I'm powering my 12v accessory port off the heated grip connector, and the voltmeter off the accessory connector. But, if you didn't order both leads together (EB has a minimum order and fairly high S&H if I recall), you can just do as Trepidator suggested. What you can do is check the voltage directly at the battery using a VOM under various electrical loads and RPMs for comparison to the reduced reading on the voltmeter.
2014 V-Strom 1000 - Desert Khaki
2011 Burgman 400
2007 Honda Helix
2013 V-Strom 650 - sold