[Richard230]

Here is a little chart that I was sent by Batterystuff.com which shows the relationship between measured voltage and the remaining level of charge of a typical lead-acid battery:

State of Charge    Voltage
100%                 12.7 - 13.2
75%                 12.4
50%                 12.1
25%                 11.7
Discharged         0 - 11.6

[axman88]

That's quite a wide range of voltages for the "fully charged" state, which is why, I think some battery maintainers don't perform as intended in some cases.  I understand that AGM and wet batteries have voltage vs state of charge profiles that are different, and that different designs of battery can vary, even when based on the same technology.

Also characteristic voltage can change over the life of the battery.

I learned a lot from reading at Battery University, a non-profit educational site created by a guy who founded a company that makes high tech battery testers and chargers.    https://batteryuniversity.com/articles