I add EDT+ to the water which is a cleaning agent that keeps the lead plates clean.
I read about it in MBM 1998.
I put some in to the 3 old units when I bought my boat 6 years ago, I dont know how old they were then but at best only gave 12 v.
Within days they were giving 13.5 v and they are still going strong, they have to be at least 8 years old now.
EDT+ works all the time and was not very dear £6 per battery I think.
If you can't find your old copy of mbm send me a pm and I will look out details tonight.