Workings Of A MPPT Controller

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MarkK

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Pursuant to many discussions with jimindenver, john61ct, ChetLazar, and tothers, and taking up Jim's offer for a discussion on the workings of the mppt style controller, this thread is being started to see what evidence Jim can share with us, or anyone else for that matter.

If I understand it correctly, the question is whether panels wired in series as opposed to parallel, will offer more volts/amps to a bank of batteries under no load in bulk mode.

And it is likely I got something wrong so will stand by for corrections.
 

SoulRaven

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MarkK said:
Pursuant to many discussions with jimindenver, john61ct, ChetLazar, and tothers, and taking up Jim's offer for a discussion on the workings of the mppt style controller, this thread is being started to see what evidence Jim can share with us, or anyone else for that matter.

If I understand it correctly, the question is whether panels wired in series as opposed to parallel, will offer more volts/amps to a bank of batteries under no load in bulk mode.

And it is likely I got something wrong so will stand by for corrections.

I get it conceptually.  Jim has tried to explain it to me only about a million times.  I've either got a mental block somewhere, or I'm finally just too old to learn.  I will enjoy playing voyeur on this discussion.  I might finally get it.

Regards,

Pat
 

regis101

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To further understand, the Series, Parallel, Series-Parallel, I will link this YouTube vid.
It has helped clarify a few questions I had.  I believe I am acting cordially with the intent of this thread.


Then grab a beverage and view this.


Hope I'm helping.
 

ChetLazar

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regis101 said:
To further understand, the Series, Parallel, Series-Parallel, I will link this YouTube vid.
It has helped clarify a few questions I had.  

Near the end of the first video the presenter discusses the resistance of the wire connecting the panel array to the charge controller.  He suggests reducing the current in that wire with a series panel connection to reduce the power lost to this resistance.
 

regis101

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At about the six minute mark he speaks of increasing the voltage through series which will in turn reduce current to get through any loss due to wire resistance.

Anywho, this is all I can bring to the table, being a noob and all.  I look forward to gaining continued knowledge.
Has anyone seen my corkscrew??
 

MarkK

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regis101 said:
At about the six minute mark he speaks of increasing the voltage through series which will in turn reduce current to get through any loss due to wire resistance.
Hi Regis.

I watched the videos as well. One thing I have learned, is I can use smaller wire when running my set up in series. Using the online calculator it called for 8AWG when wired in parallel, but in series it calls for 14AWG. Waiting to hear back on that info here or on the other thread about doubling the wire.
 

jonyjoe303

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From my own actual use of pwm (wincong 20 amp) and mppt (ecoworthy 20 amp) on both a 120 watt (21 volt) and a 240 watt (36 volt) panels. Both panels flat on the roof, below is what I got when connected to lead acid battery.

120 watt panel
pwm = 6 amps
mppt = 6 amps

240 watt panel
pwm = 6 amps
mppt = 12 amps 

One thing I always wonder is if I connect 2 x 120 watt (21 volt) panels in parallel to a pwm or mppt what would I get?  I suspect I would get 6 amps
I see many kits sold with 2 panels and a pwm controller and wonder if that is the most efficient system.

I know connecting 2 low voltage panels in series will give me 42 volts, mppt will be able to work with the higher voltage and give me at least 12 amps. 
I think if you have 2 low voltage panels it might make more sense to just use 2 pwm controllers and connect them separately, that will get you 12 amps total. 2 pwm controller might cost less than 40 dollars. Cheaper than most mppt controllers.
 

jimindenver

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About running panels in series. Each MPPT controller has a Voc limit, DO NOT exceed it. Many MPPT controllers can clip excess amps when they exceed the controllers capacity. This is called over paneling and a practice I disagree with but that's another discussion. What a MPPT controller is intolerant of is exceeding the Voc limit. It will fry most of them except a few that will stop charging if the limit is exceeded. In fact a good buffer should be observed because bumping right up to it could cause failure. Panels are rated at sea level and easily exceed their ratings at the high altitudes I camp at. So much so that if you are pushing your controllers Voc limit, it can die. Bob speaks of just such a incident where a drive over a 12,000 ft pass popped a controller. Cold can produce the same effect. I have watched a panel rated at 36v jump to over 40v when exposed to bright sunlight on a freezing cold day. Combine altitude and cold to see bigger spikes and amps out like you have never seen if your controller can take it.

Another thing about running in series is the efficiency of the conversion. I will use my trailer as a example. I have three 250w panels rated at 35v running in series for 105 Voc. In theory they should put out just as much power in series as they do parallel. The thing is each panel is capable of 18.5a in parallel yet I only see 50a in series. Where did 5.5a go? Simply put the conversion from 35v to 12v is more efficient than it is at 105v. Not only do I loose power but the inefficient conversion causes heat to the point that the controller will reduce output to protect itself. I want to run a test between two 12v panels in parallel vs series to see if a 12v panel on its own has too little excess voltage for the buck converter to do much with. I am sure it is on the net but I would rather see it in person than regurgitate it.

This is my favorite link on MPPT

https://www.solar-electric.com/lear...d-charging/mppt-solar-charge-controllers.html

Thank goodness there are you tubes and links like this because as Pat elluded to, I am not very good at explaining it. The important thing is not to convince people that MPPT is the only way to go but rather make it so they can make a more educated choice. There are some misconceptions like what happens in various charging modes. There are advantages to a PWM system when it comes to flexibility of panel combinations. A open, non defensive discussion could benefit many. Everyone finds their balance and I'd rather understand it without them needing to defend it.
 

jonsun

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low end controllers time limit the MPPT function usually 120 minutes, 180 user defined. during this time wiring series is pretty good benefit. During the rest of the day.....a controller in stage 2 is operating a constant voltage and current limiting. It's not both boosting and limiting at the same time (unless someone has proof otherwise). 3 100w 18v panels in series in stage 2 are outputting 1.8a versus 300w @ 18v - 16.7A
 

MarkK

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bardo said:
low end controllers time limit the MPPT function usually 120 minutes, 180 user defined. during this time wiring series is pretty good benefit. During the rest of the day.....a controller in stage 2 is operating a constant voltage and current limiting. It's not both boosting and limiting at the same time (unless someone has proof otherwise). 3 100w 18v panels in series in stage 2 are outputting 1.8a versus 300w @ 18v - 16.7A

Verifying your point is once bulk is done, and the CC is on absorb mode, wiring panels in series limits amps to the batteries and becomes @50% as efficient as the same set up wired in paralell in absorb, and even float modes?
 

jimindenver

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Bardo

The buck converter makes 100% of the panels output available in all modes. Your battery may not be accepting it but it is there and can be used to power a load. That's how a 230w panel can put out 15a in float while it is rated for just over 8a. That is why I can run the air conditioner and yet never lower the banks voltage even in float. Turn off the loads and the batteries are accepting hardly anything, turn on a load and the output of the controller will rise to cover it and keep the batteries voltage stable.

This is one of the misconceptions about MPPT that I hope to clear up.
 

ChetLazar

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MarkK said:
Verifying your point is once bulk is done, and the CC is on absorb mode, wiring panels in series limits amps to the batteries and becomes @50% as efficient as the same set up wired in paralell in absorb, and even float modes?

That is not what happens.

As the battery gets full the battery charging charging current goes down.  That's what happens to maintain 14.3 volts absorb at the battery.  The battery takes less as it gets full.

If your fridge compressor comes on and takes 5 amps the MPPT controller steps down the volts and steps up the amps.  In the absorb mode the controller keeps the voltage at 14.3  The battery could be taking 2 amps and the fridge taking 5 for a total of seven.  When the fridge cuts off those amps stop flowing to keep the absorb voltage constant.

If your fridge thermostat is off (cold enough) and you unplug all phones and computers, and turn off all lights and TVs and the battery is full taking 1/10 amp in float mode then of the 8 amps possible, only 1/10 amp will flow.  With no other load what should happen to the 7.9 unwanted amps?  The amps are available.  If the fridge compressor comes on the amps will flow.  When the compressor shuts off the current goes back down.
 

MarkK

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ChetLazar said:
That is not what happens.

As the battery gets full the battery charging charging current goes down.  That's what happens to maintain 14.3 volts absorb at the battery.  The battery takes less as it gets full.

If your fridge compressor comes on and takes 5 amps the MPPT controller steps down the volts and steps up the amps.  In the absorb mode the controller keeps the voltage at 14.3  The battery could be taking 2 amps and the fridge taking 5 for a total of seven.  When the fridge cuts off those amps stop flowing to keep the absorb voltage constant.

If your fridge thermostat is off (cold enough) and you unplug all phones and computers, and turn off all lights and TVs and the battery is full taking 1/10 amp in float mode then of the 8 amps possible, only 1/10 amp will flow.  With no other load what should happen to the 7.9 unwanted amps?  The amps are available.  If the fridge compressor comes on the amps will flow.  When the compressor shuts off the current goes back down.

I think I'm good with this explanation, but I was trying to verify whether this was what Bardo was getting at. Seems to me his point was that the MPPT doesn't boost the amps with the extra current except for the bulk stage. Essentially that absorb does push the selected volts but without the benefit of the conversion of extra volts to amps.
 

MarkK

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How does a MPPT controller, such as the Victron 75/15 know to deliver volts for a 12V battery bank and not a 24V battery bank when it receives higher voltage due to series wiring and if the battery bank is 12V?
 

jimindenver

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Some controllers select the voltage of the bank when you connect it, others require you to tell it what voltage the bank is. This is why you hook up the controller to the batteries first. Otherwise what is coming in from the panels is converted to what the bank needs by the buck converter.
 

frater secessus

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MarkK said:
...I was trying to verify whether this was what Bardo was getting at. Seems to me his point was that the MPPT doesn't boost the amps with the extra current except for the bulk stage.

That is a common misperception (or perhaps misstatement) which I think Bardo was repeating.  Even MaineSail and Handybob have repeated it as an oversimiplification while discussing the (real) limitations of MPPT controllers.  A more correct expression of the concept might be something like:


Under usual conditions the maximum benefit of MPPT controllers will be observed during Bulk phase.


Jim's assessment is correct, as can be easily proven by loading the controller down during non-Bulk phases and watching the controller hold ~Vmp and trading volts for amps.
 

frater secessus

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Consider the following:  yesterday I was working in the van and observed my system idling along in Float.   The CC had moved panel voltage upwards toward Voc to decrease panel output to match the decreased needs of Float + minimal load.

For the purposes of this demonstration I I turned on the roof vent, a 12v fan, and a small inverter to run a CFL drop light.  The CC responded by moving panel voltage back down toward Vmp to increase power to match the needs of Float + increased load.




Here we have the defining characteristics of MPPT (power point tracking and DC-DC downconversion) operating in a mode other than Bulk. 

Q.E.D.
 

jimindenver

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At the time Solar Bob wrote his blog a high voltage panel retailed for over $700 and the least expensive controller to run it was $325. It was easy to see that a MPPT system like it cost much more than a similar sized PWM system. Since then the cost of the panels have plummeted and the MPPT controller to run it is under $100. Find the right deal on the panel and you can have a sub 300w system for under $200. The reality is the economics have changed making the benefit of MPPT affordable and a no brainer. The last bastion of those sticking to the old school way of thinking is to discredit the advantages of the controller while they really don't understand how it works.

A series of side by side test on two systems of the same size would show the differences but for all of the effort I have found that those that do not want to hear it will discredit the test, the equipment and even the tester. Those that are willing generally will listen and not need the test as proof.
 

jonsun

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A) Bulk Charging
In this stage, the battery voltage has not yet reached constant voltage (Equalize or
Boost Voltage), the controller operates in constant current mode, delivering its
maximum current to the batteries (MPPT Charging).
B) Constant Charging
When the battery voltage reaches the constant voltage setpoint, the controller will
start to operate in constant charging mode, this process is no longer MPPT
charging, and in the meantime the charging current will drop gradually, the
process is not the MPPT charging. The Constant Charging has 2 stages,
equalize and boost. These two stages are not carried out constantly in a full
charge process to avoid too much gas precipitation or overheating of battery

http://www.epsolarpv.com/en/uploads/news/201512/1449040581526220.pdf


this is why you see people with sooo much solar for normal loads. they were sold high voltage, high wattage panels for cheap or wired panels in series thinking they were bucking the system. Then think that 1/3 output is normal because you only get 4hrs in the summer of good sun. WRONG. Then they bought huge battery banks that never drop below stage 2. So the 600w+ on the roof never even uses MPPT and they only get sub 100A a day. All of which they ended up paying 3x as much for.

get the lowest voltage panels you can find and get any controller that has user voltage settings. you will pay less and get much more charge. MPPT is and always was a gimmick right along with selling 200w 29v panels for $100 less. people go "yeah but if you get mppt it will convert that excess voltage you're saving money and getting more efficiency!!!" It's sad. you're spending a dollar to save a nickel.
 
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