Single hot cell

Definition

In a healthy panel the series-connected cells carry the same current and run at about the same temperature. When one cell cannot produce as much current as the others, it is driven in reverse and dissipates power as heat instead of producing it. When the heating covers the whole cell, the anomaly is classified as a single hot cell.

Signs in the imagery

In the thermal image it shows as a single warm square or rectangle bounded by the cell edges, while the rest of the panel is uniform. The RGB image often shows nothing. If there is dirt or a bird dropping on the cell, it can also be identified in the RGB image.

Possible causes

• A micro-crack or break in the cell
• A broken cell interconnect ribbon or solder joint
• Cell mismatch from manufacturing
• Local dirt, a leaf or a bird dropping covering the cell

Effect on production and safety

The panel's output is limited by the current the hot cell can carry. The loss is usually small. If the heating persists, the encapsulant around the cell can brown and the backsheet can age. As the temperature difference grows, so does the chance that the bypass diode switches in or that the cell develops into a hot spot.

Recommended action

First check the RGB image for a covering that can be cleaned. If there is none, put the panel on a watch list and check at the next flight, on the same panel record, whether the temperature difference has grown. If it is growing, plan a replacement and a warranty claim.