It's up to you.
Most people wait 'til it fails which usually occurs at inconvenient
times or locations. Sorta like a crap shoot. Often, battery failures
give little or no warning.
At any rate, your time is getting closer...
You failed to tell us the most important detail. If you live in a hot
climate (ie:TEXAS), you're overdue to change it. If you're in a
temperate climate, you're still close to expiration date. Battery shops
will do a load test and show you if it's on it's last legs (free).
An old battery will tend to wear down the alternator more
quickly. From my reading, it is better to replace
pre-emptively rather than wait until it is completely dead.
I use the following clues to guide me:
Many, including myself, can hear when a battery is
struggling to start a car. Pay attention to how long it
takes for the engine to "catch" (via the battery driving the
starting motor turning the crankshaft), and you might detect
when the battery is dying.
Age. Five years or more, strongly consider replacing.
How many jumpstarts? Running a battery down to the point
that it will not start the car decreases its life
A simple multi-meter check with the engine off. Should read
at least 12 volts, and probably closer to 12.5 volts. My now
3.3 year old battery read 12.7 volts when new and read the
same last week.
I think Interstate batteries (which are OEM) are the way to
go, from reading here.
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