battery swelling and leaking

[JRSully]

Took the rod out the other day and when I got back, noticed the battery case was swelled and was just starting to leak a tiny bit of acid through the seams, glad I got it home before kaboom. Never had an overcharging issue ever, battery is a sealed Delco about 5-6 years old, stays on a battery tender most of the time, never any battery issues prior to this. Pirkle rebuilt V.reg and Alt have always worked fine with only about 5-6k miles on the car in last 5 years or so.? Thoughts.? Tired battery.? Thx