Let’s look at the case of this pair of “100 Ah” batteries. They were barely three years old I tested them for Ah capacity. They had tested capacity of just 69 & 70 Ah’s respectively as tested at 77F under a 5A constant load. The owner of this bank still had the battery monitor programmed for 200Ah’s, as many boaters often do at three years.
If we add 69 Ah’s + 70 Ah’s we can see that his actual capacity was now just 139 Ah’s not the 200 Ah’s he had assumed it was. Herein lies problem, the owner was still discharging this bank as if it was still a 200 Ah battery bank.