Mismatch for Cells Connected in series

As most PV modules are series-connected, series mismatches are the most common type of mismatch encountered. Of the two simplest types of mismatch considered (mismatch in short-circuit current or in open-circuit voltage), a mismatch in the short-circuit current is more common, as it can easily be caused by shading part of the module. This type of mismatch is also the most severe.

For two cells connected in series, the current through the two cells is the same. The total voltage produced is the sum of the individual cell voltages. Since the current must be the same, a mismatch in current means that the total current from the configuration is equal to the lowest current.

Open Circuit Voltage Mismatch for Cells Connected in Series

A mismatch in the open-circuit voltage of series-connected cells is a relatively benign form of mismatch. As shown in the animation below, at short-circuit current, the overall current from the PV module is unaffected. At the maximum power point, the overall power is reduced because the poor cell is generating less power. As the two cells are connected in series, the current through the two solar cells is the same, and the overall voltage is found by adding the two voltages at a particular current.

In the animation, cell 2 has a lower output voltage than cell 1.

Short-Circuit Current Mismatch for Cells Connected in Series

A mismatch in the short-circuit current of series connected solar cells can, depending on the operating point of the module and the degree of mismatch, have a drastic impact on the PV module. As shown in the animation below, at open-circuit voltage, the impact of a reduced short-circuit current is relatively minor. There is a minor change in the open-circuit voltage due to the logarithmic dependence of open-circuit voltage on short-circuit current. However, as the current through the two cells must be the same, the overall current from the combination cannot exceed that of the poor cell. Therefore, the current from the combination cannot exceed the short-circuit current of the poor cell. At low voltages where this condition is likely to occur, the extra current-generating capability of the good cells is not dissipated in each individual cell (as would normally occur at short circuit), but instead is dissipated in the poor cell.

Overall, in a series connected configuration with current mismatch, severe power reductions are experienced if the poor cell produces less current than the maximum power current of the good cells and also if the combination is operated at short circuit or low voltages, the high power dissipation in the poor cell can cause irreversible damage to the module. These effects are illustrated in the two animations below.

Current mismatch for two cells in series can be quite serious and quite common. The I_sc of the combination is limited to the I_sc of the lowest cell.

An easy method of calculating the combined short-circuit current of series connected mismatched cells. The current at the point of intersection represents the short-circuit current of the series combination (ie. V1+V2=0).