Well, there's no way to know for sure unless you test them out. If I were you I would take the batteries to a parts store that will test them for free to see what kind of life they might have left in them (whether or not they can be recharged). Being as old as they are sounds like they will probably be shot. If you want you an also pull the alternator and have them bench test it while you are there - that way you will know the status of your alternator and it won't take running a new battery dead to figure it out. Aside from a bad battery or bad alternator, the other alternative is that you have some parisitic drain somewhere that you will have to identify - but don't worry about that until you have determined the status of the battery and alternator.

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman