Standby Current (Parasitic Draw)
The sum of the small currents that stay in the circuit even when all devices appear to be off. It comes from sources such as the alarm, the gas detector, control panels and the standby circuit of the inverter. Even a total of a few tens of milliamps can seriously flatten the service battery within weeks.
The arithmetic is straightforward: a continuous 50 mA is 1.2 Ah per day, roughly 36 Ah per month. Since the usable share of a 100 Ah lead-acid battery is around 50 Ah, the battery reaches a critical level within six weeks if the vehicle is left disconnected from shore power. This is usually the mechanism that kills batteries in winter storage.
To measure it, disconnect the negative battery terminal and connect an ammeter in series; the value read with all loads off is the standby current. If it comes out higher than expected, pull the fuses one by one to find which circuit the source is on.
In an RV the most practical measure is to fit a main isolator between the battery and the distribution panel. If the vehicle will not be used for a long period the switch is opened, so the standby current goes to zero and only the battery's self-discharge remains. If continuously supplied control units such as the levelling system are also left behind this switch, they create no extra consumption during the storage period.
Context in which this term is used: RV automatic levelling guide