Thanks Troy. I decided to try Epson salt, because it is also freely available locally.
Unfortunately I did not have any luck so far. I tried to rescue three batteries, and after I have added Epson salt yesterday, I drained the batteries today, and refilled all with new battery acid today.
The voltage seem to be slightly more on two of the batteries, while the voltage on the third battery is much better - however, none of the three batteries seem to hold current - after a while the voltage start dropping again.
I must admit -the first two batteries were lying around for a couple of years, and I did not expect a miricle.....
Regards
Danie