I spoke to the tech representative at Trojan on exactly this question. The data you want is given in the bottom curve at this site: http://www.trojanbattery.com/pdf/T875-AGM_Reliant_DS_Final.pdf Some points are: 20% 2800 cycles 30% 1800 cycles 40% 1300 cycles 50% 900 cycles These are more optimistic numbers than given in Nigel Calder's book (3d and 2d edition. 1st edition has the curve all wrong) Calder's curve gives 20% 800 cycles 30% 500 cycles ​ Personally I never discharge deeper than 20%. unless on anniversaries or birthdays. If you minimize the D of D. your batteries will probably die of old age. I use the boat for 90 days each year. That is 90 cycles per year. According to Trojans so-called data those batteries will last me 31 years! That is not going to happen. With Calder's data I should expect 9-10 years. My Trojan's are presently 7 years old but I have not yet made the capacity test to determine their state of health. If your boat has the normal front loading poorly insulated fridge it will be difficult to limit the depth of discharge. Richard​