The operation of buck converter or any converter is clear if the load is resistive. However, when the load is as a voltage source or current source as a battery. I think I struggle in understanding the limitations of the buck converter operations. in the case of the input source is a solar panel, the relationship between the output and input voltages is by the duty cycle; Vbattery = D*Vpv. Now if the I want to use MPPT, I can change the duty cycle in increase if the power increases and vice versa under the condition that output resistance is less the input resistance . Now by changing the duty cycle, I know the Vpv will change because the buck converter changes the impedance seen from the solar panel by changing the solar panel. Now in changing the duty cycle, does the output voltage changes also? or it is just clipped to V Battery voltage that is in Bulk charge. We also know that V battery voltage is increasing as the SOC increases, hence its impedance changes. Also, how can the equations of the buck converter if the buck is connected to a battery. Note that there is one loop that is adjusting the duty cycle based on the sensing of voltage and current of PV and adjust it for MPPT,

Let summarize it in this:
Let's say the battery is in bulk stage and I want to charge it with MPP and let's also consider Temperature and irradiance are constant , we increase the current only by varying the duty cycle and the output voltage is clamped to the battery voltage. During the Bulk stage, the battery voltage increases and we cannot regulate this output voltage. What the effect of this increase in voltage in the MPPT operation and the DC-DC converter in general?

Can we also set a constant duty cycle that regulate the output voltage after bulk stage assuming the input is constant ?
lets now ignore the safety (for max current ) and output voltage regulation after bulk stage for now.

 

Bump

 

Bump

Hi Dickcappels,
I don't understand your comment.

 

The Buck Converter will very slowly increase the voltage of the battery, as the battery charges.

But the MPPT wants to maximize Watts_Input = Volts_Input x Amps_Input.
The MPPT logic occurs on the PV / Input side of the Buck Converter.
You need to understand the I-V Curve of Solar Cells, with a superimposed Watts curve.
MPPT must actively hunt, or be perturbed periodically, to find & maintain the Solar Cells at "Peak Power Point".

Note:
There is nothing in the above, discussing the battery's voltage during Bulk Charge.
During Bulk Charge the battery's voltage is below the Setpoint Voltage needed to switch from Bulk Stage to Absorb Stage.
When you maximize the Input Watts, extracted from the Solar Cells, then you also maximize the amps flowing into the battery.
The voltage on the battery will increase, as the SOC increases.


Now, after the Bulk Stage is Absorb Stage.
Absorb Stage is a CV mode = Constant Voltage.
But Absorb Stage is still not a constant Duty Cycle because the amps flowing into the battery are now dropping.

 

THe "Bump" was to bump the post closer to the top of the list of posts so more people would see it.

 

The Buck Converter will very slowly increase the voltage of the battery, as the battery charges.

But the MPPT wants to maximize Watts_Input = Volts_Input x Amps_Input.
The MPPT logic occurs on the PV / Input side of the Buck Converter.
You need to understand the I-V Curve of Solar Cells, with a superimposed Watts curve.
MPPT must actively hunt, or be perturbed periodically, to find & maintain the Solar Cells at "Peak Power Point".

Note:
There is nothing in the above, discussing the battery's voltage during Bulk Charge.
During Bulk Charge the battery's voltage is below the Setpoint Voltage needed to switch from Bulk Stage to Absorb Stage.
When you maximize the Input Watts, extracted from the Solar Cells, then you also maximize the amps flowing into the battery.
The voltage on the battery will increase, as the SOC increases.


Now, after the Bulk Stage is Absorb Stage.
Absorb Stage is a CV mode = Constant Voltage.
But Absorb Stage is still not a constant Duty Cycle because the amps flowing into the battery are now dropping.

Thank you very much for your reply. I understand what are saying. But you said, as the SOC increases, the battery voltage increases. I asked The battery manufacture for the charge stage Vs voltage, they gave this table :
Charging voltage rates:
Float Charge 6.75V

Absorb charge 7.275V

Bulk charge 7.425V

Equalize 7.875V

Which does not make sense. Are they wrong or do they mean something else than the battery voltage.

 

THe "Bump" was to bump the post closer to the top of the list of posts so more people would see it.

THe "Bump" was to bump the post closer to the top of the list of posts so more people would see it.

Thank you