MPPT question, please take a look

[TekUT]

mppt algorithm

Hi all,
I know that for a PV system the converter attached to the solar cells must have a MPPT control strategy in order to extract the maximum available power from the cell but I've a trouble when I'm thinking that the same converter has to match the output voltage with the battery backup voltage.
So, for example, suppose to have a solar cell of about 17.3 V at the maximum power point and that you like to use it in order to charge a lead acid battery of 12 V located on the output of a suitable converter.
From a efficiency point of view, of course, we've to use a switching DC-DC converter, suitable topology can be the BUCK (step-down) because the output voltage is less than the input one.
At this point I've a question; the output voltage value should be regulated in order to meet the battery specification, then not exceed 14.3 V (full charge), also I think that we can expect a output voltage variation of the battery depending the state of charge which can be between 11.5 V and 14.3 V.
At the same time I've to use a duty-cycle able to reach the maximum power point for the solar cell, to did this I've to read the cell current and voltage and after that make the multiplication in order to calculate the power extracted from the cell. About the MPPT control I can use the well know P&O algorithm and make the duty-cycle changing in order to achieve the condition of the maximum power.
Well, but I've also another constraint about the battery which is on the converter output side.
Here is my question, on the solar cell side (converter input) is clear what I've to do in order to set the right duty-cycle to achieve the maximum power point (MPPT algorithm), but I've also to be sure that the duty-cycle value is valid regarding the battery voltage (to not overcharge the battery).
From this point of view then I can simply implement the MPPT algorithm and shut down the converter when the duty-cycle on the maximum power point exceed the maximum output voltage for the battery or I've to implement a duty-cycle decreasing in order to meet the maximum battery voltage (of course in this situation the converter will work outside the maximum power point).
What is your think about this question, could you please evaluate a little around this question?

Thanks in advance for everyone that have some time for my question.
Pow

 

[VVV]

mppt duty cycle

Charging a battery can be done in several stages. It all depends on the system.

In a solar application, it is preferable to use a thress-stage algorithm: first do a bulk charge, that is inject maximum current until the battery voltage reaches about 14.4V (at 20C). This is stage 1. Then maintain the voltage at this level until the current into the battery levels off, that is it no longer drops. This was the second stage. The third stage is maintaining the battery at the float voltage, which is about 13.8V (at 20C).

So as you can see, it makes sense to used the MPPT algorithm during the first stage, where you should inject as much current into the battery as possible. Then, as you noted, you can no longer use MPPT, since the output voltage will dictate the duty-cycle.

However, there may be situations where the load on the battery is heavy so that it makes sense to use MPPT, to make sure you deliver the most current to the battery/ load combination. So it's really a matter of how much current is needed at the output of the converter. At light loads you cannot use MPPT, with heavier loads you should.
I would make that decision based on the ability to maintain the desired output voltage: if the output voltage is at the set level, then you don't need to do MPPT. If the output voltage is below the desired level, then the load is heavy and the maximum current is needed from your converter, so switch to MPPT.

 

[TekUT]

mppt algorithms constant voltage

@VVV

Thanks for your reply, well, my doubt was about how to mitigate the two requirement MPPT on the input side of the converter and the battery voltage on the output side.
From your reply some of my things are confirmed then not all the time the MPPT are used to charging the battery, of course I've to remove the MPPT control strategy when the battery is full charged.
At now I've also another question related the transient behaviour of the converter, let me suppose the converter working on MPPT at the input side of the DC-DC converter, then now suppose to have a battery with a voltage equal to 12 V. Also suppose that the duty-cycle value multiplied by the cell voltage at the maximum power point is different than the battary voltage, e.g. the theoretical voltage is equal to 13 V. In this scenario I've the converter with a theoretical output voltage of 13 V and the battery with 12 V then as a final result I can expect a high current flowing trough the battery because isn't meet the steady state condition for the converter. In this situation, transient behaviour, I've also to look the current trough the battery and when it is over some safe value detach from the MPPT control stage? Could you give me some idea about it?
My things is that is difficult make some reasoning with the converter by using the classical theory, in this scenario I've on the input side the solar cell that should be driven to obtain the maximum power but at this situation the output voltage from the solar cell multiplied with the duty-cycle needed to reach the MPPT may not meet the actual battery voltage then, if the output voltage will be greater than the actual battery voltage some sort of high current should result from the output of the converter into the battery.... I'm right? From a control point of view the load can be view as a voltage source then I've to take in account that the output voltage should be kept constant and then changing the duty-cycle should change the input voltage accordling the solar cell I-V characteristics.

From above consideration I'm thinking to use this control algorithm strategy:

- read the input converter voltage and current and calculate the input Power
- calculate the new Duty-Cycle from the P&O algorithm
- set the new Duty-Cycle
- check if the output voltage is lower than the maximum value
if yes I can step again trough the P&O algorithm
if not then if Vo is greater than the Vomax I can reduce the duty-cycle or
disconnect the converter from the battery backup

have some sense? Could you please give me some suggest?

Thanks again if you've time to reply.

Bye
Pow

 

[VVV]

mppt algorithm + p&o

As far as I know the MPPT is usually achieved by slightly varying the duty-cycle around the steady-state value and measuring the input power. If the power increases, then you are approaching the peak, else you are moving away from it. So you always disturb the system with this low-frequency "move" and always search the peak this way.

Like I said, I think you should simply attempt to keep the battery voltage at a set level, say the bulkd charge voltage. If that is not possible (batt voltage is less than the setpoint) then do the MPPT algorithm. If the battery voltage is at the setpoint, then there is no need to do the MPPT algorithm.

This is what I would do.