Parasitic Loads

Just when you thought you knew everything there is to know about proper maintenance of aircraft batteries, there may be something new you haven’t had to deal with: Parasitic loads.

Parasitic loads on aircraft are not uncommon, but they are getting more attention these days because many aircraft are spending more time on the ground and less time in the air. This operational change means more time for the parasitic load to drain the battery.

The deeper the battery is drained between flights, the greater impact it can have on the aircraft electrical system and on the battery itself. A partially discharged battery can make it harder to start the aircraft engine, especially turbine engines. Repetitive cycling of the battery due to parasitic drain can shorten its service life. In extreme cases, the battery may be completely drained by the parasitic load and the aircraft may have to be grounded until a replacement battery is installed.

What is a parasitic load?

A parasitic load is a small, continuous flow of DC current that takes power from the battery even when the aircraft master switch is turned off. Parasitic loads are present, to a greater or lesser degree, in almost all modern aircraft. Examples of parasitic loads are relays, clocks, radios, avionics equipment, and on-board computers. These loads are generally low amperage (typically under 100 milliamperes), but since they are continuously present they can deplete the battery’s capacity if the aircraft is inactive for an extended time. In some aircraft, the parasitic drain is so high that the battery becomes noticeably depleted within a few days.

The parasitic load slowly drains the battery capacity. In the case of lead acid batteries, this drain causes the plates to become sulfated. Sulfated plates make the battery harder to recharge and can lead to premature failure of the battery. A long-term, low drain rate can deeply discharge the battery, deeper than what can occur with normal aircraft loads. Repeated deep discharges of this nature will shorten the battery life substantially. A battery deeply discharged in this manner may not be recoverable with normal charging methods. If the parasitic drain is high, the battery will become completely discharged in a few days and render the aircraft inoperable.

Can the parasitic load be eliminated?

The items responsible for the parasitic load are generally imbedded in the aircraft electrical system and alteration may affect FAA certification. In some aircraft, modifications can be made to reduce or eliminate the parasitic drain. The aircraft manufacturer should be contacted for more information on this subject. Another option is to disconnect the battery plug from the battery, which stops all parasitic loads. This procedure is often referenced as part of the aircraft maintenance manual or pilot’s operating handbook. However, if not specifically referenced, the aircraft manufacturer should be contacted to verify acceptability of this practice.

How to measure the parasitic load

The parasitic drain of an aircraft battery can be directly measured with a digital multimeter (DMM) equipped with an ammeter function. Most DMM’s have separate jacks for low current (typically 200 mA maximum) and high current (typically 10A maximum) measurements. The ammeter jacks contain a fuse to prevent damaging the internal electronics. It’s always best to start measurements using the high current jack, and then switch to the low current jack if the measured current does not exceed the low current jack’s rating. The low current jack will give a more accurate measurement of typical parasitic currents (generally 1-100 mA).

Concorde has recently developed a Parasitic Load Test Adapter (PLTA) that makes it very easy to measure the parasitic load of aircraft batteries equipped with an MS3509 style quick disconnect receptacle. The PLTA is designed to connect between the battery receptacle and the aircraft’s mating plug, with separate test leads for connection to the DMM. The PLTA is rated for loads up to 10 amperes. Step-by-step instructions for the PLTA can be found in document number 5-0409 available on Concorde’s web site (

Protecting the battery from parasitic loads

To protect the battery from being depleted by the parasitic load, the following procedure is recommended:

Measure the parasitic load as described previously. Determine how long it will take to deplete 10 percent of the battery capacity using the following formula: