I'd say replace before failure. My anecdotal reading indicates that a failing
battery puts a strain on the alternator and may contribute to premature
alternator failure. E.g.

"A weak battery that is not storing enough power will cause the alternator to
work harder and possibly cause premature alternator failure. Dirty or corroded
battery terminals can severely reduce the lifespan of the battery and
alternator." http://www.trustmymechanic.com/35.html

" ...depleted batteries make recharging much more difficult, shortening
alternator life." http://www.intra-tech.com/news/homeapriltwentnine.html

"By maintaining batteries in peak condition alternators do not work as hard,
batteries accept charge more readily so alternator life is extended. Healthy
batteries start engines easier, so your starter should also last longer."
http://www.megapulse.net/faq.htm

I (imprudently) let the first two batteries on my 1991 Civic (152k+ miles) stay
until they died completely. Each lasted 4.5 years. I had a few jumpstarts on
each. Jumpstarts also shorten battery life. I am on my second alternator (hmm).

The first battery was OEM and the second was the best quality battery Sears
offered for my car.

===============================
My Criteria for Replacing My Present Battery
===============================
My present battery (the third, another Sears battery) is approaching 4 years in
age. I recently took "benchmark" voltage readings on it: They're 12.4 volts when
the car is not running, and 14.5 volts when it is.

Naturally, any voltage below 12 volts when the car is not running (and the
battery is supposedly fully charged) indicates a suspect battery. Also, sites
like http://www.justbajan.com/cars/mainta...01-01/volt.htm suggest about
14.4 volts when the car is running is one indication of a satisfactorily
operating battery.

At the four year mark, I plan to check the battery voltages monthly. I'll be
looking for voltage drops below 12 volts (not running) or under 14 volts
(running).

Also, as others here said, I will be watching for signs of hard starting.

These voltage checks are only a crude gage of how the battery is doing. A
certified mechanic would also probably check the battery with a hydrometer (if
possible; some batteries are sealed) and the battery's ability to deliver
current. But I don't want to pay for this (though places like Autozone might do
it for free), as I think I now have a pretty good feel for when the battery is
dying.

If the voltages are still satisfactory at 4.5 years, I'll probably replace the
battery simply as a precaution.

I note also that any given make of battery will be specified for "cold climate"
or "hot climate." I have always had "cold climate" batteries in my car, as I
have lived in the Northern U.S. until a year ago. Now I'm in a much warmer
climate, so the battery currently in my car is probably not quite right.

Also, heat is harder on batteries than cold.

I likely will buy an Interstate battery next. I understand this is what Honda
puts in its new cars. They're more expensive than what I can get at Sears (by
around $15), but I'm told the Interstate battery will last longer.

tflfb wrote
> Do you waite till your battery fails or replace it when you figure its about
> to.