I was tightening the negative terminal tonight, using a Gedore 5 - 50 Nm torque wrench set to 10 Nm (the manual says tighten to 12 Nm). The bolt inside the inverter snapped off :-(
And this is what the other half of the bolt looks like - kind of powdery inside?
I am a little bit unhappy, since we have regular power failures in South Africa at the moment, and I have no idea how long this will take to have fixed (the inverter is less than a year old).