Looking at the total amp-hour yield over the cycle life of the battery provides some insight. The 24V Telecel brand LiFePo4 batteries I use have this cycle life rating. For simplicity, the calculations for ah over lifetime below are for a 1 amp-hour rated battery. These calculations assume the capacity remains constant over the life of the battery though it would be slowly declining over that time. End of life is defined by most manufacturers as the point when the battery loses half of its rated capacity. If I get a chance, I redo the calculation on a spreadsheet to account for the diminishing capacity.
80% depth-of-discharge, 2000 cycles (1600 ah over lifetime)
70% depth-of-discharge, 3000 cycles (2100 ah over lifetime)
25% depth-of-discharge, 8000 cycles (2000 ah over lifetime)
There is minimal (5%) difference in ah yield over the lifetime of the battery for 25-75% depth-of-discharge, but the yield drops by 25% when the depth-of-discharge is extended to 80% and likely drops even further with deeper discharge. It would be interesting to see cycle life data from other LiFePo4 batteries for comparison. In a nutshell, this data indicates discharge depth between 25% and 75% yield roughly the same cycle life while extended discharge depth to 80% reduces life by 25%.
