Does the photovoltaic junction box heat up even if it is not poorly welded? The truth behind the appearance of rows or scattered pieces is revealed in one article!

In the operation and maintenance of photovoltaic power stations, many people fall into a misunderstanding: heating of the junction box = false soldering.

However, during the actual on-site inspection, it was found that many overheated junction boxes, when disassembled for inspection, showed no signs of false soldering at all. Even more strangely, the glue inside the junction boxes was spotlessly white. Some cases of fever are absent in a row, some occur in rows or patches, and some appear sporadically. Moreover, drones cannot capture it at high altitudes, while handheld infrared cameras can capture about 80%. Drones can capture it comprehensively at low altitudes.

In fact, the core answers are just two: heat generation is strongly correlated with current, and the detection difference is directly linked to distance and shooting Angle. Today, no empty talk. It's all about the real stuff. Photovoltaic professionals can directly save it for future reference to avoid pitfalls in troubleshooting!

I.Dispel the misunderstanding: If there is no false soldering, why does it still get hot? (Core Logic)

Remember a core formula: Heat generation = contact resistance/abnormal diode + current flowing through (Q=I²Rt)

False soldering is just one of the reasons for heating. In a non-false soldering state, these three situations are the triggers, and all are related to the current - the greater the current, the more obvious the heating, and it may be completely invisible in weak light and low temperature.

1. Increased contact resistance (most common, accounting for over 80%)

It is not a false soldering, but rather the oxidation of the internal terminals of the junction box, the loosening of the MC4 plug, the corrosion of the copper bar, or insufficient bonding force, which causes the metal contact surface to change from "surface contact" to "point contact", and the contact resistance surges (from the normal < 5mΩ to tens or even hundreds of mΩ).

As long as there is current flowing through, the resistance will generate Joule heat. The greater the current, the more intense the heat generation. For instance, when the string current approaches the rated value, even a slight resistance anomaly will quickly accumulate heat.

2. Abnormal bypass diode (easily overlooked)

The diode is not completely broken down but shows leakage, aging, or a high forward voltage drop. As long as there is current passing through, it will continue to heat up.

Especially when the components have hidden cracks or power mismatch, the bypass diode will conduct prematurely and work under current for a long time. The heat will be directly conducted to the junction box. It seems that the junction box is heating up, but in essence, it is an abnormality of the diode. Of course, the most fundamental essence is the hot spot on the battery cell.

3. Cable/Connection Loss (Detail Pitfall)

Insufficient diameter of DC cables, broken strands of multiple conductors, or aging of the cables can lead to an increase in line resistance, with current concentrating at the interface between the junction box and the cable, thereby causing local heating. This has nothing to do with false soldering.

II. Key Question: Why do some rows not heat up while others heat up in rows?

This phenomenon is essentially the result of uneven current distribution in strings, coupled with the inherent defects of the junction box. The combined effect of these two factors can be accurately judged in three situations:

1. Row heating: The entire series of current is "uniformly raised" or "uniformly defective"

As long as there is heating in rows or patches, there is no need to check individual junction boxes. First, check the entire series current and the environment.

• The orientation and inclination Angle of the entire string of components are consistent, and the lighting is uniform → the current of the entire string is consistent, and the junction boxes with slight defects are triggered to heat up simultaneously.

• Improper string configuration (such as short strings), low MPPT voltage of the inverter → the current of the entire string approaches or exceeds the rated value, the I²R effect is amplified, and the entire row of junction boxes heat up together.

• The quality of the junction boxes in the same batch has defects → the aging trend is consistent. As soon as the current rises, they will heat up simultaneously.

Conclusion: Row heating = consistent current environment + consistent junction box defects, and has nothing to do with a single fault.

2. One row has no heat at all: low current or no defects

The reason is quite simple. The "heating condition" is not met:

Either the orientation of this string of components is poor, the light is weak, or the string is too long, resulting in a smaller current. Either the junction box is of good quality, free from oxidation and loosening, the component matching degree is high, the diode does not conduct - R (resistance) is fine, I (current) is small, and naturally it will not heat up.

3. Sporadic fever: Individual defect, unrelated to the entire series of current

Only a few junction boxes are overheating, which is most likely a single-point issue: for instance, the seal of a single junction box fails, water ingress causes corrosion, a single MC4 plug is loose, or a single component has a hidden crack causing the diode to conduct - the entire series current is normal, but only the single-point resistor/diode is abnormal, so it occurs intermittently.

III. Infrared Detection Myths: Can't Be Captured at High Altitudes, But at Low Altitudes/Handheld?

Many operation and maintenance personnel have reported that during high-altitude inspections by drones, they cannot see the junction boxes heating up. It can be captured by hovering at a low altitude, but it may not be possible to capture it by holding an infrared camera. The core issue is not about the equipment but three fundamental principles. Once you read them, you'll understand:

1. Inability to capture at high altitudes (drone > 30m) : Insufficient pixels + signal attenuation

At high altitudes, the distance is long, and a single junction box only occupies 1 to 3 pixels in the infrared image. The heating area (usually < 5cm×5cm) is "pixel-fused", and the slight temperature difference of 3 to 8 degrees Celsius is averaged out, making it impossible to stand out at all. In addition, water vapor and dust in the atmosphere scatter infrared signals, resulting in an insufficient signal-to-noise ratio. Naturally, it cannot be captured.

2. Low-altitude/handheld shooting (low-altitude < 10m, handheld < 2m) : Sufficient resolution + minimal interference

At close range, the junction box can occupy 10 to 100 pixels in the infrared image, with the temperature gradient and heating contour clearly distinguishable. It can be accurately identified with a temperature difference of more than 3℃. Moreover, the distance is short, the infrared radiation attenuation is reduced, the signal is sufficient, and the drone can be directly positioned at the front area of the junction box, making the heating area clearly visible.

3.Why is the front side glowing with infrared light but the back side not?

Understand the Differences among Three Detection methods with one Table (for direct comparison)

IV. On-site Practical Operation: 3-step inspection, No Detours (Land directly)

Without complex instruments, follow this logic to quickly locate the problem:

1. For row heating → First check: string current, branch current, then the orientation/occlusion of the components, and the batch of the junction box.

2. Sporadic heating → Priority inspection: Sealing of a single junction box /MC4 plug, hidden cracking of a single component, diode status;

3. Infrared detection → High-altitude initial screening of hot spots on components, low-altitude (5-10m) hovering to check junction boxes, handheld infrared rechecking of suspected points (abnormal determination when temperature difference > 5℃, immediate shutdown when temperature difference > 10℃).

In conclusion, the photovoltaic junction box is not due to false soldering and overheating. The core issue is "abnormal resistance + current flow". If they appear in rows or scattered, check whether the current in the entire series is consistent. The differences in infrared detection are essentially issues of distance and resolution.