Electronic – How do MPPT charge controllers curtail the power if the load is less than the solar generation

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I am trying to understand how MPPT charge controllers work when the load is less than the solar power generation. Let's assume an off-grid system with battery backup and no dump load. In my understanding, the following situations can happen:

  • solar < load: solar will partially fulfill load requirements, rest will come from the battery
  • solar = load: all of the load is fulfilled by the solar directly, battery status remains unchanged
  • solar > load (battery not fully charged): solar will power the load and rest will go to the battery.
  • solar > load (battery fully charged): In this case, the charge controllers curtail the power generation to match it exactly with the
    load. The excess solar energy is lost in the form of heat.

I want to understand how this curtailment happens in actual MPPT charge controllers. Do the MPPT techniques/chips leave the MPPT mode and the operating point of the solar panel is controlled by the current drawn? In that scenario, how does this whole circuit (DC-DC converter) knows how much demand is out there? Is there any implicit feedback loop?

I will highly appreciate your detailed response on the question. Please let me know if the question is ambiguous.

Best Answer

MPPT controllers are not about power balancing, but power maximization.

Let's say you have a system like this:

  • Solar panels ⇒ MPPT controller ⇒ battery charger ⇒ battery ⇒ load

If the load on the system is smaller than the incoming solar power, the battery voltage will rise.

When the battery voltage reaches a point where the battery charger decides that the battery is full, the charger will cut out, and stop drawing power from the MPPT controller. In other words, the battery charger will present a high impedance to the MPPT controller.

The MPPT could react to this by presenting a high impedance to the solar panels, effectively stopping any significant power draw from the solar panels.

When the battery is not being charged, the load will cause the battery voltage to fall. Then the charger will start drawing power from the MPPT again, and the MPPT will start behaving like an MPPT again. And the cycle repeats. The system can cycle back and forth between these two states several times a minute, or even thousands of times a second, all depending on the time constants (battery size or capacitance) and hysteresis in the system.

You may have a commercial product called an "MPPT" that has more than an MPPT controller. For example it could have an integrated battery charger and maybe even an integrated voltage regulator for the battery output. Don't let any oversimplified marketing jargon confuse you. The MPPT part of the device only handles MPPT, and if the device also actually charges a battery in a proper manner, then it must also have a battery charger built into it.

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