Bike Power

So my 2000 Sporty 883 runs great. No problems, the other day, I had it on my trailer outside for about 20 mins, and it was raining. Took it off the trailer and it wouldn't start. No power at all. Then i took the battery off the tray, still hooked up though, and then it started right up. Put the battery back in the tray, tried to start it and nothing. No power. took it back out and it started right up. I noticed the negative cable was a little loose so I tightened the post back up. But when I put the battery back on the tray no power. WHAT THE HECK IS GOING ON??? It has something to do with the water for sure. I took the battery off the tray to let the water dry up. Is there something else?
Thanks for anything.

You didn't say how old the battery is & it's conceivable that the case is damaged (age, vibration) which allows a dead short to ground when you stick it back in the tray. Your problem certainly does sound water-related, but there are other possibilites too, including the fact that the battery cables can break inside the insulation & you won't see it from the outside. In other words, when the cable is stretched out with the battery out of it's tray you've got enough connection to start the bike, but when you stuff the battery back in, the cables bend around enough that you lose the connection you had. And/or there's enough moisture inside the cables that it ruins the connection - but only when tha cables are in a certain position.

I tighten the posts up on the battery and so far it seemed to work. It starts up fine the last 6-7 times I tried. Maybe it was just a coincidence with the rain and not starting.
Thank you for the info.

What should i use to clean? steel brush?

Auto Zone and Advanced Auto both sell special battery terminal brushes that are differently configured steel brushes that aid in cleaning battery terminals and posts. Or you can buy the standard steel brush which will make things a heack of alot easier on your wallet if need be.

steel wool from under the sink, a pocket knife, screwdriver just about any thing that will scrape lead to a shine

and put a little spray paint over the terminals afterwards to slow the oxidation, or petrojell that kinda stuff

If you clean the terminals and still have the same issue, have your battery tested. If the battery is good, then think about replacing the cables.
They are cheap compared to the headaches they can cause. Good luck!

thanks everyone!! So far so good. It still starts.