the weather is getting colder and the current Interstate battery starts the car fine. However, I noticed the purchase sticker on the battery was Jan 1999. It would make it close to 5 years old.

It took it to the local Autozone to be tested and it came out "BAD". I was suspicious so I walked away from buying a new one.

How long do these batteries functionally last? What does "BAD" mean? It starts the car fine with one crank.

Should i get a new one or should I save my money for when the current battery gives me the signs of impending death? What do you think?

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$500 1982 240d owner "Gunter"
125k miles,4 speed manual
crank windows, MB Tex, all original
no a/c, daily driver
Scorecard so far....
$230 Maaco paint job
$27 dimmer switch, $40 shutoff valve, $45 brake MC, $260 calipers, $40 brake hoses, $40 side fender trim, $20 balancing, $109 lifetime alignment, $10 fuel return lines, $115 oil&fuel filters for next 30k miles