Does my battery have a parasite?

Do you park your vehicle for a few days; maybe a week or so and when you go to use it again the battery is dead? Let me guess, your battery is new? What is wrong?

Unfortunately that week of nonuse is typically when you park your vehicle at the airport and you finish your vacation with a call to a towing company for a jumpstart. While they are there they try to sell you another new battery. Good for Teck, I suppose.

The problem is likely simple. The solution maybe not so simple. Parasitic draw is likely the cause of your woes. There is something using power from your battery while your vehicle is not being used. Yes, you could be leaving your lights on but that would be obvious and it only would take a few hours to kill your battery.

Maybe the power parasite is a reading lamp left on by a passenger. Maybe a door left one notch open. Maybe you left an electrical accessory like a GPS, satellite radio or some type of charging device plugged in.

These customer caused problems are less likely on modern automobiles. Many automobile’s electrical systems go out of their way to protect you from killing your battery. Many or all of the power outlets in your vehicle are turned off after a specific amount of time. All courtesy lights only stay on for a set amount of time doors open or not.

Finding out whether your vehicle will protect itself from you leaving a door open can be a daunting task. Checking the owner’s manual is akin to reading the bible less the spiritual rewards.

Accessories that were added to your vehicle after the fact are likely parasites. Remote starting systems, fancy or replacement stereo systems with extra amplifiers, and trailer wiring are some of the possibilities. The installers of these items really need to know what they are up against with modern vehicles.

Your mechanic likely deals with this type of problem a lot. Solving these type of problems can be very challenging and incredibly time consuming especially on late model electronically complicated vehicles.

An ammeter we connected in series at your battery tells us how much current is flowing from the battery. Some vehicles even have a built in sensor and computer to monitor this value. Ultimately for most vehicles the allowable limit is fifty milliamperes. Anything above this causes trouble. One hundred milliamperes will give your one week dead battery at the airport scenario.

So we always start at the battery. Unfortunately modern automobiles take time to reach a state of minimum parasitic draw. The network of computers used to run your vehicle do not all shut down as soon as you pull your key out of the ignition. In the case of smart keys and push button start vehicles it is even more involved.

Complete shutdown can take up to an hour. Until that happens we cannot attempt diagnosis. Computers that do not go to sleep are often causes of excess parasitic draw. Finding the culprit computer is very difficult especially if the problem is not always there. Patience and logical process of elimination has to be part of our repertoire.

Trail’s Ron Nutini is a licensed automotive technician and graduate of mechanical engineering from UBC. E-mail:

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