RoyB - Good posts.
Before I had a battery monitor I used voltage under load to determine when to charge batteries. Maybe this doesn't sound right to all of us "experts" but it to was also real world experience.
How? By paying attention one can determine with experience a reasonably idea of the amount of discharge. I initially made several no load voltage and hydrometer checks in the morning after a typical evening for example and from there it was seat of the pants knowledge about battery state.
OP You can do something similar.
