Parasitic Drain
This is the current that keeps being drawn from the battery even when the system is off. Standby circuits, clocks and memories make a small consumption normal; however a wrongly connected relay or a board left permanently energised raises it to a level that will flatten the battery within days.
The measurement is made by connecting an ammeter in series between the battery terminal and the line. After connecting, you wait for the system to go into sleep mode; some electronic units draw high current for several minutes and an early reading is misleading.
- Method: no door, cover or switch may be opened during the measurement; if the circuit is broken once, the waiting time starts over.
- Narrowing down: pull the fuses one by one to find which circuit drops the current. The fuse at which the current falls noticeably points to the guilty circuit.
- Choice of instrument: a series ammeter is required for low currents; the resolution of a clamp meter is usually insufficient at this level. Do not crank the engine or run a high-current load while the ammeter is in series; the instrument's fuse will blow.
- Permanent solution: fitting a main isolator switch on vehicles left unused for long periods, or taking the device supply through the ignition.
In caravans, agricultural vehicles and seasonally used machinery, the complaint "the battery keeps going flat" is most often caused by a supply line wired to permanent positive during installation.
Context in which this term is used: Installation, redundancy and field continuity