Some years back my company did work for a battery manufacturer, we were able to get some insite on 12 volt batteries. What I have been told by these battery folks is a typical starting battery should never drop below 12 volts or permanent damage ( not able to fully recover) will be done. Under load these batteries can drop below 11 volts temporarily but should return to 12 volts plus when the heavy draw is removed...a deep cycle battery can be drawn down to 11.5 volts without permanent damage but should not be used where a heavy draw would rapidly draw down the charge as that would permanetly damage the battery. In my many years of boating there have been ocassions where I abused my batteries and those statements proved to be true!!! On the batteries that I have monitored ( in other words I took care of them) they have all lasted at least 4 years or longer, in a couple rare cases I had 7 years of good service !! Yes there were a couple times a battery failed before its life should have ended, but typically the batteries gave great service when cared for properly and not deep discharged, this is the #1 cause of battery failure........
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