100 Watt Solar Panels

rmathers

Im trying to find problem, seems the panels are only putting out 9 voltage should be more like 12 to 14, I have two hooked to a 40 amp controller to 12V batteries any idea on problem?

mike95490

Check the battery in your volt meter.  
A bare panel with nothing else should read about 19V, unless it's badly damaged.
 2 panels giving goofy readings, I'm guessing the meter is wrong

BB.

Measure the voltage of your 12 volt battery (or better--your car battery with the engine idling? Typically, around 14.2 to 14.4 volts).

-Bill

rmathers

Ill check that but the volt meter works, the panels are connected together going into rpod on one lead to controller, would that make a difference?

BB.

It is possible that the solar panel has a shorted By-Pass diode... That will "short out" about 1/3 or 1/2 of the panel's output voltage (~19-21 Volts Voc--voltage open circuit).

If the J-Box on the bank of the panel can be opened (some simply have screws, others are full of glue/silicon and cannot be opened without causing possible damage to panel), you will see 2-3 diodes (typically). With the panel "in the dark", you can use an Ohm Meter to measure the diodes (working, one direction will be low resistance, the other the diode will be very high resistance--A failed shorted diode will show low resistance either direction).

https://vandogtraveller.com/wiring-solar-panels-van-mppt30/


By the way, both of those are "bypass" diodes (no blocking diodes).

-Bill

RCinFLA

Bypass diodes on cheap panels are often insufficient to due their job.  Their job is primarily safety when part of the panel is shaded.  It is normal for cells to have shunt resistance defects.  During normal operation the cells have 0.5v to 0.65 v of voltage across them so the shunt defects may not be significant.  When part of the panel is shaded, the cells in the shaded section can receive a high reverse bias from the other cells that are not shaded.  A defect shunt that is insignificant in normal forward operation can produce a lot of heat when subjected to higher reverse voltage during partial shading.

The bypass diodes are placed across groups of typically no more then 20 series connected cells.  They are typically connected across every two long row pairs in panel so they can be connected at one end of panel.  This allows then to be connected in or near the junction box. It is also why you typically see even number of long rows in a panel.

Problem is the diodes can be required to carry near the full Isc current of the panel.  With their forward diode drop and 5 to 8 amps of carrying current they can get very very, blistering hot.  Higher quality panels use multiple Schottky diodes in series to keep their heating down.  Bypass diodes can fail open or shorted.  Bypass diode's reverse breakdown voltage rating need to be greater then highest array voltage (typically 600v) and carry the rated current of the panel.

If you never get shading then you will not likely see problems with bypass diodes. You could possibly get a lightning hit that blows a bypass diode but that is rare.

Check Voc, open circuit voltage will detect shorted bypass diodes.  Should be about 0.6v x number of series cells in panel.  There is a lot of 'chop shopping' for mono cells where half or third of a cell is used to keep costs down so sometimes it is hard to tell just by counting cells visible in panel.  Use Voc rating label if it is there.

Open bypass diodes are harder to trouble shoot.  Can be checked with DVM if visible in junction box.  Many times they are encapulated with coating to protect against moisture. A panel may seem normal with an open bypass diode but if shaded it might burn a hot spot causing the sealing plastic on back of panel to melt an open hole allowing humidity intrusion.  In the worse case it could start a fire.

BB.

This site has a nice diagram that shows where/how the bypass diodes work:

https://www.digikey.ca/en/articles/active-bypass-diodes-improve-solar-panel-efficiency-and-performance

-Bill

NANOcontrol

Bypass diode's reverse breakdown voltage rating need to be greater then highest array voltage (typically 600v) and carry the rated current of the panel.

NO, bypass diodes should be 45V breakdown.  There is a penalty for using higher voltage Schottky diodes because the forward voltage is higher producing even more heat.