I replaced the battery 4 years ago on my 1989 300SE with a new OEM one. I drive the car on weekends, and usually for long distances, and have not had a problem starting the car until this past weekend. The first start on Saturday was weak and it got progressively worse to the point of being a non-starter by Sunday morning (thankfully in the garage). Can a battery give-in in 4 years? When the car was running power appeared good...lights ok, radio, etc. Can the alternator be causing a gradual drain of the battery? How can I test the battery? Any advice greatly appreciated.

Thanks.

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2004 E320 Wagon
1989 300SE
1998 Range Rover 4.0 SE