Operation With Batteries
At night, an off-grid PV power system may use batteries to supply loads. Although the fully charged battery pack voltage may be close to the PV array's maximum power point voltage, this is unlikely to be true at sunrise when the battery has been partially discharged. Charging may begin at a voltage considerably below the array maximum power point voltage, and an MPPT can resolve this mismatch.
When the batteries in an off-grid system are fully charged and PV production exceeds local loads, an MPPT can no longer operate the array at its maximum power point as the excess power has no load to absorb it. The MPPT must then shift the array operating point away from the peak power point until production exactly matches demand. (An alternative approach commonly used in spacecraft is to divert surplus PV power into a resistive load, allowing the array to operate continuously at its peak power point.)
In a grid-tied photovoltaic system, the grid must be forced to absorb all excess power delivered from solar panels. The MPPT in a grid tied PV system will always attempt to operate the array at its maximum power point.
Read more about this topic: Maximum Power Point Tracking
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