Actually open circuit voltage on a 12 volt that has been off charge for a hour should be around 12.72 volts 2.12 vdc per cell and on charge fully charged the vdc will be between 14.75-15. vdc good way to check to see if your charger is working properly

Depending on the equipment and if it has a kick out voltage most things will run to 10vdc which is 1.66 vpc Things have changed abit since I was in the battery field with the gel cells and the AGM batteries. But regular flooded cells and motive power batteries (forklift) would discharge to 80% which is 1.37 vdc but that was for dc motors so I dont know what the inverters are design to accept.
When you charge your batteries is when they wear out, if you charge it for a hour is the same as a 8 hr. charge the same amount of materials falls from the plates and settles on the bottom(flooded type) glass mat and gel are a different make up of sulfuric acids and calciums usally a higher specific gravity around 1300-1310 much higher ah capacity and run time but a shorter life span...Sorry been A while since I was talking batteries and I am rambling

A little snow, Please!