Batteries, once cabled together to form a single bank, will only charge and discharge equally if care is taken in the initial design and configuration of the bank, and consideration is taken with the maintenance of the bank. When designing the bank it is critical that all intra (series) and inter (parallel) battery cables are large diameter cables. The parallel cables should be as large as the cables leading to the switch, and as close to the same length as practical, and the series cables should be as short and direct as possible without causing strain on the lug crimps, and no more than one gauge smaller than the parallel cabling. The goal is to keep resistances between all terminals on every battery as similar as possible.
All batteries have internal resistances as well, and these resistance values will change over time, and resistance values will vary between battery types, and also between battery manufacturers of the apparently same type of battery. It is for these reasons that batteries intended to function as a single bank should all be of the same type, from the same manufacturer, and purchased and installed at the same time. It is not advised to change a portion of a battery bank out, even with identical batteries from the same manufacturer as the higher internal resistances of the older batteries in the bank may cause unequal charging and discharging when combined to the newly added lower resistance batteries.