Charging multiple glider batteries

Our club (US) keeps 8 batteries charged in two banks of 4 batteries. We
use two Battery Tender Junior chargers, one for each bank. The charging
rate is not very fast, but they have at least 20 hours between use and
often a week. Mostly this has worked pretty well, but we are more often
finding batteries that are not fully charged, even though the battery
tender is green indicating full charge. Once fully charged, the charger
just floats them. The batteries are the popular glider batteries, 12
volt 7 amp hour sealed lead acid.

How do other clubs, commercial ops keep multiple batteries charged up?

I realize charging problems can occur when one battery is damaged, won't
take a full charge or is simply dead. Or if the charging rate varies
markedly from batt to batt. It may be possible to overcharge the other
ones. Ideally each battery would be charged individually, but that is
expensive and takes more space. What solutions are out there?