MPPT Charge Controller for LifePO4 Batteries

[mrm]

Originally Posted by dennisail

That proof is a joke. Proof would require some solar panels. As terry says, VMP will be way lower in the real world due to heat.

No, it is not a joke, it is a well designed experiment. Many posters seem to be confused with what it presents and are lost in nitpicking over Volts and Amperes. Try to clear your mind, take a step back and look again.

Forget Amps and Volts. Think of a solar panel as a power source. Now look at a set of panel performance curves several posts above. You will agree, that for a given panel temperature, there is only one point on the corresponding curve where the power output from a panel is at maximum (a Maximum Power Point). You will also observe, that this point can not be guessed in advance, because it moves with panel temperature (and is influenced by other external factors, but that is beyond the scope here).

Now go back to Maine's experiment. What he really did there? He set a bench power supply as a voltage and current limited power source. It should be observed, that the maximum power point of this source is when both current and voltage are right at their upper limits. Any other combination and the power taken from the source is less than the source could deliver.

Then he connected the MPPT power converter and demonstrated, that it was able to determine the voltage and current of maximum power point of a power supply it was connected to and stay at that point, thus constantly demanding maximum power from the supply.

He also connected a so called PWM regulator and demonstrated, that it simply shunted the power supply to the battery, which resulted in sourcing less power from the supply than it was capable of delivering.

So the main point of his experiment was to show the difference in operation between MPPT power converter and 'PWM' charge 'regulator', where MPPT power converter can locate and use the maximum power point of an unknown power source.

The secondary point he also demonstrated was that when you can source more power from the same source, you can put more power into the load (a battery in this case), but that is really obvious, isn't it?

[socaldmax]

Originally Posted by T1 Terry

It is obvious you don't understand much about solar panels. Unless you place the solar panel on ice the STC figure will never be a real world figure so using STC figures for theoretical test results will always be flawed.
A solar panel in the sun gets hot, at the point within the panel where the action is happening is not exposed to any cooling at all, it has a sheet of glass on top that is being heated by the sun and a sheet of reflective material on the back to maximise the cell efficiency, it gets real hot in there. The solar panel works best and gives it's max output when perfectly aligned with the sun, this is also where the max heat is transferred to the panel, how on earth could the panel temp remain at 25 deg C?

Next issue is how you measure input and output from an MPPT controller. Have you ever connected a CRO (cathode ray oscilloscope) to the input side of an MPPT controller? If you understand what you see on the screen you would understand why attempting to use digital equipment to read such a mix of frequencies is extremely inaccurate without a massive amount of filtering to reduce the signals to the ones the digital equipment can handle without giving false readings. Try putting an analogue volt meter and amp meter on the same connection points and do the math, this will at least reduce the error factor of the digital equipment due to the type of loads within there 2 analogue instruments, but the figures will still not match, nor will the figures be as high as they were without the analogue meters attached, hopefully then you will realise your test methods are flawed and any results are meaningless. But no doubt you will have it set in your mind the digital readings were correct and the analogue meters are causing interference and false readings

The bottom line is, if you want to charge your batteries using solar panels in the real world, you need to do the testing in the real world, not a lab bench or in the theory books, but in the real world using instrumentation that will accurately show the true values either side of the controller or more accurately, at the battery terminals and the entry point of the controller. Only then can you determine the percentage, if any, one controller type has over the other.
The simple one is by reading the Ah into the battery at the end of the day from each controller type, alternate controllers on alternate days, readings over a 1 mth, 2 mth and 12 mth period if you really want the full story, average out the reading and determine what the real difference is between one controller and the other.

T1 Terry

Great idea!

You take 2 panels, connect them to the PWM controller of your choice.

Take 2 identical panels, mount them right next to the other 2, and connect them to an MPPT controller. Use identical cabling and lengths. Mount both controllers side by side and use calibrated test equipment for the measurements. Record the whole thing, post it on youtube, then I'll believe the results. I'm thinking you will have to believe the results as well, since you are the one who controlled the test conditions.

[socaldmax]

I understand that you guys are stating that Vpm will decrease at a known rate as cell temp increases, but I think you're overstating the effect in the real world, especially in a marine environment where you may have a good breeze and possibly salt spray as a cooling effect.

I have 2 sets of solar arrays here in southern CA, and the ambient temp is around 100-105*F in the summer. According to your numbers, I should be seeing a huge drop in solar panel output, and it's just not happening. Any time an object is significantly above the ambient temp, it's going to radiate, or conduct, (or both) that heat into the surrounding objects and area. Even with only 2" of clearance under the panels, they just aren't losing that much power out.

[Maine Sail]

Guys the testing I was referencing was NOT that demonstration. That demonstration does not translate into real wold but it shows how an MPPT works, which was the point. I believe I mentioned as much in that demonstration..

The test I was referring to was done using two identical 140W Kyocera panels one fed through a PWM and one through MPPT. Wire lengths & gauges were identical.

Each system had a shunt in the neg feed to the LFP bank. Each shunt was measured by a Bogart Engineering Pentametric monitoring system which can simultaneously monitor multiple shunts and multiple voltages. The bank started at 80% DOD and both controllers remained in bulk. I repeated the test over multiple different days including a rainy day. MPPT beat PWM in that test..

I did the test for my own curiosity to pick a controller to use with my LFP bank.. I will apparently need to redo it and record the performance so I can speak more accurately to specific performance differences, especially with this much interest...

Fortunately I have two Kyocera 140's in my shop and plenty of MPPT's and PWM.. Need to wait till it gets a tad warmer though..

[noelex 77]

Originally Posted by socaldmax

I understand that you guys are stating that Vpm will decrease at a known rate as cell temp increases, but I think you're overstating the effect in the real world, especially in a marine environment where you may have a good breeze and possibly salt spray as a cooling effect.

I have 2 sets of solar arrays here in southern CA, and the ambient temp is around 100-105*F in the summer. According to your numbers, I should be seeing a huge drop in solar panel output, and it's just not happening. Any time an object is significantly above the ambient temp, it's going to radiate, or conduct, (or both) that heat into the surrounding objects and area. Even with only 2" of clearance under the panels, they just aren't losing that much power out.

It is easy to measure the cell temp with an IR thermometer (every boat should have one anyway)

Ambient temperature is a factor, but the cells heat up reasonably well in bright conditions with moderate air temperatures. The NOCT test is conducted at only 20c ambient temperature, but the cell temperature is typically 40-45c.

The power loss from the rise in cell temperature (above STC conditions) is only mild. The significance is that the predominant effect is to reduce Vmp. This has little effect on PWM controllers, but has a greater effect on the output of MPPT controllers. It is therefore important to use real world conditions ( or NOCT, rather than STC specs) when comparing the output of MPPT and simpler PWM controllers

[noelex 77]

Originally Posted by Maine Sail

I did the test for my own curiosity to pick a controller to use with my LFP bank.. I will apparently need to redo it and record the performance so I can speak more accurately to specific performance differences, especially with this much interest...
.

. That would be great.

[dennisail]

Originally Posted by Maine Sail

The test I was referring to was done using two identical 140W Kyocera panels one fed through a PWM and one through MPPT. Wire lengths & gauges were identical.

Each system had a shunt in the neg feed to the LFP bank. Each shunt was measured by a Bogart Engineering Pentametric monitoring system which can simultaneously monitor multiple shunts and multiple voltages. The bank started at 80% DOD and both controllers remained in bulk. I repeated the test over multiple different days including a rainy day. MPPT beat PWM in that test..

I did the test for my own curiosity to pick a controller to use with my LFP bank.

That is a good and realistic test. Do you have detailed results? How hot was it? The bench power supply test is good at showing what MPPT is capable of, but is not an indication of how it will behave in the real world. I live in tropical and hot climate where I could almost cook an egg on my panels A test like the one you speak of will be the only realistic test to go by, and for the results to be accurate one must also take the local conditions into account.

[socaldmax]

Originally Posted by noelex 77

It is easy to measure the cell temp with an IR thermometer (every boat should have one anyway)

Ambient temperature is a factor, but the cells heat up reasonably well in bright conditions with moderate air temperatures. The NOCT test is conducted at only 20c ambient temperature, but the cell temperature is typically 40-45c.

The power loss from the rise in cell temperature (above STC conditions) is only mild. The significance is that the predominant effect is to reduce Vmp. This has little effect on PWM controllers, but has a greater effect on the output of MPPT controllers. It is therefore important to use real world conditions ( or NOCT, rather than STC specs) when comparing the output of MPPT and simpler PWM controllers

Just so I understand, are you saying that a system with a PWM controller will put out more power than an MPPT system when the solar panel cell temp. rises above (and Vmp drops below) a certain point?

Do you have any links to youtube videos of this effect, or any articles, sources, etc?

[noelex 77]

Originally Posted by socaldmax

Just so I understand, are you saying that a system with a PWM controller will put out more power than an MPPT system when the solar panel cell temp. rises above (and Vmp drops below) a certain point?

No. The relative improvement in output gained from the MPPT voltage conversion falls as the the cell temperature rises and the Vmp falls.

The self consumption of MPPT controllers and the losses in voltage conversion are a bit higher than a simpler PWM controller so at very low Vmp's the output of a PWM regulator can be higher than a MPPT controller, but the conditions where this occurs is not very common and temperature alone is not likely to be the cause.

[T1 Terry]

I guess it very much depends where you are at the time. Ice bound winter doesn't really lend itself to thoughts of high temp effect, but over here we are still feeling summers bite and it's a real issue, and it will continue to become an issue all around the world as the years to come. No matter what you believe is the cause, the place is getting hotter each yr, Aust broke 150 peak temp record this yr, including a 24hr average of 35.5*C in Melbourne, 6 days straight at plus 42.5*C in Adelaide etc
Solar panel operating temps will continue to rise make no mistake, yet false figures are put forward as proof of concept. What is the likelihood of mid day summer ambient temps being 20*C? Yet this is the test conditions solar panel manufacturers use for determining NOCT (Normal Cell Operating Temp) Now considering how ridiculous that sort of scenario is, the NOCT figures are still around 50*C. Even at 50*C the Vmp is much lower then the 17.5v quoted by MPPT supporters as the Vmp they use for their calculations, look at the chart in post #122 and the photo in post #140, that brings the Vmp back to around 15v.
Please look at the claims with your eyes wide open, unless it snows in summer where you plan to use your solar panels, the claims are nothing but a work of fiction.

T1 Terry

[socaldmax]

Originally Posted by noelex 77

No. The relative improvement in output gained from the MPPT voltage conversion falls as the the cell temperature rises and the Vmp falls.

The self consumption of MPPT controllers and the losses in voltage conversion are a bit higher than a simpler PWM controller so at very low Vmp's the output of a PWM regulator can be higher than a MPPT controller, but the conditions where this occurs is not very common and temperature alone is not likely to be the cause.

It sounds like the solution to this problem is to use 24v panels for a 12v battery system, or 35 or 40v panels for a 24v battery system. This way the effect is negligible, or non-existent.

[DumnMad]

My SP's do very well in hot sunny weather. I'm thought the most important performance requirement of my solar panels was their output in cloudy conditions and mornings and evenings when an ambient temperature of 20 degrees might be quite representative.

[noelex 77]

Originally Posted by socaldmax

It sounds like the solution to this problem is to use 24v panels for a 12v battery system, or 35 or 40v panels for a 24v battery system. This way the effect is negligible, or non-existent.

The short answer is no.

There is some debate between the efficiency of series and parallel connection, particuarly with the isolated shading that is common on yachts the result is difficult to model theorically and I am not sure we have reached a difinative conclusion

24v panels are effectively just two smaller 12v panels that are connected in series in the one physical enclosure. Using a 24v panel with a 12v battery needs a MPPT controller, without this voltage voltage conversion a PWM controller would be inefficient, but the net result is similar, but not as good as two separate 12v panels and only a tiny fraction better than the same sized 12v panel.

The Vmp of 12v 36 cell panel is generally well above battery voltage even in high temperatures and low light. Under low light conditions the solar output drops to nothing because the cells can no longer produce useful current. The voltage is adequate long after the current has negligible and increasing panel voltage is of little benefit.

[T1 Terry]

The voltage is adequate long after the current has negligible and increasing panel voltage is of little benefit.

This is the part most people get confussed with grid tie inverters and low voltage DC to DC MPPT controllers, an MPPT controller can't change volts in to amps.
Watts is the measurement of power, 36v @ 0 amps equals 0 watts = 0 power. The only time a higher voltage panel array will be of benefit is when the Vmp would otherwise drop below the battery terminal voltage, then there would still be some output no matter whether an MPPT or PWM controller was used, but with a voltage mismatch like that, the MPPT controller would be the better choice as the overall output through a full charging day would be higher.
There are solar panels with high amp outputs and 16v mentioned, this 16v is open circuit voltage, STC Vmp is around 13v, if you could keep the panels at 25*C these would be the best panels of all, yet ayone who has bought them by mistake for charging 12v battery will tell you they were useless, the NOCT voltage is down around 11.5v, such is the effect of the sun heating the panels. So, if someone comes up with an effective method of bolting a heat exchanger to the panels and pumping seawater through it, then these panels will be in big demand :lol:

T1 Terry

[goboatingnow]

Originally Posted by T1 Terry

This is the part most people get confussed with grid tie inverters and low voltage DC to DC MPPT controllers, an MPPT controller can't change volts in to amps.
Watts is the measurement of power, 36v @ 0 amps equals 0 watts = 0 power. The only time a higher voltage panel array will be of benefit is when the Vmp would otherwise drop below the battery terminal voltage, then there would still be some output no matter whether an MPPT or PWM controller was used, but with a voltage mismatch like that, the MPPT controller would be the better choice as the overall output through a full charging day would be higher.
There are solar panels with high amp outputs and 16v mentioned, this 16v is open circuit voltage, STC Vmp is around 13v, if you could keep the panels at 25*C these would be the best panels of all, yet ayone who has bought them by mistake for charging 12v battery will tell you they were useless, the NOCT voltage is down around 11.5v, such is the effect of the sun heating the panels. So, if someone comes up with an effective method of bolting a heat exchanger to the panels and pumping seawater through it, then these panels will be in big demand :lol:

T1 Terry

No issue designing a SEPIC mppt to handle the low voltages can't see why someone hasn't din it

Dave


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