I just pulled the seat and followed the leads, checking with my voltage meter. The insulated or covered lead when you pull the cover off of the pigtail is positive...

 

Battery Tender Plus for 5 years with no problems.

 

http://www.lectriclimited.com/battery-butler.htm

 

Another option. I have two of these that I've used for 5+ years on both my bikes and cars. Work great and no problems.

Yuasa SmartShot Battery Charger
1.5 Amp - 6/12 Volt Charger/Maintainer
Model 12V1500

For a real powerhouse charger YUASA’s 1.5 amp SmartShot features five-stage charge circuitry that’ll bring a completely discharged and partially sulfated battery back to life. SmartShot’s 28-day charge recycle circuit automatically re-evaluates the battery’s status when it’s set on the shelf for long-term storage.

Has the ability to charge both 6V and 12V batteries
3 LED indicator display
Spark-free operation
Switches automatically to float mode, built in timer for battery protection
28-day charge recycle re-evaluates battery condition for long-term storage
Provides up to 16V for recovery of sulfated batteries
Reverse polarity protected
Easy, error-proof operation; designed to prevent overcharging
Charger ring connector available separately
Output 6/12VDC @ 1.5 ADC
Input 120 VAC @ 60Hz

 

2ND THAT, BEEN USING FOR 5 YRS. NO ISSUES 24-7 365 DAYS A YEAR. When im not riding the bike, its on the TENDER!!!