Clearly, there is a difference in ring wear on this alternator. I don't think it's due to polarity, though. As mentioned, the current flow through each is the same. There is some excitation differences and it is true that slip rings will wear differently, but on a small generator like this, we're talking microns over the 5000 hours of operation an alternator might see. On much larger (multi-KW) generators, slip ring wear is indeed monitored and accounted for. Polarities are switched on a schedule, but the currents are much higher, they're in constant operation and service lives are measured in decades. More than likely, it's that the alternator works in the worst environment imaginable for a brushed machine. It's hot, oily, and dirty all of which will increase brush and slip ring wear on a much more meaningful and noticeable scale.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
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