negative earth spark & loss of charge 508d

Hello, currently putting a 1982 508d back on the road and having a few electrical problems.
Everytime i start the van i get a spark on one of the 2 12v 95ah
batteries. This has previously caused one of the negative terminals to corrode and reduce in size.......
The van is 24v start and 12v for everything else. Once it is running the batteries are both charging fine at 14v. In the past i have repalced the charge regulator / switchover and all has been fine, but has just started losing power and sparking........ any ideas gratefully received...
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