Little help would be great. I have a 2016 Commander XT-P 27 miles and 3.5 hours. Battery died while out riding. Hooked up Voltmeter to battery when I charged up, had 11.5V with engine running with key in Lights on position. Revved up motor, no change to voltage level. Put key in just ON position, no lights, had 12.5V. Revved up engine and voltaged rose some, then lowered to 12ish volts at idle. Ideas??

THanks in advance
Roland