Thank you Bill, now I'm tryng to give you more informations.
I think it depends of my boat because the phenomenon has been detected in 2 different marinas.
Measuring in AC the Volts are 3,10V with the Battteriy Charger connected and 0.30V with the Battery C.. off, In DC the volts are 2.3V with B. C. connected and 0.8V disconnected.
I think the guilty is the Battery .C. because, if I exclude it, the phenomenon ends.
The Ground isolator is connected between shore power and general input switch of AC in the boat.
It's mandatory to connect the ground nipple of the carcass of the Battery Charger to the groung system of the boat? The engeneer who installled the new Batteruy Charger didn't make that connection, must I do it?
Thanks again for your advices. Good winds. Enio