By aux outlet, do you mean like cigarette light socket? If you put a positive volt meter probe on the positive pin of your Battery Tender wiring harness, and neg probe connect to car body, and have the car off, you should be getting accurate read of the battery volts. To get a more in-depth read of the battery condition, some sorta electronic load tester*, connected directly to the battery posts, is what's required. I appreciate that's a pita with the c though. * Solar BA9 would be my pref. They look to be out on Amazon.com though.