Inside the plug, it looks something like this...
(Attachment Link)
Whatever it is that little circuit board does in there, it works really well. That VW solar panel floats the genset starting battery all day at 13.25 volts - it never goes over that. I have checked it now and then to make sure this is going to work long-term. I now have no doubts it is working fine, and will not harm the battery by over-charging it or anything.
It is unlikely from this point on that the generator will be run at all thru the summer months, except for its monthly exercise for 30 minutes. It's pretty amazing that the generator electronics draw about the same amount of power as the little panel is rated at for output, but it still puts out enough "extra" to keep the battery right up to 100% SOC.
The circuit in the VW PVs (the ones that have the circuit in there) usually regulate to about 13.7V average, maybe 13.5 to 13.9V.
The hysteresis is about 0.3V to 0.4V, with serious slop from one to the next.
Meaning (in good sun) it can cycle on a full charged 7AH 12V SLA (in decent condition) maybe 3 to 20 times a minute? Cycling also depends on the actual SoC of the battery, not just the terminal voltage.
What you are seeing now is very similar to what you saw earlier.....
The battery was at 12.44 volts when I plugged it in. It took about 30 minutes and it had already come up to 12.92 but it wouldn't go over that.
Now, the power it is putting into the battery is basically balancing out to keep the battery at 13.25V.
Earlier, it was balancing out at 12.92V.
Later, mid summer, it will be higher, and may even start to show the hysteresis if it cycles from 13.8 to 13.5 to 13.8 to 13.5....
If the battery or drain is too large, it may never make it up to the point of shutting off.
The controller in it is not overly fancy, and the PV is small enough to hve a low risk of overcharging a car battery with any drain at all.
VW must have figured that out at some point, or some of their PV maufactures did, and thats why most do not have the built-in controller.
Most VW PVs only have a blocking diode.
All that pertains to the old crystaline PVs. I don't know about the newer amorphous type, or the newer newer amorphous type with the computer OBD plug end.
G-