Something is not adding up. I run an 880Ah leisure battery bank on a barge through a new Victron Multiplus 3000/120, and 400w of solar through a Victron MPPT Controller 100/15. Putting amps back in using either engine or genny, when i reach a full state of charge i watch as the voltage drops from 12.60 down to 12.20v where the voltage stays but only if the inverter and every appliance it powers is switched off. With inverter on, the voltage can drop to 11.95v at 99% full. In short, newish batteries are discharging fast, according to the BMV700 monitor, without any significant load on them (other than a few 1.8w downlights) .
Something is wrong. Before i infer that the batteries are shot to bits after only 6 months, is there something i can do to maintain a healthier voltage, not that the batteries are working hard at all?