We explain how to choose an MPP (maximum power point) voltage for solar charge controller chips that use a ‘constant voltage’ regulation method, such as TI’s bq24650 or Linear Technology’s LT3652. We also let you know what we chose to use with our 6 Volt panels and the accompanying trade-offs.

Importance of Setting Voltage

Every solar panel has a maximum power point, and it’s at this specific voltage and current that the panel operates most effectively. TI’s bq24650 and Linear’s LT3652 throttle the current to maintain a set voltage–ideally the MPP voltage–at the input. If the load is not properly matched with these properties, then the power output of the panel will be sub-optimal or even zero.

If you set the voltage too low, you waste power in prime conditions as you’ll be operating below the maximum power point.

If you set the voltage too high, the panel current may drop to zero and your system will generate zero power.

Our goal is to strike a balance so that we give up as little power as possible in good conditions, but don’t drop out in poor ones.

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Real World Challenges: Why Solar Spec Sheets Are Wrong

Solar panels, including Voltaic’s, have a spec sheet which lists VMPP, which is the peak voltage. However, there are a number of reasons the VMPP is different and you should not use this number without further testing and analysis.

  1. VMPP is based on the specifications from the cell manufacturer – cells in smaller panels are cut by laser, soldered together and covered with a protective coating.
  2. VMPP is based on standard test conditions of 1000 W/m² @ 25°C – as the ambient temperature increases and the panel heats up, the voltage will drop. Our panels may reach 60°C on a warm day.
  3. The voltage will drop when it is cloudy, shaded or angled away from the sun.

You can build your own curve, but we’ve found that our VMPP is about 80% of the open current voltage (VOC) at any given time . It can range from 70-80% of VOC.  We suggest that you ensure that the voltage setting is never below lowest expected VOC  for your panel over the course of the year.

Let’s see how these conditions affect the maximum power point.

Effect of Temperature on MPP Selection

Let’s say that we choose 15V as the MPP voltage for this example panel. I’ve drawn a vertical line at this voltage and made crosses where the power lines cross it. This would be an accurate and effective MPP voltage when it is 75 degrees–imagine we’re optimizing for a semi-tropical climate–but in every other temperature this is not the MPP voltage. So what happens when the temperature changes?

Take a look at the graph above. If the temperature were to increase to 100 degrees (the blue line), the MPP voltage would shift to 13V. At this voltage, the panel could output 95W, but since our voltage is still set to 15V, the panel instead only outputs 82W or so (~13% loss).

On the other hand, if the temperature were to decrease to 0 degrees (the brown line), the MPP voltage would shift to 20V. At this voltage, the panel could output 140W, but with our 15V setting, the panel instead outputs 108W (~22% loss).

Now let’s take a look at another example. Say we were to optimize for an arctic environment, at 0 degrees, an we set our MPP voltage to be 20V. When it is actually 0 degrees, the controller performs perfectly, with a fantastic 140W or so being generated.

But if it heats up to 50 degrees, it’s around a 50% loss. But if the temperature were to somehow heat up to 75 degrees, then the panel wouldn’t generate any power at all. For this reason, it is important to note the open circuit voltage of the panel in extreme heat; you shouldn’t set the VMPP past this voltage as no power will be generated. Optimizing for cold weather where there is large temperature variation could lead to the panel dropping out completely in warmer conditions.

Effect of Irradiance on MPP Selection

irradiance effect on solar panel voltage

While the effect of irradiance isn’t as strong as temperature, there is still one and it might be worth looking into depending on the situation. 1000 W/m² corresponds to a clear sunny day, with the panel facing directly into the sun. This is the condition in which the VMPP is found on the specifications. However, we see that VMPP decreases with less irradiance. On a cloudy day (200 W/m²), the VMPP dropped a full Volt. In areas that don’t get full bright sunlight–San Francisco perhaps–it might be worth looking into to see if dropping the set voltage could help with power efficiency.

VMPP for Voltaic 6 Volt Solar panels

We chose 5.9V as the maximum power point setting on our upcoming TI bq24650 prototyping board.

If you look at the specifications of our 6 Volt, urethane coated panels, the VMPP is listed as 6.5V. This is based on our cell manufacturer’s specifications and due to natural variation, cutting the cells, and coating is higher than what we’ve measured in the field. The VMPP of our panel at STC is closer to 6.2V, which is about 80% of VOC. Given that we want to be able to accommodate all of our customers’ needs and environments, which can range from the cold, low light conditions of a Maine winter to the heat of an Arizona summer, we had to do our best to balance two extremes.

There is no one right answer. The MPP voltage of the panel at 150°F is around 5.3V, whereas at 36°F it can go up to 6.2V or higher.

We want to ensure that the controller doesn’t drop out in the heat, and with a 5.3V VMPP we can extrapolate and see that the VOC is approximately 6.6V. With a 5.9V MPP voltage, the system will still be creating 88% of maximum power.

 

We need to strike a balance between 5.3V and 6.6V, but not too low as the VMPP in extreme cold is 6.1V or so. So the effective range of MPP voltages we can choose for our panels is somewhere between 5.3V and 6.1V. Knowing that closer to 6.1V still won’t drop out in the heat and will produce good power in the cold, we opted to lean toward the higher side, at 5.9V.

The balance between them is about 5.75V, but as the average yearly temperature in the continental US is 53°F, noticeably closer to the cooler side, we bounced it up a bit to 5.9V.

Maximum Power Point Setting Conclusion

While we realize a general purpose MPP voltage means lower efficiency in certain conditions, it is an important trade-off to make for overall performance. In addition, the solar charge controller board mentioned above has spot for a custom MPP voltage to be set. Whether you’re using it for a different panel, or our panel in a very specific environment, this option allows you to tailor the board using the information provided in this article.

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