Almost certainly the typical Tenere regulator/rectifier fault.

You will need to either: (a) Replace the loom and the reg/rec unit with OE parts - expensive (unless its in warranty of course). Or (b) Cut off the existing plug on the loom and replace the reg/rec unit with an FZS600 item (eBay) and solder the connections and heat-shrink (or make up your own connector).

http://www.xt660.com/showthread.php?t=17054

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