Odyssey Dry Cell Life/Failure Modes

I installed an Odyssey Dry Cell battery in my Fly Baby about four years ago, and
I've been very happy with it.

Lately, though, my voltmeter has been running a bit low, both before the engine
starts and when the plane is cruising. My Microair transponder has a voltmeter
function, and I've verified that the battery is running about a half-volt lower
than I'm used to.

The engine starts so quickly I can't really tell if the battery charge is low.

Anyway, I was figuring I had more problems with the generator/regulator, but
when I put an external charger on the plane this morning, the initial charge
current was quite a bit lower than I'm used to seeing when a battery is low on
charge.

So my questions run down to:

1. What kind of life are people getting with these Odyssey batteries, and
2. When they fail, what kind of indicators do you see? Does it just suddenly
refuse to hold a charge, or are there early warning signs?

Ron Wanttaja