Caravan Batteries (talking AGM here) change their parameters with age and cycling. All being equal, you get optimum performance with matched cells.
Say you have two batteries in series. If their capacity is different (unmatched due to age, cycling or even different brand or models) and you want to avoid discharging below 50% (for long life), you will be limited to the capacity of the weakest battery in the bank. So if placing a “new” and “old” battery in series, the old battery will limit the capacity. Another issue is that old batteries have higher “self discharge” than new. When you have two batteries in series, the older battery will self discharge faster and become “out of balance” with the new battery.
If you have a “mix” of unmatched cells/batteries in your bank, you will have to keep a close eye on them as you will likely experience a failure of one of your battery before the other/s. If the failing battery isn’t diagnosed early you risk severely degrading the whole system.
