I know, a full battery reads 12.8 open circuit but ...

The "fuel" bar indicator in the Nissan Leaf also shows battery life. There have been a few instances where there's been the loss of only one bar. When you don't have to deal with Peukert and temperatures are regulated, you can get pretty accurate on cycle life estimates for Lithium Ion batteries. Probably not possible on lead acid without a built in hydrometer and something to monitor the plate shedding and thickness (and probably some other stuff).