Hello- I have a 1998 Mercedes e320. The battery dies if the car isn't driven for 3+days. The battery is newer. Has anybody had this problem?

Exactly how old is your "newer" battery?

I replaced the battery less than a year ago because of the same problem.

People have had similar problems, but I don't recall reading about a specific W210 with this issue.

One of your consumers is taking power all of the time and probably needs to be replaced. You can connect an amp meter to the battery and monitor the parasitic loss, then start pulling fuses. Once you find a fuse that reduces the loss, find what modules it powers.

What resolved the issue a year ago?

You may have a battery drain somewhere, pull one fuse at a time until the drain goes away.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Search on "battery drain". You will find the top 100 ideas on what causes it.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

And don't forget to pull the seat to check the battery cables are on tight.

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Prost!