MPPT vs PWM Solar Regulators

[kaimusailing]

I see an example shown with the battery voltage at 14.4 and the MPPT panel voltage at 16. This is very unusual. The most common MPPT operating volatage I've seen is 17.5 and my batteries are at 13.8 when not under load. 14.4 is more like an equalization voltage, not the bulk charging voltage. If you take a more typical state of a battery needing recharge, it would be in the high 12 to 13 volt range. I agree that adding more panels is the cheapest way to add more power, but when you run out of room, an MPPT controller will make a big difference, not a little, 5-10 percent difference.

[Dsanduril]

I see an example shown with the battery voltage at 14.4 and the MPPT panel voltage at 16. This is very unusual. The most common MPPT operating volatage I've seen is 17.5

The most common VMpp for a 36-cell panel is about 17.5V - 18.5V at 25 degrees C panel temperature. Panels normally operate at about 20 degrees C above ambient temperature, and most panels have a voltage coefficient of about -0.35%/C. So, if ambient temperature is 25C then operating voltage of the panel is about 1.2-1.3V less than the rated VMpp, which puts you squarely in the 16-17V range.

This is a very typical real-world operating range for a panel. It takes an air temperature of about 5C to get a panel to operate at its rated VMpp. In the tropics (where many like to sail) panels frequently operate at temperatures above 50C and the voltage drop is even greater.

[noelex 77]

Good advice from Dsannduril. The manufactures of MPPT don't want you too know about the temperature coefficients. At the rated illuminences cell temperature is likely to 40C+, not the 25C that the cell voltages are quoted for.

The tests for standard output are produced with brief flashes of light to overcome the heating effects.

Add some shadow and lower illuminences and the Vmp is generally in the 15-16.5v range in real life, unless you are cruising very cold, but bright areas.

[cfarrar]

Kaimu has a point: output voltage only approaches 14.2-14.4 when the batteries near full charge. Low 13s is more typical if batteries are partially dischared, and maybe even lower if the batteries are under load. My panels are rated at 12.5amps @ 17.5 volts, but plenty of times this summer my Blue Sky controller was putting out 14-15 amps - that's with air temp about 15C, low humidity, a nice breeze to cool the panels, batteries partically discharged, and me working on my laptop, WIFI, etc.

[T1 Terry]

The whole STC rating of panels is theoretical nonsense, smoke an mirrors stuff, no panel can stay at 25 deg C under full sun unless it's been packed in ice or something. The true panel output is the NOCT (normal operating cell temp) spec but solar panels look rather sad at those specs so resellers use the STC specs knowing full well only 70% of that can be achieved in the real world.
The possible 10% gain from an MPPT controller is out of that 30% lost, not on the STC rated output. In a 1.5kW or higher system charging lead acid battery, worth it, a smaller system or charging lithium batteries, not only over priced for the results but sometimes produces less than a PWM controller over a full days output.
Boats have a huge advantage when it comes to solar, with light scattered cloud the sun light will reflect off the water, then off the cloud giving a double hit to the panels. I live between a large lake and the ocean, in the right weather conditions I can get greater than the panels STC rating via a PWM controller so spot measurements aren't an accurate method of measure.
Direct charging a 12v battery with 13v across the terminals (4 cell lithium battery) will produce higher amp reading than charging a 15v battery with 16.5v across the terminals (5 cell lithium battery) so an MPPT controller reading amps in at 16v plus and amps out at 13v or lower will look good but are a distortion of what would really happen.
I have trialled a PWM controller v a good quality MPPT controller using a battery switch between the solar panels and the 2 controllers, at certain periods during the day each controller has a better output than the other, neither are better for the full day. In the middle of the day under full sun the MPPT controller was behind the PWM controller, using an infra red thermometer on the back of the panels read 52 deg C, the Vmpp was 14v, the PWM controller was in full pass mode, the MPPT controller still had to power it's circuitry and cool itself but at 14v charging a battery at 14v across the terminals there were no gains, only losses.
A similar trial on a rainy day, rarely was the MPPT ahead of the PWM controller, again, that circuitry needs to be powered, if the panel output is low there is no gain, only losses. Start up in the morning and last thing in the evening before shut down, the PWM is always in front, again, that power to drive the circuitry must come from some where so the low input stages only produce enough to power the MPPT controller, none left to charge the battery. The smaller the panel array the greater the problem.

The debate between PWM and MPPT will go on for ever, generally it comes down to the person who has bought an MPPT controller can see figures displayed to justify their purchase so they are happy, PWM owners have charged batteries at the end of the day so they are happy.

The reflected sun between the water and the clouds will always give a higher output no matter which controller you use.

I'm now trialling going beyond using a solar controller at all a such, but that's a whole different story.

T1 Terry

[Captain Bill]

I had my xantrex C40 PWM controller die on me in June so I replaced it with a Bluesky 2512x and an IPN-proremote. I find that I am actually less efficient with the MPPT that the pwm controller. The program that contols the charge on the Bluesky appears to be too simple minded for the real world. The unit goes into float mode way too quickly. It interprets any drop below the acceptance threshold as a battery full condition, when it is clearly not. The most common cause is simply a cloud going by. The panel out drops below the acceptance threshold and the unit interprets this as battery full and shuts down the panel output. It will not go back in to acceptance mode until you've drained enough power from the batteries to go back into bulk and the voltage gets back up to acceptance. My xantrex was not this stupid and kept the batteries in acceptance mode until the charge was at the acceptance threshold for a significant period of time. I thought my unit was defective when I first got it, but Bluesky tech support assures me it is working as designed.

The only time I saw output amps exceeding input amps on my unit was in Maine late in August when the temperature dropped. The remainder of the summer my output amps seemed to be significantly less than what I was seeing historically with the Xantrex. Since I wasn't doing a side by side comparison I can't say for sure, but in NC in June I would regularly see 15-16 amps on the Xantrex and never saw above 13 on the Bluesky. At this point I wish that I had just bought another PWM and saved a bunch of money.

[Aussie_Sequoia]

Just a quick note. My experience comes from 16 years in the domestic solar industry (I've worked for Trace [which turned into Xantrex], SMA America and even OutBack)... not so much on boats, but 2 things that I like to keep in mind when other yachties ask my advice is this:

If you are cruising near the equator (as many of us do) temperature of the panel will be higher and will cause the output to drop - through less efficient transfer of the semiconductor itself... this can often translate into further reductions in output in a MPPT.

Also, if you purchase a higher voltage MPPT controller (all solar innovations are designed for the domestic market not the tiny boat market) this was made for high voltage strings (all in series) of many panels on house roofs that never move. Boats move. If you are putting your panels in series to attain high voltage and you have them spread across your deck or on either side of your rails or on your arch they will inevitably be in the shade for part or half the day due to the mast/sail configuration (I guess trawlers are exempt here?). This will reduce the output to: ZERO.

So, if you put your panels in series... to gain 10%-20% (factory claimed) by using a MPPT, you easily can loose 50% of total output but having the panels in series but having one in the shade. This is a false economy if I ever have heard of one.

It is my opinion that series connections have no business on boats.

[onestepcsy37]

first a quick background. i have 450ah lead acid house bank. i have light electric loads - the biggest being an adler barbour fridge - total load being maybe 80 amps a day max. i have one 135 watt kyocera panel. it is DIRECTLY wired to the battery bank with just a 10amp fuse and an on/off switch in between. there is no controller of any kind.

i can do this because i'm guessing the panel puts out 40-50 amps a day and the fridge alone draws that much so i can't really overcharge the bank. every couple of days i run the honda 1000 to a battery charger and bring the bank up to maybe 90% or so.

two questions.

1. am i putting more into the battery bank because i have no controller in the way?

2. occasionally i find the 10amp fuse blown for no special reason. any ideas why?

thanks for your help. please keep replies simple as befits a simple sailor...

[T1 Terry]

During the freak conditions I mentioned in an earlier post that aren't quite so freak in a boat, the sun reflecting from the water to light cloud and back to the panel, the output could be as high as 15 amps for very brief periods. The fuse needs to be big enough to protect the cable from melting in the case of a dead short so if decent sized cable was used a 20 amp fuse would be a better choice.
A fuse can never protect the panels, the logistics say the fuse needs to be at least 10% greater than the max current it will carry to prevent it for failing due to heat.

As far as no regulator, as long as the batteries don't back feed the panels at night there is no problem if the load will always exceed the charge current over a given period and there is sufficient capacity for storage during the periods charging is greater than load and load is greater than charging, it sounds like you have found that balance. A simple amp meter or even a digital multimeter in series in the solar cable will tell you if the battery is back feeding the panels over night. If this is the case, a Schottky diode rated at more than double the max. current in series with one of the cables will solve the problem. These diodes have a very small voltage drop across them so they are best suited for solar connection.

T1 Terry

[Jimbo485]

[QUOTE=onestepcsy37;

two questions.

1. am i putting more into the battery bank because i have no controller in the way?

2. occasionally i find the 10amp fuse blown for no special reason. any ideas why?

thanks for your help. please keep replies simple as befits a simple sailor...[/QUOTE]

1. Yes.
2. Fuse is too small.

[noelex 77]

Originally Posted by onestepcsy37

1. am i putting more into the battery bank because i have no controller in the way?

2. occasionally i find the 10amp fuse blown for no special reason. any ideas why?

thanks for your help. please keep replies simple as befits a simple sailor...

1. Basically the same. A good non MPPT will deliver very slightly less due to the very small self consumption, but their will also be a slight gain eliminating the slight discharge at night without a blocking diode.
A good MPPT regulator will deliver slightly more, but the gain would be small.

2. The fuse should not be blowing. The Imp of most 135w panels is about 7.7A. Without a regulator the current delivery will be a bit higher than this (because the voltage is lower), but even the 1sc is well below this at about 8.4A.
It is possible for panels to exceed these numbers in exceptionally bright conditions, but not over 10A without voltage conversion, unless your panel is a lower voltage 32 cell panel?
I would look carefully for an intermittent short in the system.

[Sumner]

Originally Posted by Captain Bill

I had my xantrex C40 PWM controller die on me in June so I replaced it with a Bluesky 2512x and an IPN-proremote. I find that I am actually less efficient with the MPPT that the pwm controller.....

That is interesting. I don't have a PWM controller to compare against, but we bought the Blue Sky 2512iX for our Mac that had 180 watts on it (now 200) and in 3 months of cruising with it I don't remember the cloud thing being an issue. The amps would drop as the battery came up on charge. Most of the time in good sun about 9-12 amps and felt that was good due to the low sun angles (Nov.-Dec) and (Mar.-Apr.).

I don't have the IPN-proremote but use...




... the digital amp and volt meter above. The amp meter is a shunt type.

We moved up to the Blue Sky 3024iL for use with the 480 watts on the Endeavour. Right now I only have one panel of six attached to it at the boatyard to keep the start battery charged that is connected to the bilge pump.

I feel that the MPPT's are probably good for 5%-10% and accept that, and will also take that,

Sum

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[belizesailor]

Originally Posted by Aussie_Sequoia

J...If you are putting your panels in series to attain high voltage and you have them spread across your deck or on either side of your rails or on your arch they will inevitably be in the shade for part or half the day due to the mast/sail configuration (I guess trawlers are exempt here?). This will reduce the output to: ZERO.
...
It is my opinion that series connections have no business on boats.

Good point on series connections. Thanks for posting your experience.

[belizesailor]

Originally Posted by onestepcsy37

two questions.

1. am i putting more into the battery bank because i have no controller in the way?

More importantly remember that controllers serve a purpose....to control the charge. Direct panel output voltages are too high for battery charging and will literally cook your batteries. Also, with no control of the charge profile charging is not going to be as effective or appropriate for the battery type.

[onestepcsy37]

belizesailor -

hadn't thought about that one - panel output voltage is about 22 volts and i've wired it directly to the battery bank. i'm no electrical genius but i can see where, if i have no load drawing down the battery bank the panel could quickly fry it. and yet i never see voltage over about 13.7 or so when charging.

an example. i have all loads turned off. this is strictly panel to battery bank. before i turn the panel on i measure 12.5 on my digital voltmeter. turn on the panel. the voltage begins climbing slowly for the next half hour until it reaches about 13.7, and then it stays there for quite a while. after several hours i get nervous and shut it down. the next morning (still no load on the battery bank) i measure 12.7 on my battery bank. i can't say for certain if the high voltage charge is hurting the batteries because even though the voltage is high the amperage is only about 7.5 max. the battery bank is four golf carts or about 450 amp hours, so the charge rate is less than 2% of the battery capacity, which i've been told is a safe rate.

ok, that's all i know. this amp/volt/watt stuff makes my head spin...