I also have dropping voltage from my single 130 watt Kyocera pv panel installed aboard my sailboat.
Yesterday I had it connected up for the first time ( wired 12 ga. wire direct from pv panel to SunSaver 10 then wired direct to two Rolls 4-D batteries. Voltage within a couple of hours was up to 14+ during mostly hazy and diminishing sunlight.
Today, a bright and sunny day, pv panel aimed, it will not get past 13.3 volts.
With the pv panel wires open, I see 20.9 volts, and load at 13.3
Did I damage my panel when I started my diesel engine last night? Did I run voltage from my alternator (14.
back into the pv panel and ruin it?
Same case as with Jorge above. Everything is working exactly as it should. Because you're not using a charge controller with MPPT technology then your PV is forced to operate at the same voltage as your batteries. The highest your PV voltage will rise to will be the full voltage of your batteries, which is around 14 plus volts.
Running your diesel engine to charge up the battery bank would not damage your PV nor your charge controller. It would just make your batteries full and happy really fast.
Also, remember that a battery bank's voltage changes when you hook a load up to it. At rest, the battery bank full charged may be say 14.4V. But the moment you run a load on that battery bank (e.g. a bilge pump, lights, radio, etc) that voltage will dip a bit ... like down to 13.3V. As soon as you remove that load you'll see the voltage will actually creep up again. If you only ran the load for a very short period of time, the voltage may rise right back to the full voltage (14.4V). That's all ok and normal.