Dead battery after driving all day with no problem?

Batteries sometimes do that. I had one car battery and one motorcycle battery do it. The bike battery read 12 volts until any load was put on it. Couldn't even push start it. I guess they came apart inside.

I don't know if you posted your question 2x intentionally, but like myself and tenspeed said before....The plates get warped and/or the electrolytes become contaminated, or a regulator has overcharged it...do you detect an unpleasant odor? If so, it's trash, it probably is anyway