## 4 Discussion  
### Reverse-Bias Voltage Measurement  
In this study, we verified a highly accurate voltage calculation method for PV modules and element cells in PV modules based on non-contact potential measurements using surface voltmeters. Using this method, we investigated a method for detecting degraded cells in PV modules.  
The open-circuit voltage Voc of the PV module in the open state, the potential difference of the No. 36 cell, and the potential difference and voltage per element cell were compared. Thus, from both the glass surface and the backsheet surface, the potential difference and open-circuit voltage Voc of the No. 36 cell and the potential difference and voltage per cell matched with a small error. In addition, the potential difference and voltage per cell agreed at \(\pm 0.1\) V. Therefore, it is effective for monitoring the operating conditions of each solar cell in the PV modules without contact.  
The PV modules under short-circuited conditions experienced voltage drops in the internal low-current cells. Although the difference in the current value between the low-current cell and the other cells was minimal, a clear difference appeared in the voltage per cell. Therefore, when the potential difference per cell and voltage match, it is possible to identify low-performance cells in the PV module. However, the maximum error increased to 0.3 V due of the decreased irradiance of the LED bulb used as the light source. Therefore,  
Figure 26: The current of the No. 8 and No. 15 cells in each shaded area.  
if the potentials of all the cells can be measured simultaneously in a uniform illuminance distribution, a more precise detection of performance differences can be expected.  
One of the interests in applying this method for inspecting the module outside (on the roof of houses or the rack of the solar power plants) is that solar irradiance is not always 1000 W/m\({}^{2}\). A question arose: How to correct the measured reverse-bias voltage corresponding to the solar irradiance level in the field? The reverse-bias voltage is the \(V_{pm}-(I_{pm0}-I_{pm})R_{sh}\), or \(V_{\infty}-(I_{sc0}-I_{sc})R_{sh}\), depending on the load resistance, and before the junction does not reach (partial) breakdown. The correction of the \(V_{pm}\) and \(V_{\infty}\) are approximated as adding the equivalent voltage of ln(ratio of the solar irradiance to the standard value of 1000 W/m\({}^{2}\)) \(\times\) (0.027 V). The correction of all the current parameters is proportional to the solar irradiance to the standard value of 1000 W/m\({}^{2}\).  
Another practical question is the impact of the bypass diodes. The number of diodes in the module is usually three and includes 20 or 24 solar cells (depending on the module with 60 or 72 cells). There are also three diodes in the half-cut cell modules, but three series strings comprise two groups of parallel half-cut cells. A question is if a difference in configuration affects the measurement.  
The function of this Inspection is to estimate a reverse-bias voltage caused by current mismatching of the module. The reverse-bias voltage is the \(V_{pm}-(I_{pm0}-I_{pm})R_{sh}\), or \(V_{\infty}-(I_{sc0}-I_{sc})R_{sh}\), depending on the load resistance, and before the junction does not reach (partial) breakdown. We measured it under short-circuit conditions so that the second formula will be applied. \(I_{pm0}\) and \(I_{sc0}\) are normal \(I_{pm}\) and \(I_{sc}\). The function of the bypass diodes to the circuit behavior is to clip the \(I_{sc}\) to \(I_{sc0}\). Nevertheless, the number of bypass diodes is the detected reverse-bias voltage of the degraded solar cell, and the current mismatching will be the same.