Had a great day down in Cornwall laning (UK), when i could smell rotten eggs, asked the twins, they said it wasnt them, then seeing steam coming from under the bonnet, knew it must be the battery boiling, that nasty acidy smell. Stopped and lifted bonnet, to see a twisted battery, with jets of acid steam coming out of it.
Drove 55 miles home with the dash lighting up like a xmas tree, but made it.
Checked charge voltage maxed out at 15 volts, so alternator ok, so why????

 

I think 15 volts is what cooked your battery. At that voltage, battery temp needs to be monitored.
Get the charging system looked at.

You may want to wash down under the bonnet with a liberal dose of baking soda/water mixture to neutralize the acid.

 

Is that an Optima Yellow top? How old is it?

 

The 15 volts was with the dead battery still in, so the alternator would be kicking out max, so 14.9-15 volts is ok.
When i fit new one, i will check again to ensure that with a charged battery it drops down to 12 volts.
As for age, it was 3 1/2 years old, so it did well, but in all my years building, and racing 4x4s, i have never seen a battery look like that.

 

A good point about washing down under bonnet.

 

So 3 1/2 years is considered good for the Optima? What ever happened to batteries that were good for 5 + years. I'm personally a bit miffed when my battery dies in under 5 but would be pissed if I'd shelled out for a premium battery that only went 3 1/2...

 

Man, I agree 100% with phantomandy

 

Agree also that the newer Optima's are junk, although my 6 year old RedTop has been drained down to the point of my needing a jump twice, and still starting my 94XJ today. The first RedTop bought back in 96 lasted 9 years.
Now that said, back to the OP's battery. This sure does look like an overcharge, especially with the statement about jets of acid steam coming out of the battery.
What is the ambient temp there? The reason I ask is that a normal charging system should output between 13.8-14.2 VDC at idle around 80*F, with the lights and accessories off. The lower the temperature the higher the output may be, and it may be as much as 15.2 at idle but that would be about 0 to 20*F.
The alternator (160 amp) on our 07 JK unlimited output about 13.9 to 14.6 according to my Fluke 87, depending on the weather and temperature. Our factory battery lasted about 4 years, and I recently installed a DieHard Platinum. The alternator output reads the same as it did with the stock battery.
And yes, if it was streaming gas, scrub the inside down as described.

 

I envy your battery life. The heat out here in AZ kills them. Getting 2 years is about normal for the average battery. My car (not my jeep) is on its 4th year and I am scared as hell. The other thing is that when a battery dies out here you are lucky to get one warning (1 semi slow start) and it is usually in the summer.