Totally Weird Electrical Problem

[GZrider21]

Hello all, Im new here and Im hoping somebody might have some insight to my very perplexing electrical problem.

I have a 2008 gz and I bought it about a year and a half ago with less than a thousand miles on it. It looked and rode like it was brand new. I had to leave town for work and it has been parked for almost a year.

Yesterday I pulled it out and surprise surprise it did not start, no lights no horn no power. I figured hey easy fix lets give it a jump. Hooked it up to a car battery and after cranking out all the cobwebs it started right up, however when I took off and gave it some throttle it started to bog down badly and did not get any better. It would idle fine but when I gave it gas it sounded like it was skipping, bogging down, or like it was not getting enough gas, or like there was a timing issue if there is such a thing on the gz. After I turned off the bike and tried to start it again it was completely dead. Started it again with a jump and still had same problem. Decided to put the battery on a trickle charger overnight.

24 hours later charger said the battery was charged and I checked it with a volt meter and said it had 13volts. Turned the key praying for a green neutral light but got nothing, totally dead, but how can this be when my meter said it had 13v?

Jumped it again and it started with no problem. I noticed that the lights would get dimmer when I gave it a little gas which I thought was weird. Tried riding it again with same problem. Took a meter reading while it was idling and it said 11v and when i took a reading with a little throttle it went down to 6v. Turned it off got reading of 12v but acted as if it was deader than a doornail.

Now this is really weird....

I decided to take a meter reading after turning the key on. Meter read 0 volts! turned the key off 12 volts. WTF?

I am far from knowledgeable about motorbikes and a friend of mine said he had a similar problem although it was a truck. He said it was an alternator issue, the alternator was draining the battery. Do these even have alternators? I assume they've got to have some sort of electrical charging system but as I said I am pretty clueless when it comes to this kind of stuff. BTW I have cleaned and tightened the terminals, looked for any loose wiring connections but everything seems to be alright

Any help, insight, suggestions would be GREATLY APPRECIATED! Thanks in advance.