mixing solar panels with MPPT controller? [1]

[ In the first part of this series, we introduce you to the MPPT technology, and to our solar array that we use to collect some comparative data.  I wrote about lost power without an MPPT solar charger in the past.   Now I have one!  Now its time tell you how it really works ]

I’ve been using and taking measurements with my solar panels and my new Outback FM80 MPPT solar controller.  This is a different animal from the older, passive, Trace C40 controller.  I have questions about this kind of charger, what voltage is best, and as I collect data to answer these questions, you will see it here first.  Unfortunately, my C40 controller was destroyed by the August lightning strikes at our home but I can calculate the gain and yoy may find my commentary on the topic useful.

Using one of the newer Multi Point Power Tracking (MPPT) solar chargers with my existing set of solar panels promises a 20-30% improvement in captured and stored solar power.  However, it does open up a series of questions about which wiring arrangement is best with a collection of different panel types.

Standard situation

In most new applications using an MPPT controller, all the new solar panels would be the same.  The the simple wiring answer would be to string them all in series, keeping the peak string voltage under 145V.  For example, if you had 7 panels with a maximum open circuit voltage (Voc) of twenty volts,  and say 70W each you could do this.  The peak loop voltage would be 140V and the peak current in the loop would be the peak current for any  in the loop.

Our solar array

Unfortunately our solar array is a collection of various panels we have been able to add over the first 5 years on the mountain.  To understand the fixed solar array we are testing with, I recorded the video below.  As you can see it is a fixed position array without any solar tracking capabilities.

Where does the MPPT gain come from?

Much of the advantage of the MPPT controller comes from the fact that the peak solar power occurs at voltages higher than the battery bank voltage.  For example, my 50W panel will produce a maximum of 3.3A in good sunshine.  Now when I connect it to my partly discharged battery bank at 12V it will trickle in 3.3A charging the batteries.

But wait, thats not 50W! Yep, 3.3A times 12V is a little less than 40W solar power.  Did they sell me a bad panel? shouldn’t it be   4.2A??  No, the panels are rated for peak or optimum conditions.  This panel has a Voc of 18V and will produce 3.3A at 16V which is your 50W.  The solar current does not change with voltage much since the panel’s short circuit current (Isc) is 3.3A.   Power is the product of voltage and current;  the fact that the solar panel operates at the same current for lower voltage means that power is lost at lower voltage.  This explains where the gain comes from.

How about a non-technical picture?

Huh?  Ok, we’ll go with a simple analogy to explain the gain.  At first I thought about a water and paddle wheel kind of example, but settled on this catapult analogy.  In both cases, passive and MPPT chargers the solar array produces the same current for a given amount of sunshine.  The current is represented by the equal size blue and green balls.

In the case of the passive charger (green ball) we are limited to the battery voltage of 12V.  With the MPPT charger the panel is not voltage limited by anything, and has the same current available at 16V.

Its easy to see in the diagram which colored ball will launch the catapult payload the farthest, or work with the greatest power.

Measuring the gain

One real way to measure this gain would be to swap between passive and MPPT controllers.  That’s impractical and impossible since the passive one is dead.  However the Outback FM80 that I’m using displays lots of useful information including the array voltage, array current, the charging voltage, and the charging current.  With this information we can calculate at any moment the array power = charging power.  If we take the array current and multiply it times the charging voltage we have what the array power would be with a passive charger.  For example, I took a quick look this morning and found:

• Array Voltage =14.8V , Array current = 18.7A, Charging Voltage = 11.9V, and Charging current = 23.2A
• Array power ~ charging power = 14.8 x 18.7 = 277W
• Passive charging power = 11.9 x 18.7 = 223W
• MPPT charging gain = 54W or  24% increase

I will be rewiring the solar panels for different voltages and capturing information about what time charging starts and ends and well as power captured.

How are your genverter/solar power plans going?  Do you think the data in this series will be useful in your design?  Tell us in the comments below!  Re-Tweet this article for your friends.

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1. mixing solar panels with MPPT controller? [3]
2. mixing solar panels with MPPT controller? [2]
3. Caution: Solar Panels Wasted without MPPT charger
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5. Solar Voltage Choices, Regret, and MPPT