It's my opinion that the 80% point is used not because the batteries can't or shouldn't go to 100%, but to minimize the chances of bad customer perceptions if batteries go south. Not for DoD reasons but the amplified effects of instrumentation accuracy when "riding the line" at the very edge of performance. You can have a 10% error at 80% DoD and have little effect, but a 10% error at 100% takes you over the cliff. Even I add a little bit of margin at the bottom end to account for my ability to accurately determine 100%, because once you cross that line I do believe there can be an increased impact on service life. I have a discharge chart for Trojan batteries that shows the 100% point. I can't attribute the source of this specifically, I have a *lot* of files in my battery folder after 30 years. My contacts at Trojan and US Battery have since moved on or retired, so I've lost my connections there for any "inside information". Battery Ah are spec'd to the 100% DoD point, not 80% so the spec itself is based on 100% DoD. What's "recommended" is likely just as much a marketing limit as an engineering one.
Here's a delivered Ah chart for T105's to 100% DoD. Not included is the reduction in Ah curve near end of life (because Trojan wouldn't give me that data) but that doesn't affect the response curve, only the absolute Ah number.
From here it's an academic exercise to plug in the cycle life and Ah data from any battery spec sheet and see the net result. For the sample above the 50% Rule optimum point is actually 30%, and this moves around a bit depending on the battery. It's such a minimal gain there is limited return for bothering to pursue it, which is another fallacy of the 50% Rule of pursuing "optimum". A point can be made that these numbers are under controlled test conditions and that in the field "your mileage may vary" due to the myriad of variables that influence cycles and capacity. It's impossible to quantify these so you just go with the lab data for at least a means of comparison. There is no indication of performance or life degradation anywhere between 20% to 100% DoD. There very well may be a particular battery that does take a turn at a particular DoD point. But that's not all batteries and I would offer even most of them, so the 50% Rule falls apart right there. At best for the sample above it would be a "30% suggestion". There can be a
use strategy of picking a specific DoD point. RV folks often use a DoD point as a power reserve threshold, so that they know their fridge or CPAP machine won't give up when they need it, and that's perfectly fine. Another is a well defined and controlled application like off grid solar. There you literally are cycling your battery every single day and you are optimizing the number of cycles to a given number of Ah, to maximize $ per Ah for a given bank size. That is a very narrow application requiring a very deliberate and well controlled set of operating conditions that is only possible in fixed installations- nowhere near what an RV battery will see in real life. So stick a fork in it, in RV service the battery is a consumable like tires and oil. You buy them to run your stuff and any notion of "maximizing" battery life through DoD is rearranging deck chairs on the titanic and amounts to a distraction. The focus should be on having enough Ah to carry you through the duration you need, then proper charging and maintenance. Do these and they'll serve you reliably for their practical service life. If people paid as much attention to maintaining their batteries as they do anguishing over tire pressures and oil changes, I doubt much of this battery minutia would ever be discussed.
Mark B.
Albuquerque, NM