How can keep output voltage constant in pv system with MPPT (FOCV method) and buck converter?

I simulate pv array (P=213 W , Voc=36V , Isc=7.84) with MPPT (fractional open circuit voltage method + PI controller) by using buck converter , and it's work very good with power efficiency 95% with varies load. BUT voltage not constant at output !

now I need keep output voltage constant at 12 V to connect 12 V battrey . it can be connect battery direct like the load ?

and how can keep output voltage constant Ex:12V for varies load with maximum power?

Thank you :)

• show work.. block diagram, schematic & necessary specs Essentially MPT charger is regulated source current to match source impedance of PV and regulated output voltage to battery both with independent switched chokes for cycle storage and C at source to lower Zc(f), so output current *V is limited by input power vs Solarity. So you need two regulators. May 5, 2017 at 18:49
• there are different methods. YOu can share 1 choke with dual controls or use hunting on input PV voltage to optimize MPT vs Vin May 5, 2017 at 18:55

Generally, the output voltage in a solar charging system is held constant by the battery itself. For a first order model, the output of your system is connected to a voltage souce! The MPPT regulates the input voltage, without even considering the output voltage, much as a traditional voltage regulator regulates the output voltage and doesn't care what the input voltage is.

Of course, a complete (and safe) charger design should include secondary loops or controls to disable charging when the battery is charged all the way up, and also possibly to limit the current into the battery if the input power may exceed the maximum charge rate of the battery.

Edit: I should clarify that, as Tony details in his response, there are many implementations of MPPT, some of which might monitor output voltage and current as part of their sophisticated slow-loop that optimizes the bias point of the cell. However, the feedback for the primary DC-DC converter loop is generally at the input. Unsophisticated MPPT solar chargers simply use a pre-programmed input regulation voltage that is reasonably optimal for the expected conditions.

• I upload images of my work , if replace load R1 with battery 12v this right thing or not ? May 5, 2017 at 19:36
• Yes, that will allow you simulate the basic behavior with a battery as the load. If you want a more sophisticated model for the battery, you can include its equivalent series impedance, develop a model that varies the voltage as a function of state of charge, etc. May 5, 2017 at 20:52
• The battery voltage is generally not constant. An MPPT cannot regulate the input voltage. That is a function of the PV cells. Not so "much as a traditional voltage regulator" because it IS a traditional voltage regulator. " without even considering the output voltage"???? It MUST regulate the output, that is what it does. May 6, 2017 at 7:13
• @nsrdeentaher use both the load and the battery in parallel. The battery voltage should be more like 13.7v. May 6, 2017 at 7:25
• @Misunderstood , i did have success doing this, as my invetrer was over volting and shutting down due to mppt peaks during float. but you have to set your final charge voltage higher as the controller will see a higher voltage at its terminals than the battery terminals due to current draw by load on battery Dec 6, 2019 at 19:18

A 12V battery is NOT optimal for a simple Open loop charger from 36Voc , but 80% +- would be or 24 to 28.8V. Thus both input AND output (current and Voltage ) regulation is needed (4) for MPT control.

You need to sense output & output current and voltage in order to measure and control efficiency.

There are many methods of MPT

• PD solar sense and compare estimate MPT from k*Isc (momentary Isc test)
• P+O Perturb and Observe
• ΔESR or InC, incremental conductance
• FOCV fractional open-circuit voltage
• FSCC fractional short-circuit current for k1
• Inc hunt realtime Vin*In microsteps
• combinations of above.

The best methods come with multiple stages of MPT and known characteristics of given PV ....

• a) Measure momentary Isc without C added to PV, estimate MPT
• b) P+O loop with microsteps
• c) track ESR changes with ΔVin/ΔIin and goto a) if limit exceeded.

Regulate output current available to regulate battery voltage during PWM for each cycle using energy in choke.

• No, wrong. An MPPT is a battery charger. May 6, 2017 at 7:15
• MPPT just means max power point transfer. could be to drive a motor. But it means input current and output current are both regulated to match input impedance and drive current demand, such that demand never exceeds supply at MPT. so I disagree (-1) @Misunderstood May 12, 2017 at 0:13
• I'm not easily baffled with BS. The OP chose his approach (FOCV) and is asking about the output voltage. He did not ask for a list of other methods. What is it that determines the output voltage of the typical MPPT? Or did you think the MPPT typically drives the load directly? You mention a few obscure irrelevant things but fail to mention anything about the max power point. It would be nice if there were a device that could regulate demand so that it never exceeds supply. You could sell that. May 12, 2017 at 18:05
• This is not as you think and I use the generic undertanding of what MPT means, It could be that MPT regulator is NOT a battery charger at all but a grid connected DC-AC converter. Curb your enthusiasm and open your eyes. May 12, 2017 at 18:44
• You misunderstand, the gain box in his added images for PV V(36) G=0.8 reflects my previous comments about voltage ratio of 80% +/-% is correct. This is why a simple Buck or direct attach is wrong due to voltage ratio 12/36 is 1/3 when a simple charger should be 80%of Voc. Thus MPT regulates V/I or the impedance to maximize power( match impedance) by matching 80% of Voc. Examine his added block diagram closer and curb your negative inflammatory incorrect comments [please] The triangle sweep then hunts to find this by modulating PWM on the Buck switcher. May 13, 2017 at 12:33

You should use a buck boost controller for when there is insufficient sunlight.

Your output voltage should be set according to the chemistry of the battery and the charging stage.

If you need a regulated 12V output you need to add a regulator to the MPPT output.

If you are using two stage charging, output voltage should be between 14 and 15.4 V during the absorption stage and 13.4 and 13.7 during float stage.

If the battery is not fully charged it will pull down the output to its discharge voltage level.

From my Morningstar, TriStar-30 MPPT manual:

I use a Diode OR circuit to switch to a power supply when the solar voltage is insufficient. I will likely add a 48 volt regulator on the input of the Diode OR board.

• did you find a reg ?? Dec 6, 2019 at 19:19