Multiple battery banks not balancing evenly

Battery banks that don't stay balanced (one bank consistently reads lower voltage than another connected bank) points to either a wiring imbalance (unequal cable lengths/resistance between banks) or one bank having a weaker cell dragging the average down. True parallel battery banks should read identical voltage at rest — if they don't after settling for a few hours, that's worth investigating rather than ignoring. How much voltage difference are you seeing, and how long have the banks been in service?