Well it was my turn to experience this issue too.
Measured battery while engine OFF: 10.8v
Measured battery while engine ON: 9.8v
Hopefully it's a broken yellow wire.
In the other case of being the regulator I wonder how I can access it without the burden of removing the rhs panel.
Has anyone done so?
Charged it overninght with charger.
12.8v engine off - 12.4 engine on.
So I have a drop of voltage as expected when starting the engine and most probably the regulator part is defect.
Partially removing the rhs rear panel is less time consuming than trying to replace the regulator without removing it imho. Likewise lots of C1 jobs. Just get a Torx bit mounted on an electric screwdriver and wiz out all the screws. Feeding the battery cables in and out of the battery box is a bit of a fiddle. Don’t bother completely removing the rhs panel. Just release all the back screws as far forward as the seat and hold the panel out while you work on the electricals.
I finally partialy lifted the rear right panel.
Much easier than removing battery, battery box etc in my opinion.
Measured with ohm meter and found all combinations having the same reading so I conclude this is most probably a regulator problem.
I ordered one for 15 euros from Ebay (Voltage Regulator Rectifier for BMW F650).
I connected it today.
It works. Only difference is that one screw can not be screwed (the second hole does not align)
the importance of a good thermal bond between the rect/charger and bike chassis was previously discussed on the forum . i recall that bmw mounts the unit on a narrow flat metal strip to provide two mounting holes but preventing direct contact to the bike chassis. bad design imho. i replaced the strip with a 4mm thick aluminium plate, even using silicon thermal paste.
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