Interesting "reminder comments" above on lead acid battery internal DC resistance!
DC resistance can be measured using two different applied voltages along with recording the resulting two different currents with a given load. Internal battery resistance can then be calculated by dividing the difference in the two voltages divided by the difference in the two currents. (Even I Remember That -> resistance equals voltage divided by current ... but DC resistance also equals delta-voltage divided by delta-current.) Boy, I hope I remembered all of that right, since I long-ago sold my textbooks.
Now ... what got my thoughts going was ... does lead acid battery internal resistance change (increase?) over time as the battery ages?
So what I think you were hinting at is: If RV battery internal resistance could somehow be measured periodically and conveniently (with some installed permanent RV system) while the battery is left in-place in the RV .... could an amp hour meter designer then cleverly use this periodic data input to have the meter also show percentage of ORIGINAL CAPACITY left that the battery was capable of when new? If so ... wa-la ... you would have a battery monitoring system that not only shows when to recharge, but also shows when to replace!
P.S. I'm not a hybrid vehicle owner, but I assume that their design includes some monitoring system with a dash light indicator that lets the owner know when their internal batteries need replacing.
