Im not specifically sure on the exact cause of your battery issue. But it sounds like it was a slow on going process really. I actually have about eight batteries sitting unused from various vehicles that ive scrapped over time. If the battery is dead when I get it, its goes in the junk pile. I dont try to revive em. But if it shows signs of life, ill charge it to what I think is fully charged. At that point, about once6 or twice a week, I change a maintainer from battery to battery. Note: I only have 1 maintainer. When I change to the next battery, it shows charging then at a point goes to fully charged. Just the other day I had a battery trigger to maintainer to "bad batt", same as you. Now that battery is in the junk pile also. But my point was that ive seen some batteries last only a few years and some last 10 or more. Im not sure if there is a rime or reason, but they all die sometime. Even when regularly maintained....
