So, you have an 1983 Mercedes 300D that you are someone installed a Glow Plug Relay with the Afterglow feature?

My favorite parts place shows a 65 Amp Alternator for your year and model.

The tech data book has an 88 amp hour Battery for Diesels. 90 amp hour for Canada, Norway and Finland.

What does the voltage drop to when the Glow Plugs are applied and the Engine is not cranking or running?
Next, what is the Voltage when Cranking (the Glow Plugs are also on during cranking if the wiring is stock)?
In general (on a regular Car Battery) the further down the Voltage drops when under a load the shorter the Battery life is.

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84 300D, 82 Volvo 244Gl Diesel