output at about 42V (bulk) is 327/42V = 7.8A.  Don't confuse the bulk setting with the actual bulk voltage.  With controller set to 44.5V max, it will start charging a depleted battery at about 40-41V.  As it reaches full charge, the voltage will go up to 44.5V (you set that).  As the charging cycle approaches full, the controller tapers off the amps as it reaches max voltage.  So, you are likely to see max amps at a somewhat lower voltage of around 40V or so.  The MPPT controls all that.

So, your single panel is really only capable of putting out ~8A, whereas, your battery wants 28A.  Four of those panels, maybe wired 2S2P, would provide the charge density you need.

Keep in mind these numbers are for the battery I picked as the example.  What we really need here is the exact battery that you have.  Though the numbers will change, the math stays the same.  Another factor is the real-world conversion factor.  Remember that panels are watt-rated in an test chamber with artificial light at exactly 1000W/square meter.  Real sunlight is usually a bit less.  I like to de-rate the panel specs by 85% to real-world output.  So, the math for four panels is ((327W x 4 PANELS)/40Vcharging) X 85% = 27.8A.

If four of those panels is too much to stomach right now, maybe at least two (wired in series) will allow you to just get by.  I would however plan on upgrading to four once finances allow.
T105-Capture.PNG
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David Baillie wrote:  So... if you are using a lead acid battery the absorb charge voltage is higher than the rated voltage of the battery. roughly speaking if its a sealed lead acid absorb voltage for a 36 volt bank would be 42.6. Your victron would do bulk charging but without absorb you would sulphate your batteries and they would have a shorter life. the charge controller I listed is a type that is being used in a lot of carts. I do not have first hand experience with that particular unit but it would be your best bet for a single panel cart mounted setup.


So in the table posted by Michael Q. below, is Absorb charge the same as Equalize charge?  
So this notion that the Victron would not do an absorb charge.....how do you know that?  Is there something in the specs of the charge controller that indicates the presence or lack of optimized charging steps.  My quite possibly false assumption was that these 'smart' controllers take care of almost everything during the charge as long as the batteries are in reasonably good shape, comply with the specs of the controller, and are of a fairly standard type/chemistry, but again I'm kind of jumping into this feet first.  Is there some reason they would not have included this kind of charging profile into their charger?  By comparison, the charger that I use now that plugs into a 120V (AC) outlet is shown below.  Strange that it says its output is 36V....shouldn't that be higher or is this just telling the user that the charger is for a 36V bank of batteries?  I'm also referring to the link below from a golf cart use and maintenance site....but don't know how accurate the information is.

https://www.chargingchargers.com/tutorials/36-volt-charger.php#:~:text=Recharge%20time%20can%20be%20approximated,2.22%20hour%20recharge%20time%20estimate.


Michael Qulek wrote:
1)    Could you please post the specs/pics of the batteries in this golf-cart?



So this is where the nightmare *really* begins! You both will be channeling Edvard Munch's "The Scream" famous painting at the battery labels on what is in this golf cart.  Please keep in mind that it was a relatively inexpensive purchase by an exceedingly inexperienced user ( :-/  ).  It even took me several weeks to realize that the original set of 6 batteries (all 6V) were actually mixed Ah ratings.  So there were 4 Durastart batteries at 186 Ah and 2 Interstate batteries at 210 Ah.  Two of the Durastart batteries were recently determined to be pretty bad....individually dropping total charge from full down to 6V overnight with no load.  So I saw a used pair of 225 Ah Interstates for sale for $100 and these hold charge nicely....these were used to replace the 2 'bad' Durastart.  So now the family is comprised of 2 Durastarts @ 186 Ah, 2 Interstates @ 210 Ah, and 2 Interstates @ 225 Ah.  Understood by my reading that this is NOT a good idea....mixing brands, Ah ratings, and different ages of batteries.  Again, my goal down the road would be to eventually replace these with LiFePO4 batteries possibly even next year as time and finances permit.

Michael Qulek wrote:
Flooded lead-acid likes to be charged best at ~ 1/8th of C.  Since C is 225Ah, then it wants 225Ah/8 = 28.1A of charging current.  Your panel is 327W, so it's output at about 42V (bulk) is 327/42V = 7.8A.  Don't confuse the bulk setting with the actual bulk voltage.  With controller set to 44.5V max, it will start charging a depleted battery at about 40-41V.  As it reaches full charge, the voltage will go up to 44.5V (you set that).  As the charging cycle approaches full, the controller tapers off the amps as it reaches max voltage.  So, you are likely to see max amps at a somewhat lower voltage of around 40V or so.  The MPPT controls all that.

2)      So, your single panel is really only capable of putting out ~8A, whereas, your battery wants 28A.  Four of those panels, maybe wired 2S2P, would provide the charge density you need.



This is a bit confusing to me....doesn't the rated amperage on the panel specs dictate the produced amps by the panel?  If the panel is rated for 6 - 6.5 A, I can see how panel decline could  lead to less amps being produced, but how is it capable of producing more?  Additionally, would those batteries want 28 A throughout the entire charging process or only during the bulk charging phase?  Also, why would they have made the charger that is sold with the cart only capable of putting out 21 A if the batteries would have been more optimally charged by ~28 A?  With the exception of circumstances where I drain the batteries down to near 50 - 60% SOC, it typically takes that plug-in charger less than 30' to indicate 'charge complete' (auto shut-off).  When this is achieved, the battery SOC meter reads 90 - 95%....Probably not optimal, but does give an acceptable amount of driving before the need for recharge.
Finally, I can certainly envision that at some state of discharge, the battery bank will be too depleted to allow for too small of a panel to charge it up, but if I charge to full overnight from the wall charger, wouldn't the panel be able to push amps into a nearly-full bank of batteries for much of the day (optimal sunlight of course) and keep them topped up?  Isn't this the basis for those small solar trickle chargers that are too small to *charge* a car battery, but are of sufficient charge to *maintain* a nearly charged battery?

Thanks for the continued education here.....hoping other newbies may be benefiting as well.
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BatteryDurastart186.JPG
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BatteryInterstate225.JPG
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David Baillie
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I think Michael was offering you and idealized setup. It's perfectly ok to use the single solar panel to extend range or slowly top up your battery bank and use the wall charger to do the fine tuning. It might shorten longevity slightly but these things are always about balance and responding to each person's unique goals. My understanding is you want to mount your single panel on the cart and add it's generation to the bank to add flexibility. If I'm correct I stand by my choices with the caveat of I have not used that charger yet but it or one like it is coming as standard on a growing number of carts.
Cheers,
David Baillie
 
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Thanks again, David, for this input and excellent information.  It's becoming clear as I float the panel specs to a few other manufacturers of controllers that one needs to be careful about matching panels and controllers properly to get not only the best result possible for the panel, but even to make sure the combination will work at all.  I supposed the following could be said for any petroleum base car as well, but when I think about the current scramble to provide the platforms for tomorrow's EV market, there's an amazing amount of technology behind the scenes that goes into devices that the consumer just plugs in "and it works!"....

Hoping to update later when I get an install in place... Thanks!
 
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This is a bit confusing to me....doesn't the rated amperage on the panel specs dictate the produced amps by the panel?  If the panel is rated for 6 - 6.5 A, I can see how panel decline could  lead to less amps being produced, but how is it capable of producing more?  Additionally, would those batteries want 28 A throughout the entire charging process or only during the bulk charging phase?  Also, why would they have made the charger that is sold with the cart only capable of putting out 21 A if the batteries would have been more optimally charged by ~28 A?

No, not with the latest electronics, what you are referring to is how older PWM controllers work.  Basically, with the older PWM controllers, you MUST carefully match the voltage of the panel to charge a particular battery.  They were basically a fancy on/off switch, sending only the raw amps coming out of the panel to the battery.  They still make PWM controllers, but they are relegated to the cheapest low-budget systems.

Modern MPPT controllers act as a transformer, taking the raw high-voltage, and transforming down to the battery charging voltage that YOU set.  The extra volts get converted into extra amps.  That's why MPPT controllers are better in every single way, except the price.

They are so much better, that some dishonest manufacturers of older PWM controllers claim that they are MPPT.  That's what I mentioned in the post above about fake MPPTs.

The only thing you need to remember is that the transformation can only work from higher voltage to lower voltage, not for lower to higher.  So, that means you always wire your panel string to a voltage higher than battery voltage.  Usually at least 30% higher.  So, for a 36V battery charging at ~42V, you want a panel string putting out >55V.  Keep in mind though that you must keep a healthy margin below the maximum voltage the controller can handle.  Since your controller can handle up to 150V, a string operating somewhere between 60-120V is fine.

For the commercial charger, did you actually MEASURE 21A, or does it just SAY 21A.  Most likely that is the maximum it can put out at a certain price point, which is not necessarily what is best for the battery.  The maximal amps happens when the battery is at a state of charge <80%.  That's bulk when the charger is flooding the battery with as many amps as possible.  As the battery approaches full charge, the amps going in drops down, until it reaches the float stage.  Then the amps are just dribbling in.  Keep in mind that there are many, many different sizes of batteries, and they aren't going to make a different charger for each and every battery.
 
John Weiland
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Just an update and thanks all for prior comments and assistance.

Well......maybe some success?

The photos below illustrate the show so far.  Initially I had purchased a controller that seemed to fit the project, but I was having a hard time connecting it with the accompanying phone app and there were no other controls or buttons on the controller to monitor how the connections were functioning.  In the end, I settled on a BougeRV controller ( https://www.bougerv.com/products/40a-mppt-solar-charge-controller ) after consulting with their tech services and presenting them with the panel specs.  All of my approaches perhaps not ideal, but juggling a lot of other factors around this project right now.  (Disclaimer that I have no financial or other compensatory interest in the products referred to in this post. )

At any rate, the golf cart roof was removed and I used perforated galvanized angle metal to serve as the panel support.  The controller was mounted inside the battery compartment and I will be adding a fuse between the controller and the batteries as well as an on/off toggle between the panel and the controller.  Installation was about as plug and play as you can get:  #8 AWG from controller to the charging terminals first and then #10 AWG with clip connectors to connect the panel to the controller.  The display on the controller indicated recognition at each step and no fault codes or alarms occurred in the process.  Even the app download and controller recognition went pretty smoothly.  After backing the cart out of the garage into the driveway and under an overcast sky, the the registered volts from the panel were around 51V and the charging amps coming from the charger were around 4.5 - 5 A.  Even under these conditions, it was gratifying to see the charge state of the batteries, monitored both by the phone app and the meter installed on the cart, were increasing as I was at a stand-still.  The manual for the controller indicates the charging profile that it performs for lead acid batteries.....don't know if it will be doing this for the panel provided or if the panel and situation for some reason will not permit this cycle to happen...???

Some photos below, with the charge controller shown in the battery compartment.  Please note that this cart is never left outside and we've always brought it in during rain so I'm cautiously optimistic that this NON-WATERPROOF controller won't meet with ill fate.  Let's hope not.  Even given the recent discussion on another thread of the pros and cons of lithium batteries, I do hope to convert to LiFePO4 batteries next year and will see if I wish to change the controller out at that time.  Here's hoping this phase of the project is going to ease our moving about the farmyard in a more energy sustainable way!
PanelOnCart.jpg
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ControllerInPlace.jpg
[Thumbnail for ControllerInPlace.jpg]
ChargingParametersController.JPG
[Thumbnail for ChargingParametersController.JPG]
 
David Baillie
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Congrats on getting it done.
 
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I see I'm late to the party.  At any rate, glad to see your system functioning.

Just a few tidbits to add, if I may;

I've been completely off-grid for 4 years now.  My solar storage bank is basically 48 Volts nominal (4x110Ah 12V deep cycle FLA's (flooded lead acid) batteries, all connected in series. I have six 12V (nominal), 100W rated panels, also connected in series for an average open Voc of 133.8V (each PV produces, on average, 22.3Voc each.  I use a Victron Blue 150-35 controller. (the same controller you initially purchased).

I see your panel is rated at 64.9Voc (open circuit voltage). So, connecting two of these identical panels in series should yield approx. 129.9Voc, which is well under your controller's 150v maximun input voltage. Obviously, connecting three (or more) like PVs would exceed the controller's 150V maximum input voltage - and may even damage it.

The Victron 150-35 would've been OK with your 36v (nominal) system, but much better with two, series connected PVs. BTW, the Victron controller has 4 charging stages - Bulk, Absorb, Float and Equalize. It is also compatible with Lithium Ion.

Lastly, testing a PV's maximum current output (Isc=Short Circuit), is self explanatory; Shade the PV from direct sunlight first, then Just connect a short 12" 10 gauge jumper wire across both PV's output terminals. Once connected, uncover the PV and aim it directly at the sun.  Use a DC capable clamp meter  ( https://www.amazon.com/AstroAI-Multimeter-Auto-ranging-Resistance-Capacitance/dp/B08MTCMWLB/ref=sr_1_3?keywords=dc%2Bclamp%2Bmeter&qid=1694491992&sr=8-3&th=1  ) around the jumper wire. A must-have tool when tinkering with electrons!
 
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Thanks for continued input on this topic.  Truthfully, the build so far has exceeded my expectations and for our particular uses....buzzing around farm-yard doing daily chores and projects, it's been amazing.  Not being immersed in solar/battery technology, I can't say how this charging/discharging cycling is affecting battery health, but I will continue to monitor the batteries for fluid levels, cable cleanliness, etc.  Just really liking the automatic nature of the charge controller and its interfacing with batteries and panels.

If I were to do this all over again for our purposes in a rural farmyard setting, I would

a)   choose a 48V golf cart.  Many available out there on the used market.....a bit more expensive than a 36V cart, but not by much.  It's not about the driving power....36V has been fine in that regard.  It's more about....

b)  getting a 48V inverter that could run corded 120V power tools along side of our cordless tools.

c)  try to buy a cart that has a lifted frame for better ground clearance.  In our farm-yard, no problems with a standard unlifted golf cart.  But out in pastures and other bumpy terrain, would be nice to have better clearance and plan to do that to our current 36V cart.

d)  The current charge controller is not sold as 'all weather'.  It's positioned under the seat and we are diligent about getting the cart out of the rain, but it just would give peace of mind to know it was built for inclement weather.

Just seems to be a great 'appropriate technology' whether going lead-acid, lithium, or future variations on that theme.
 
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Eugene Kenny wrote:I see I'm late to the party.  At any rate, glad to see your system functioning.

Just a few tidbits to add, if I may;

I've been completely off-grid for 4 years now.  My solar storage bank is basically 48 Volts nominal (4x110Ah 12V deep cycle FLA's (flooded lead acid) batteries, all connected in series. I have six 12V (nominal), 100W rated panels, also connected in series for an average open Voc of 133.8V (each PV produces, on average, 22.3Voc each.  I use a Victron Blue 150-35 controller. (the same controller you initially purchased).

I see your panel is rated at 64.9Voc (open circuit voltage). So, connecting two of these identical panels in series should yield approx. 129.9Voc, which is well under your controller's 150v maximun input voltage. Obviously, connecting three (or more) like PVs would exceed the controller's 150V maximum input voltage - and may even damage it.

The Victron 150-35 would've been OK with your 36v (nominal) system, but much better with two, series connected PVs. BTW, the Victron controller has 4 charging stages - Bulk, Absorb, Float and Equalize. It is also compatible with Lithium Ion.

Lastly, testing a PV's maximum current output (Isc=Short Circuit), is self explanatory; Shade the PV from direct sunlight first, then Just connect a short 12" 10 gauge jumper wire across both PV's output terminals. Once connected, uncover the PV and aim it directly at the sun.  Use a DC capable clamp meter  ( https://www.amazon.com/AstroAI-Multimeter-Auto-ranging-Resistance-Capacitance/dp/B08MTCMWLB/ref=sr_1_3?keywords=dc%2Bclamp%2Bmeter&qid=1694491992&sr=8-3&th=1  ) around the jumper wire. A must-have tool when tinkering with electrons!


Eugene, A good rule of thumb for max working voltage on controllers is panel string VOCx1.25.  When you have a cold day and the panels have not yet started producing the initial voltage easily goes over the VOC. Most MPPT Controllers can shed extra amps but not extra voltage.  I have replaced 4 controllers for people for that very reason, all in winter.  Its less of an issue with the new high voltage controllers but the 150 and 135 volt units are susceptible to it. Blue smoke of death is not good...
Cheers,  David
 
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David Baillie wrote:[......  When you have a cold day and the panels have not yet started producing the initial voltage easily goes over the VOC. Most MPPT Controllers can shed extra amps but not extra voltage. ....



David, more excellent information.  Can you elaborate on this effect?  Why, between a cold or warm weather situation with the panels receiving the same solar input, would the cold panels crank up voltage without producing amps (if I'm interpreting this phenomenon correctly)?  Again, forgive me my rudimentary understanding of the forces involved, but without amps, what contributes to controller burn-up without the involvement of amperage?  Thanks!....
 
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David Baillie wrote:Eugene, A good rule of thumb for max working voltage on controllers is panel string VOCx1.25.  When you have a cold day and the panels have not yet started producing the initial voltage easily goes over the VOC. Most MPPT Controllers can shed extra amps but not extra voltage.


Yes, I am aware of the voltage variations of PVs, based on changing ambient temperatures.  My 5 YO panels have noticeably aged, losing at least 0.03-0.06% output (the typical voltage increase when cold, for fresh panels).

David Baillie wrote:I have replaced 4 controllers for people for that very reason, all in winter.  Its less of an issue with the new high voltage controllers but the 150 and 135 volt units are susceptible to it. Blue smoke of death is not good...
Cheers,  David


Odd, I've not heard of anyone around here (or via the web) that has negligently toasted a Victron controller. The Victron Owner's manual is crystal-clear regarding maximum voltage inputs - that said, I suspect most owners do read the manual.
 
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John Weiland wrote:

David Baillie wrote:[......  When you have a cold day and the panels have not yet started producing the initial voltage easily goes over the VOC. Most MPPT Controllers can shed extra amps but not extra voltage. ....



David, more excellent information.  Can you elaborate on this effect?  Why, between a cold or warm weather situation with the panels receiving the same solar input, would the cold panels crank up voltage without producing amps (if I'm interpreting this phenomenon correctly)?  Again, forgive me my rudimentary understanding of the forces involved, but without amps, what contributes to controller burn-up without the involvement of amperage?  Thanks!....


My understanding of the phenomenon is the cold weather reduces internal resistance allowing for a higher initial voltage while not under load. The 1.25 safety factor rule comes from outback and midnite tech support. WHo knows maybe victron is immune but it is a common design number in my area. I do deal with temperature swings down to -40 Celcius though.
 
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Eugene Kenny wrote:

David Baillie wrote:Eugene, A good rule of thumb for max working voltage on controllers is panel string VOCx1.25.  When you have a cold day and the panels have not yet started producing the initial voltage easily goes over the VOC. Most MPPT Controllers can shed extra amps but not extra voltage.


Yes, I am aware of the voltage variations of PVs, based on changing ambient temperatures.  My 5 YO panels have noticeably aged, losing at least 0.03-0.06% output (the typical voltage increase when cold, for fresh panels).

David Baillie wrote:I have replaced 4 controllers for people for that very reason, all in winter.  Its less of an issue with the new high voltage controllers but the 150 and 135 volt units are susceptible to it. Blue smoke of death is not good...
Cheers,  David


Odd, I've not heard of anyone around here (or via the web) that has negligently toasted a Victron controller. The Victron Owner's manual is crystal-clear regarding maximum voltage inputs - that said, I suspect most owners do read the manual.

Trust me reading a manual is not typical! I am convinced half of my skill set in solar design is due to actually reading the manuals and following what they suggest. Having said that there is a lot of missing info that can only come from talking to their tech support people....
Cheers,  David
 
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This is getting off in the weeds, but the panel voltage as a function of temperature is a published coefficient on the datasheet, so you can accurately predict your peak open circuit voltage in cold. This is a better method than a rule of thumb, and I still put a little margin on the result for safety.
The physics of it has more to do with silicon semiconductor band gap voltages at lower temperatures (photons can kick the electrons to a higher potential when the electrons are more uniformly at a lower initial potential, so cold is lowering the initial state, and since all voltages are relative, this results in a higher overall panel voltage) than with resistance, though it is true that resistance is slightly lower at lower temps, but this is a second or third order effect.

That said, systems are best sized in a spreadsheet, where you can easily multiply Voc by panels, calculate and add the temp coefficient effect, and just see on virtual paper if you break the bounds of any other component. As David noted, manuals are a really good thing...

Final note, the Voc temp coefficient is NEGATIVE! That means, though, that lower temps (dT<0) means higher Voc. There are other temperature coefficients which can help you understand peak power over temperature, but the Voc coefficient is for not smoking expensive bits.
Happy homesteading,
Mark
 
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Mark Miner wrote:This is getting off in the weeds, but the panel voltage as a function of temperature is a published coefficient on the datasheet, so you can accurately predict your peak open circuit voltage in cold. This is a better method than a rule of thumb, and I still put a little margin on the result for safety.



Quite true, though a spreadsheet is beyond some people I think.  Here is a link to Midnight Solar's string calculator, which has the sheet already made.  Just fill in your numbers.  I myself have found it to predict a more accurate number than the rule of thumb.

http://www.midnitesolar.com/sizingTool/index.php
 
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Thanks for continued additions to this thread from the past year.  I'm happy to report that the 36V golf cart build is still going strong.  It sat for the winter in an unheated garage....a few -20F nights this past winter, but it was a mild one for our region.  During that period, I kept the batteries topped off every 2 weeks with the regular plug-in charger and since the MPPT controller is tolerant of 'back-fed' current, just left all of the wiring intact during this storage and re-charging period.  Storage was for 5 months and when it was taken out in late April, it started right up back where it left off.....charging up the batteries in the sun and giving me similar numbers as last year for panel output.

As noted above, I was keeping the option open for possibly upgrading to a 48V cart down the road if my 36V attempt was successful.  Thinking about that again now as we would prefer a cart with higher clearance for our farm property and for some of the attributes noted in a previous post within this thread.  My main question here is whether the current panel (see specs above in previous post) and controller could simply be transferred to a 48V cart.  The MPPT controller is switchable to charging a 48V system....but do the panel specs at 327W/~64VOC support using this panel for charging a 48V system where batteries are ~200 Ah?  I would be starting out with lead acid batteries but may be switching to LiFePO4 batteries down the road.  Thanks again for help and suggestions!
 
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