If the battery is 6 years old it is on borrowed time... definitely replace it. But to elaborate on the charging method mentioned... When a battery is fully charged the sulfur is suspended in the fluid, as it becomes discharged the sulfur soaks into the lead plates. When a battery becomes fully discharged some of the sulfur gets "stuck" in the plates (called sulfation) which translates to a lower total charge. Most "smart" battery chargers nowadays have a desulfation stage built into them where it puts extra high voltage (like 16+ volts at 40+ amps) to force the sulfur back out of the plates into the acid.

When I sold my E36 just over 2 years ago it had a new battery, but then it sat for 2 years after my friend wrecked it. He gave it back and I was able to revive that battery by doing this a couple times. (fully charged with desulfation cycle, load tested the crap out of it till it was low, then charged with desulfation cycle again) The battery has been ok since I did that about a month ago and I only started/ran the car about one a week max since.

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