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Original question:

BQ24650: STAT1 and STAT2 cycle blink when battery is fully charged

BQ24650: MPPSET configuration

Michel Fripiat
Prodigy 90 points
Part Number: BQ24650
Other Parts Discussed in Thread: LM234

Hi Support,

We use the BQ24650 in our solar home system products for few months now and we realized that the solar panels produce less than what we expected.

By investigating deeper, we found out that the MPPSET could be the issue.
Indeed, we can find from the datasheet:
If the solar panel or other input source cannot provide the total power of the system and bq24650 charger, the input voltage drops. When the voltage sensed on the MPPSET pin drops below 1.2 V, the charger maintains the input voltage by reducing the charge current.

However, this logic brings two main issues:

1. The voltage at maximum power (Vmp) decreases when the temperature rises.
Example: If Vmp=17.8V at 25°C and temperature coefficient of Pmax = -0.4%/°C. When temperature rises to 45°C, Vmp = 17.8V*(1-0.004*20) = 16.4V
This issue can be solved by measuring the panel temperature with a LM234 as explained in the datasheet. However, it is not feasible in our use case.

  1. The cable between the controller and the solar panels can be quite long sometimes, which brings voltage losses and thus the panel voltage measured by the controller is lower than in reality.
    Example: If the cable is 10m and 4mm², with a current of 10A, we measure a voltage loss of 0.8V.
    The controller measures 17.2V instead of 18V.

These two issues force us to reduce the Vmp to take in account cable losses and temperature.
But putting a lower Vmp at all time will of course reduce the solar production in certain conditions.
For example, if temperature is 25°C and no losses on cable because the panels are really close from the controller, the voltage will drop until it reaches the MPPSET.
If we take a quick hypothesis that, between 16.5 and 17.8V, the current equals the short circuit current (Isc = 9A),
Pmax = 17.8*9 = 160W
However, the voltage is 16.5V. So, the actual power is: 16,5*9 = 148,5W
There is thus a loss of around 8%.

My question is: how could we optimize the solar production in these conditions? Is it fine to reduce the MPPSET threshold to 16.5V? What are the risks?

Thank you very much for your help!

Michel

 

  • 0
    TI__Mastermind 24075 points

    Hi Michel,

       The LM234 application is a good way to account for fluctuating MPPSET based off of temperature, however if that is not feasible, then it would be hard to compensate for the temperature coefficient effect on MPP.

    There are no risks in decreasing the MPPT set point, apart from the fact that you will not be operating the solar panel at its true MPP. The input voltage regulation loop will regulate and still protect solar panel from worst-case crashing due to high load.

    When taking into cable losses:

    • identify the range of battery voltage that battery will normally charge at (for example if you have a battery protector with a cutoff voltage, then the range of battery voltage willl be shorter)
      • Depending on duty cycle, there will be a range of input currents you can expect.
    • Identify typical cable impedances
    • Measure what input voltage is sensed at the top resistor of MPPSET pin, that corresponds to the actual solar panel MPP
      • Set MPPSET to this value

    There will be tradeoffs as this is static input voltage regulation loop, and cannot be changed dynamically.

  • 0 in reply to Kedar Manishankar
    Prodigy 90 points

    Hi Kedar,

    Thank you very much for your quick reaction.

    It is good to hear that having a lower MPPSET won't bring any risks for the panel and controller.

    Maybe, do you have feedback from other clients concerning the best MPPSET when temperature feedback is not feasible?

    Furthermore, concering cable losses, I don't understand why the battery voltage range has an impact on the definition of the MPPSET. Can you give more details on this?

    Thank you and have a nice weekend,

    Michel

  • 0 in reply to Michel Fripiat
    TI__Mastermind 24075 points

    Hi Michel,

       For the same charge current, at higher duty cycles the input current draw on the solar panel/input source will be higher, results in more loading of the solar panel. This input current will result in voltage drop across the cable impedance (as well as forward voltage drop across input reverse blocking diode) between solar panel and VCC of charger. This way you are able to understand the true VCC of charger depending on your solar panel, and test conditions, and accordingly set MPPSET value.

    MPPSET also has to be set higher than battery voltage, as having the input voltage regulation set point under output voltage, would not help as charger will prioritize going into SLEEP mode.

    As MPPSET depends on system conditions and application it will be different for every client. I have not seen many designs with the temperature compensation, so the general feedback is that most people are willing to eat the efficiency loss from the Vmp fluctuation w.r.t temperature or design the MPP slightly lower.