12V Sealed lead acid battery level sensing
The "dead" or "end" voltage you are looking for is 11V (1.83V/cell) at room temperature.  

Below are typical discharge curves for a 20Ah 12V battery - note how the 1A curve ends just below 11V.  The higher current curves go below 11V because they consider the additional voltage drop due to the internal resistance of the battery.



There are two related voltages worth knowing: "bulk" and "trickle" - this article is a brief but excellent introduction. Your manufacturer's data sheets generally provide the most pertinent information for your specific battery.