MPPT charge controllers for PV panels

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In summary, the conversation discussed the use of MPPT controllers for small power applications, specifically on a boat with limited space. The participants advised against using cheap Chinese versions and suggested using a PWM controller instead, or building one's own controller. They also mentioned that the realistic power gain from MPPT is usually only 5-15%, and it may be more cost-effective to simply buy more panels. The importance of balancing the battery system and limiting depth of discharge was also mentioned for increasing battery life.
  • #1
sophiecentaur
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I know this is a 'how long is a piece of string' question but here goes, anyway.
I am planning to augment the wind turbine on my boat with a PV panel (both very low power - up to , say, 2A @ 12V).
Area is limited. MPPT controllers get more out of a panel but I wonder what the actual statistics are, for small panels.
The boat is on a swinging (drying) mooring, so direction is a factor, although the tide leaves her facing one particular way for almost half the time, usually, if the wind doesn't dominate.
You can read a lot about MPPT controllers on the web but it's mostly seller's blurb and aimed at higher power, domestic applications.
A 20W or 30W panel would be a much more convenient size than a 50W panel and I could adjust the elevation from time to time. I have read of 40% advantage with MPPTs, so I am on a bit of a cusp. Also, a small MPPT is cheaper than extra panel area.
Is there any practical experience out there in PFland? Cheers.

Omg. This should really be on the EE forum, I think. That's what you get on a smart phone. :)
 
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  • #2
If the IV power curve of the PV panels are matched to the battery or load profile and the power levels are low (<100W) then MPPT will provide little or no advantage under normal conditions.

A typical panel designed for a 12vdc system will have a power curve like this.
solar-panel-operating-point.jpg

When using a typical solar charge controller the first stage normally pushes the max possible current up to a voltage limit. This is the stage where MPPT is useful as it takes power at the panel ideal IV curve point, uses that power to provide the most input current (as a constant current source) to the battery and it's where a battery normally has it's best coulombic efficiency. If the charge/load profile falls close to where the MPPT would be anyway then you would see little or no gain in normal light conditions. If the light conditions were poor for most of the time then MPPT would give gains in current but the added power would be much lower than in good light.

Once the current into the battery causes the battery voltage to be at the max voltage setpoint of the charger the MPPT function become much less useful as we are now limiting the power input to the battery, the coulombic efficiency is usually lower, so real efficiency gains are lost.

Panel is cheap now so simply buy more panel with a rock solid PWM controller instead of an expensive MPPT type controller (the cheap Chinese knockoffs are junk) for a simple battery system.
 
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  • #3
Thanks for that. Your last comment about cheapo Chinese versions is interesting. I shall have to measure up and decide just how big I can find room for. My needs are actually quite modest, with a chartplotter and tiny compressor fridge. LED lights throughout are a very small load.
 
  • #4
The length of your string will of course depend on your operating conditions, and a quick inspection of the panel's IV curve suggests that there's quite a bit to be gained (or lost rather) by controlling the operating point of a panel. If the panel's operating temperature--and hence the voltage at which it will produce maximum power (Vmp)--is fairly constant, a simple constant-voltage type of MPPT such as the LT3652 could be suitable if your willing to build your own controller.

http://www.linear.com/product/LT3652
 
  • #5
Looks like a good idea to BMO. A bit of home construction is theraputic. I don't have access to so much equipment but this would only need my DMM.
 
  • #6
sophiecentaur said:
Thanks for that. Your last comment about cheapo Chinese versions is interesting. I shall have to measure up and decide just how big I can find room for. My needs are actually quite modest, with a chartplotter and tiny compressor fridge. LED lights throughout are a very small load.

The realistic power in/out gain from just MPPT at same input/output voltage is usually about 5-15 percent. Spending more for extra panels for low power systems is usually the better choice.
 
  • #7
nsaspook said:
The realistic power in/out gain from just MPPT at same input/output voltage is usually about 5-15 percent. Spending more for extra panels for low power systems is usually the better choice.
If the 5-15% figure is relative to the PWM approach you mentioned in your first post, where I think you are suggesting that the panels are clamped to the battery voltage, the first statement seems to rest on an assumption that the battery depth of discharge is low. This might be a valid assumption depending on the OP's setup, but in general this configuration would move the operating point away from the MPP as the battery discharges and could ultimately produce the least amount of power when you need it the most, e.g. in the case of a drained battery.

nsaspook said:
Panel is cheap now so simply buy more panel with a rock solid PWM controller instead of an expensive MPPT type controller (the cheap Chinese knockoffs are junk) for a simple battery system.
The LT3652 is available at Digikey at $6.98.
 
  • #8
gnurf said:
If the 5-15% figure is relative to the PWM approach you mentioned in your first post, where I think you are suggesting that the panels are clamped to the battery voltage, the first statement seems to rest on an assumption that the battery depth of discharge is low. This might be a valid assumption depending on the OP's setup, but in general this configuration would move the operating point away from the MPP as the battery discharges and could ultimately produce the least amount of power when you need it the most, e.g. in the case of a drained battery.

Your points are all true but usually if you want to increase battery (a expensive consumable) life with lead-acid chemistry you balance the system by limiting the DoD to about 25% with a max charge rate of about 10% of the battery capacity. My general position about solar powered systems is if you're draining your batteries heavily daily then you need more panel or fewer loads because you're murdering them. If you can achieve the effect of more panel with cost effective MPPT then that's the way to go.

I think Morningstar has a fair discussion about PWM over MPPT for small systems here.
http://www.morningstarcorp.com/en/support/library/Trad-PWM-vs-TrakStar-MPPT-April-2013.pdf [Broken]
 
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1. What is an MPPT charge controller and how does it work?

An MPPT (Maximum Power Point Tracking) charge controller is a type of electronic device used in photovoltaic (PV) systems to optimize the amount of power that can be extracted from PV panels. It works by constantly adjusting the input voltage of the PV panels to find the maximum power point, where the panels can generate the most power. This is achieved by using a DC-to-DC converter to match the voltage of the PV panels to the voltage of the battery or grid that the panels are connected to.

2. What are the benefits of using an MPPT charge controller?

There are several benefits of using an MPPT charge controller. Firstly, it can increase the efficiency of the PV panels by up to 30%, meaning more power can be generated from the same amount of sunlight. This can also lead to a decrease in the overall cost of the system, as fewer panels are required to produce the same amount of power. Additionally, MPPT charge controllers can help to prolong the lifespan of the PV panels by preventing overcharging or undercharging of the batteries.

3. How do I choose the right MPPT charge controller for my PV system?

When choosing an MPPT charge controller, there are a few factors to consider. Firstly, you should make sure that the charge controller is compatible with the voltage and current of your PV panels and battery. You should also consider the maximum power rating of the charge controller and ensure that it can handle the maximum power output of your PV panels. Additionally, you may want to look for features such as temperature compensation and communication capabilities for remote monitoring and control.

4. Can MPPT charge controllers be used with any type of PV panel?

In general, MPPT charge controllers can be used with any type of PV panel, as long as the voltage and current ratings are within the specifications of the charge controller. However, some MPPT charge controllers may be optimized for certain types of panels, such as monocrystalline or polycrystalline, so it is important to check the manufacturer's recommendations before making a purchase.

5. Are there any maintenance requirements for MPPT charge controllers?

MPPT charge controllers do not require much maintenance, but it is important to regularly check for any physical damage and ensure that the connections are secure. It is also recommended to clean the charge controller and PV panels periodically to remove any dust or debris that may hinder their performance. In the case of any issues, it is best to consult the manufacturer's instructions or seek professional assistance.

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