According to Trojan data, you should expect around 1,200 cycles at 50% DOD, and around 3,000 cycles at 20% DOD.
If the goal is solely maximum battery life expectancy, why not limit depth of discharge to 20% instead?
The difference I see is that one scenario spends more money up front, while the other spends it over time with more frequent battery replacement.
But even differences in total costs shouldn't be the main decision driver because it's not that significant on its own. As a rough example, do the math between 2 batteries that may last 1,200 cycles versus 4 batteries that last 3,000 cycles and the difference can be less than a tank of gasoline for your motorhome. And is it that important if it occurs every 5 years or longer? It wouldn't be to me.
In my opinion battery bank capacity and planned depth of discharge should be selected based on balancing all system needs. There are a lot of factors to consider when designing most anything.
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