After months of sitting it could easily be a discharged or bad battery. Sitting that long connected and without charging, the parasitic drain could kill a cell or two.
Remove it, charge it with a good charger, and take it to have load tested. If you're still running the top-post battery then it's time to get the right side-post one.
Remove it, charge it with a good charger, and take it to have load tested. If you're still running the top-post battery then it's time to get the right side-post one.