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aage 03-07-2008 12:49 PM

How effective is a solar panel MPP tracker?
the effectiveness of MPPTs was emphasized in this forum many times. But does anyone have real data? I talked to an engineer from a solar research company the other day. His opinion: Typical MPPT are too slow for sailing boats. They determine the maximum power point only every 30 to 60 minutes. This is useless under sail, not effective under anchor, and docked you don't need it. Thus the idea sounds good, but is it effective? How is your experience?

sailingdog 03-07-2008 01:10 PM

I think you're a bit confused.

MPPT stands for Maximum Power Point Tracking, and has nothing to do with the movement of the sun and the angle of the panel to the sun...and they could care less about boat movement... they're purely electronic devices that alter the voltage and amperage of the solar panel output to more closely match the charging needs of the battery bank and reduce the amount of electricity spent as waste heat. In general, using an MPPT charge controller will increase the efficiency of your solar panel/battery bank setup by anywhere from 10-20%, depending on the size of the panels, type of battery used, etc.

Here's an example: A solar panel will generally put out 17 volts or so.

Say one panel puts out 10 amps at 17 volts. A 12VDC battery during bulk charging phase only requires 14.6 volts. Without the MPPT charge controller, you'd be charging the batteries at 10 amps @ 17 VDC, and if they're wet cells, boiling off some electrolyte in the process.

Now, let's add an MPPT charge controller. The charge controller takes the solar panel output of 10 Amps @ 17 VDC and will change it to 11.64 amps @ 14.6 VDC... effectively giving the battery 16.4% more charge for the same time period. The charge controller uses a tiny bit of the solar panel's output to transform the output to the batteries—on a good MPPT charge controller, this is less than 2-3% of the power going from the panels to the batteries.

AGM batteries are really require to maximize the benefits of an MPPT charge controller, since they have much higher current acceptance rates.

tomaz_423 03-07-2008 01:27 PM

"The charge controller uses a tiny bit of the solar panel's output to transform the output to the batteries—on a good MPPT charge controller, this is less than 1% of the power going from the panels to the batteries. "

If this is true it is very effective.
From my old school days I remember that AC to AC conversion (with a simple transformer) is perhaps 95% to 98% effective (depending a lot on frequency, and ho knows what else).
I was also told that AC to AC is much more difficult to transform as you first have to do DC to AC conversion, then transform the voltage and then covert back to DC.
But of course that was analog technology.
If they can really do it now at 99% efficiency it is a great progress.
I will have to read about this.

camaraderie 03-07-2008 01:36 PM

MPT controllers are proven devices for boat solar power and as Dawg says can improve the actual deliverly of power to the batteries during full panel output (i.e. full sunlight) on the order of about 10-20% as they will allow use of power that is normally just wasted as heat by normal regulation.
I will disagree with Dawg about being useful primarily with AGM's as they can help any decently large battery bank...wet or dry depending on how many amps your panels are capable of putting out at full power.

Example: A panel rated at 200watts and 17.5V puts out 11.43 amps unregulated. Regulating this down WITHOUT a normal 14.5 volts bulk charging voltage will produce 13.79 amps.
That is the MAXIMUM current you can see with 200 watts of panels.
Since wet cells can take 15% of their rated amp hours in bulk charging amps...all that is required is 92 amp hours of battery rating...slightly less than a group31 size. MPPT controller can provide a benefit to ALL sailboat owners with solar panels that have unregulated outputs in excess of standard charging voltage of 14.5V.
Tomaz...I am not making claims for 99% conversion efficiency! Just way better than throwing usable power away as heat!!

sailingdog 03-07-2008 01:38 PM


The Outback MX60 is about 99.1% efficient at 40 amps... goes down a bit if the amperage is higher IIRC. Detailed specs as PDF file HERE.

hellosailor 03-07-2008 02:01 PM

99.1% ? Not according to that PDF file, or other information I've been given. That PDF files says 98% IF you are using a 48VDC battery and array--which isn't going to be on sailboats.

The numbers I'd gotten from one of their competitors said about a 4-6% power loss in pretty much all MPPT controllers, but that is offset by the greater efficiency and the advanced charging algorithms, which allow an MPPT controller to continually apply maximum amps at "just enough" vols, rather than a conventional 3-stage charge.

Outback is also claiming to use a "5 stage" charging process, which is frankly crude compared to a continuous charging profile that another vendor uses.

Overall? Let's say your solar panel puts out 17.5V at 30A during peak sunlight, that's 525 watts. With a REGULATOR, you will get 14.4V at 30A, or 432W, some 82% of the real available power. You've lost 18%.

But with an MPPT charge controller, that 525 watts will be converted to 14.4V at 36A, fully 20% more power than the regulator was giving you. So the controller loses are still gaining 16% and the battery may charge 10% more efficiently at the same time. Even if you figure a 16% gain--that's a lot. (MPPT controllers are putting out pulsed DC, not the pure DC from a direct panel, so while the controller absorbs some power, the batteries still charge faster because they are not gassing internally the same way that pure DC would make them.)

And since a good controller will optimize the voltage/amperage even further than that (i.e. if your battery only needs 13.6V to charge it, it could be fed 38A instead of 36A)...there are some really significant gains to be had.

Against that, the MPPT controllers are outright expensive, and there's no easy way to combine an MPPT controller and a conventional engine-driven alternator/regulator running at the same time. (Each will drive the other nutso.)

Now, if you want to talk about moveable panels and automatic solar trackers that move them to track the sun--that's not for sailboats. Sailboats rock and roll in three dimensions, there's no tracker on the planet that can swing solar panels around that way and still spend less energy than it gains.

tomaz_423 03-07-2008 02:06 PM

OK, I found this number in the link above:
"Power Conversion Efficiency Typical 98% at 60 amps with a 48 V battery and nominal 48 V solar array"
This is better then I expected. Of course conversion of 48 V is more effective then conversion of 17 V, but still - it is good.

I also found that: MPPT Solar Charge Controllers
Most modern MPPT's are around 92-97% efficient in the conversion

I was expecting at least 4 to 5% loos at 17V and perhaps 1,5 to 2% at 48V.
My logic was based on the old knowledge: whenever you do a solid state conversion you may expect a voltage drop of around 0,7V. This was true for Silicon transistors and chips, but technology must have moved forward.

So, if you only loos a few percent on a conversation this is great. You probably loose more on the wires.

sailingdog 03-07-2008 02:08 PM


The site I got the link to that PDF claims 99.1% @ 40 amps. Northern Arizona Wind & Sun.

BTW, it isn't hard to get 48 VDC out of a solar array...just connect three identical panels in series, rather than parallel. The input voltages on most MPPT charge controllers can be much higher than you'd be able to do with out one.

hellosailor 03-07-2008 02:23 PM

Don't mistake an MPPT charge controller for being a "solid state" conversion. I mean, it is solid state in the sense that there is no "buzzer" creating an AC voltage, like the old UPSes used to use to make AC from DC batteries.
But an MPPT controller simply takes DC and puts it into a high frequency switch, converting the DC into a 30KHz AC signal, which is fed into a transformer. (Pulsed DC, but that's still more AC than DC to me.)
Then the other side takes the AC power from the transformer, and sucks it back out in an optimized fashion. The fact that they are running around 30KHz allows them to be pretty efficient, this is essentially the same technology that is in every computer's "switching power supply" these days for $20-100.
The MPPT controller goes one better, in theory, by also having a cheap dedicatred CPU on board that looks at the input and output, and tries to get smart about how to deal with the conversion. (An easy task, like winning at poker all the time every time.[g])

With some panels you an get 48V out, others you can't. If the factory has used three "12"V arrays in one panel, that might get you 48 volts at peaks--if you can rewire the arrays. In order to get better seals and lower costs, more of them are coming hardwired for 12 or 24v operation and you tsake what you've got. The problem is, if you wire up 2x12 or 2x24 to get higher voltage, and EITHER panel gets any shade--you lose output from the whole array. On the other hand, if you have all the strips in each panel set up in parallel (i.e. internal and external, all set for 12V) then any shade on any one part of the array just reduces the output amperage--instead of reducing the voltage of the whole system.
And of course, on a US-market boat, you've still got to get it down to a 12V battery, not a 48V one. Going from 12 to 24 volts give you some extra efficiency, going from 24 to 48 is just gilding the lily. Like calling a four stage charge with an "off" cycle a "five stage" charger. As, ahem, some of the makers are now doing.

chucklesR 03-07-2008 02:41 PM

I'm using the Xantrex C12 charge controller I reckon I could disconnect it and check my XBM monitor, then connect it and check again, record the results and post them, but it's cold outside so I'm not going to.

Does this do the same thing?

I've never seen my house bank go below 12.1 volts, so does it really matter?

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