Battery life depends on two main factors besides keeping them charged. Number of charge/discharge cycles and how deep of a discharge during a cycle.
Starter batteries in cars are ok to discharge to 80% capacity.
Yellow top style hybrid starting/deep cycle can take 50% discharge.
True deep cycle can be discharged to 20% capacity. (Like boat trolling batteries.)
Discharge them further, and it takes life out of them.
How can you charge a battery dry?? And good luck with the "draining the electrolite" as you could never get it all out.