Parasitic Draw Savvy Bolsters Business by Paul Rupp Have you ever had to jump-start your car after letting it sit for the week- end, not once, but time and time again? And the frustration builds because you know your battery and alternator are in good shape. It can happen at the most inconvenient times, like in the airport long-term parking lot after returning from a trip. If this has happened, you may be the victim of parasitic draw. In this article I will explain parasitic draw, list some common causes, describe the methods and the common tools we use at our shop, tell a few case stories and summarize with how to market this type of work. You will be surprised how easy it can be when you break it down into steps. Parasitic Draw Defined Parasite as defined by ‘“Merriam- Webster” is the dependence on something else for existence or support without making a useful or adequate return. A very selfish existence indeed! Parasitic draw as it relates to a vehicle’s electri- cal system is an unusual or unexpected amount of electricity leaving the stor- age of a battery at an unexpected time which can occur constantly or intermit- tently. The draw is dependent on the battery to sustain life until there is no more life to give. The time it takes to experience a dead battery is dependent on the rate at which parasitic draw occurs. Other factors include the condition of a battery and the amount of the draw. Parasitic draw is measured in amperes (amp). Ohm’s Law reads V=IR, or volts equals amps times resistance. Rearranging the equation as I=V/R, reveals one amp of current equals one volt of electricity applied across one ohm of resistance. One amp also equals 1000 milliamps (ma). The draw (or load) is the resistance. Most vehicles have a normal key off rate of 40 to 50 ma draw. An excessive amount of draw would be anything more than 50 ma. On modern vehicles it’s not unusual to see up to a 300 to 500 ma draw for five to 10 minutes just after keying off the vehicle. This is due to various control modules powering down and going to sleep until the vehicle is started again. Let’s see how long it takes to run down your battery if you leave your headlights on. A car battery is rated in Capacity (amp x hour) among other things. A typical battery, rated at 60 amp x hours would supply 60 amps for one hour, or six amps for 10 hours. Say your headlights draw 12 amps when left on, then 60 amp x hour divided by 12 amps equals five hours. Your battery would be dead after five hours. In another example, if the parasitic draw were 250 ma, it would take 10 days to kill a battery. Surprisingly, many vehicles actually do have excessive draw, but they are driven often enough to allow the alternator to recharge the system. Occasionally we will acquire a new customer who states they have had to replace their battery and/or alternator every year or so. In these cases we will find excessive draw, only the vehicle is started and used often enough, the bat- tery never drops far enough down to need a jumpstart. The alternator always has to play catch-up each time the vehi- cle is started, which can shorten its life from overworking it. Sources of Draw Most vehicles are designed well and do not just “get” parasitic draw. Usually we find something and/or someone has inadvertently caused it. Some of the most common sources of parasitic draw are aftermarket alarm systems and stereos along with various control mod- ules which have been damaged. The first question I have for cus- tomers in suspect cases is “Did you recently have any aftermarket acces- sories added to your vehicle?” Most answers are “yes.” Power wiring that shorts to ground is another common source. When you take a piece of wire in your hand and bend it back and forth enough times it will finally break. On vehicle wiring, first the insulation breaks through and exposes the bare wire to potential grounds. Common locations for these types of faults are whereever something moves, like door jambs, truck lids, tilt wheels, shifters and seats. Broken motor mounts allow an engine to move back and forth as well. We find the lesser quality vehi- cles have more sharp comers for the wiring to get around. Methods and Tools The logic in finding the source of a draw is to think about the entire system. The battery is the starting point and the device which needs electricity to oper- ate is the end. There are fuses in between designed to protect circuits, and relays to protect the devices. Somewhere in the middle of all this is the problem. The initial step is to ensure the vehi- cle has a proper battery and alternator with a starting and charging system test. A battery needs to be able to maintain 12.6 volts after a charge and not drop approximately 9.4 volts when applying a load of half the CCA for 15 seconds. The alternator needs to be able to supply its rated amperage output when load is applied and the diodes must be in good shape. Next we simply test for draw with all the electrical accessorie