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I've designed a PV based solar charger controller using the MPPT algorithm to charge my 12 V tubular battery for a DC load.

I need to keep the output voltage constant at 14.5 V to connect a 12 V battery for charging purpose through directional load switches (MOSFETs). When I connect the battery to my charge controller, the output voltage of the MPPT buck converter as well as the PV input fluctuates by 3-4 V, which constantly affects the feedback logic (if Vout >= 14.5 V connect to the battery/ if Vout < 14 V disconnect the battery) that I programmed in the microcontroller. It repeatedly connects and disconnects the battery from the source.

How can I keep output voltage constant, e.g. 14.5 V, to charge the battery with maximum power?

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    \$\begingroup\$ Your MPPT algorithm/method is unstable. \$\endgroup\$
    – Andy aka
    Commented Aug 14, 2022 at 13:34
  • \$\begingroup\$ @Andyaka actually my algorithm is huge and complex ,here i just mentioned the feedback logic and the device was working fine with the existing code. \$\endgroup\$
    – Suraj Sashtri
    Commented Aug 14, 2022 at 13:57
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    \$\begingroup\$ You can't. MPPT must modify the output voltage to control the load current in order to determine the correct voltage for max power. You have expressed 2 mutually incompatible requirements here : that means, go back to the requiremenns specification, work out what you actually require, correct the requirements spec and proceed from there. \$\endgroup\$
    – user16324
    Commented Aug 14, 2022 at 14:03
  • \$\begingroup\$ What’s the peak unloaded output voltage from your solar panel? \$\endgroup\$
    – winny
    Commented Aug 15, 2022 at 8:52
  • \$\begingroup\$ ”device was working fine with the existing code” != ” the output voltage of the MPPT buck converter as well as the PV input fluctuates by 3-4 V, which constantly affects the feedback” \$\endgroup\$
    – winny
    Commented Aug 15, 2022 at 8:53

2 Answers 2

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An MPPT will maximise charging current by trying to set its input impedance at the value that makes the solar panel sit at its MPP, where the solar panel delivers its maximum power.

That is the only regulation loop it has; its output current is maximised, but there is no output current or voltage regulation towards a certain value. The output voltage is set by the battery that is connected (or the input impedance of the circuit that is connected).

You can't regulate the output voltage of an MPPT to a fixed value and maximise the output current at the same time.

Also, there is no need to regulate the MPPT's output voltage when charging a battery; you do need to monitor the battery's voltage and stop charging at a pre-set voltage, but that is not the same as regulating the output voltage.

If you insist on having a regulated output voltage, you will have to add another DC/DC converter regulating its output voltage to a fixed value of your choice. The whole circuit will charge batteries less efficiently than the one with just the MPPT though, and it will take quite the current to get a steady 14.5 V across a discharged battery.

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  • \$\begingroup\$ Yes,I got your point sir but my major headache is the voltage fluctuation that occurs between MPPT output and battery charging input terminal(14.5v to 12.1v every second). It just pulls down the solar input voltage (3-4v) at any climate condition. Any suggestions should be appreciated 😇 \$\endgroup\$
    – Suraj Sashtri
    Commented Aug 14, 2022 at 18:08
  • \$\begingroup\$ @SurajSashtri Again, that's due to the MPPT algoritm being unstable. \$\endgroup\$
    – winny
    Commented Oct 7, 2022 at 9:12
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You need a DCDC converter between the PV panel and the battery. This will allow you to separate the MPPT operation from the battery voltage.

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  • \$\begingroup\$ Could it able to provide the current Iout(max)=10A? \$\endgroup\$
    – Suraj Sashtri
    Commented Aug 14, 2022 at 18:10
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    \$\begingroup\$ That depends on the PV, DCDC, and battery. Certainly systems with that current is possible. \$\endgroup\$
    – jp314
    Commented Aug 14, 2022 at 19:43

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