I'm not sure what you mean by "the batter." However, in having done this fiberglass repair to my W123s in the past, I found that removing the battery and battery tray (or what is left of it) exposes the "culprit," which is a rusty area (usually with the beginnings or more of a hole) toward the corner behind the battery on the firewall. If you lay some fiberglass over this area with sufficient overlap to sound areas you should find your leak fixed (you may want to spray paint to exterior color before replacing battery). I don't believe that you need to involve the drainage areas with the fiberglass. Someone recently posted pictures of their repaint on a W123 on this site and the "before" picture of the area shows the "culprit" quite well.

You also may want to try a SpiralCell battery as seen in many catalogs for your next battery as they are advertised as leak proof/non-acid problem batteries. However, I have not yet tried one and would be interested if anyone else on this site has had a good experience with one.

Best of luck. Please reply if you need any more info.

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