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AIRFRAME TECHNOLOGY Parasitic Loads Are they taking a toll on your aircraft battery? J By Dr. Dave Vutetakis ust 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. 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 equip- ment, and on-board computers. These loads are generally low amperage (typi- cally under 100 mil- liamperes), but since they are continuously present they can deplete the battery's capac- ity 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 bat- The parasitic drain of an aircraft bat- tery can be directly measured with a digital multimeter (DMM) equipped with an ammeter function. Photos courtesy of Concorde Battery Corporation. 20 September 2011 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. tery capacity. In the case of lead acid batter- ies, this drain causes the plates to become sulfated. Sulfated plates make the battery harder to recharge and can lead to prema- ture 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 com- pletely 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 electri- cal system and alteration may affect FAA certi- fication. In some aircraft, modifications can be Aircraft Maintenance Technology • www.AMTonline.com • www.AMTSociety.org Concorde Battery Parasitic Load Tester