Battery lifetime/cost vs. state of charge

[Pizzazz]

It is often said on this forum that it is advisable to keep wet cell batteries above 50% state of charge to ensure maximum life. I have been analyzing my battery setup and came to the conclusion that it is cheaper to abuse the batteries than to follow the advice above.

The logic is that if we discharge the batteries to 20%, they will last half as long (650 cycles vs. 1200 cycles) but will cost half as much for the same capacity (from 80% to 20% SOC vs. from 80% to 50%; I think we all agree that charging above 80% is costly, inefficient and not practical at sea).

So why do people feel so religious about the 50% SOC cut-off? This logic also defies the case for lithium batteries. I can see many benefits to "abusing' the batteries down to 20%: you carry half the weight, bulk charging from 20% to 80% is nearly 100% efficient, i.e. the batteries take in all the current from the alternator and so on. The downside is that the batteries need to be replaced every two years vs. four years (heavy), probably more maintenance (checking the water level, etc.) and slightly higher risk that one of the cells will short circuit in the most inopportune moment (we have the starter battery for that). Any thoughts?

Regards,
SV Pizzazz