I know that you've all heard it before, but I just did it and it made a HUGE difference with cranking and all things electrical on my 580se: Remove and clean back to shiny bare metal the ground strap and everything it connects to including frame, bolts, washers etc.
My machine (580se) was cranking slow (but starting) and lights were not coming on, finally it stopped cranking altogether.
I decided to remove the starter and bench test it, it was fine , lots of pep.
What clued me in was when I checked the 12v connections with a test lamp. The test light glowed bright when connected between the + - posts of the battery but the test light was noticeably dimmer everywhere else. At that point I looked at where the ground strap connected to the bolt thru the frame on the back of the battery box and sure enough i could move it a little. I took everything off the ground path (neg battery cable, bolt/washers, woven strap) and cleaned all including the spot where it touches the frame, back to bare metal with a wire wheel and sandpaper. After reassembly, boom!, it was like a new machine.....fast cranking (outside 30F temp) and all electric components working vigorously (lights, wiper, blower, gauges etc). Since this made such a huge difference and I had overlooked it for so long, I suggest that it cant hurt to spend an hour to dismantle and clean your ground strap and all connections.
The thing is that my machine did crank and start even with this weak ground connection but it was at half the power as with a good connection. I wasn't aware of this in-between, weak-but-functioning state a poor connection can create. I had thought that an electrical connection could only be ON or OFF and so I never suspected the path from the battery to the frame had a problem. Live and learn !