[The Newell]

Even with a battery disconnect switch, batteries can self discharge over time. Obviously a disconnect switch at the battery removes all loads. "Master" switches may not disconnect all loads and leave such things as radio memory, clocks, gas detectors, etc. powered. These loads will hasten the discharge of the battery. A good quality battery maintainer that won't overcharge your batteries is a wise investment if you don't already have an inverter/charger with a "float" stage to maintain the batteries.