In 2001 we bought a '95 855 - it needed a new battery almost immediately.

We put a Volvo battery in that lasted more than 11 years (car has been sold) that included winters in Massachusetts, one at Lake Placid (Cooold) and 5 in northern Minnesota (very cooooold)

The first year in Minnesota the temperature never went above freezing from Thanksgiving until the end of March and the car was kept outside - it was garaged the last three years my daughter owned it.

Interestingly all three of our 850s needed new batteries before they were 8 years old - all got Volvo batteries and all made it to 17 years old with those batteries.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa

I got 8 years out of an Interstate in my 90 244. From what little I know Interstate is the best aftermarket battery if you end up replacing your OEM.

FWIW - I have a Volvo OEM battery in my 95 855. Bought the car 3 years ago and have no idea how old the battery is. Tested strong about a year ago. Hopefully I can get somewhere close to 18 years...:)

I got 10 years and just under 130,000 miles from the factory battery in my '86 240. It died because I left the dome light on for a couple of days. Completely discharged the battery. If that had not happened, who knows.

I've bought a handful of replacement batteries in the years since. They seem to last about 2 1/2 - 3 years. These are "aftermarket" batteries.

If your charging system is in good order, and you never (or rarely) completely discharge the battery, I suppose that 2 decades is possible. On the other hand, the things can only withstand so many charge cycles.