RP 140B - Understanding Key-Off Parasitic Loads: Background: Notes

A key-off parasitic electrical load is defined as any current which is drawn from the vehicle battery or battery pack by an electrical or electronic device while the engine and the ignition switch are off. The electrical or electronic device may be actively on or may even draw power when not active or switched off. Some devices are on timers that will stay on for a specific amount of time after the ignition key is turned off. This is to enable various systems to complete their cycles, purge liquid from a line, etc.

Key off parasitic loads can damage or degrade the performance of a vehicle's electrical system. The potential degradation of the electrical system will be influenced by both the length of time the vehicle is off (not charging) and the amount of parasitic load discharging the battery or batteries. Essentially, the more parasitic current being drawn from the batteries - and the longer the batteries are discharged without recharging - the less current will be available for starting and other functions requiring battery power.

Excessive parasitic load over an extended period results in discharged batteries. Repetitive discharging results in:

• reduced cranking power.
• cycling of the batteries.
• reduced overall battery life.
• increased susceptibility to freezing or damage in cold weather environments.
• a negative impact on a vehicle's electrical system when operating at reduced system voltage.

The following are two examples of typical parasitic loads and their impact on battery state of charge.