Mechanically Speaking: Trying to determine what is draining your battery

Automotive technician Ron Nutini helps troubleshoot reasons for a dead battery.

Many a vehicle owner has experienced a dead battery. This is especially true when your vehicle reaches the ripe old age of five years. Yes, five years is about the average life of a vehicle battery.

Maybe you left your lights on for a couple hours.

Maybe you left a door open overnight and the interior lights were on.

Maybe you were just sitting waiting for someone listening to the radio with the vehicle off. In all cases when you turned the key to start your vehicle there might have been some noise and maybe some indicator lights but there just was not enough voltage in the battery to start your car.

This time a jump start gets you going and you are back in business.

Sometimes though there seems to be no reason when you go to start your car and the battery is dead. In many cases if your battery is over five years old it is time for a new one. Just replacing it may not solve your problem though.

Sure, the problem might improve. Or you might think it is fixed.

The next time you leave your car overnight and go to start it  the next day your battery is dead again.

Bad battery? Maybe.

Bad charging system? Maybe.

Bad starting system?  Maybe.

The battery was a good first guess.

Many more guesses and you will be broke. There is an important step that must be taken. Check for a draw. A draw is a circuit that is taking energy from your battery when it is not supposed to.

Like leaving a light on. Looking at your car in a dark garage and you might see the footwell lights that are stuck on. You won’t see the light that is staying on in the trunk because the switch is broken. Draws on your battery are not only lights and therefore not always visible.

Fortunately, testing for a draw on your battery is easy. Unfortunately, finding the culprit circuit is not always very easy.

Any draw around 100 milliamps or more is going to give the owner grief.

Even with a brand new battery your vehicle will likely not start after you leave it at the airport for a week or so with this kind of draw

Finding the culprit circuit is a puzzle that requires a methodical approach.

Your mechanic might first look for the common causes. Trunk light switches, glove box light switches, or a bad alternator diode are at the top of the list.  After the obvious it is time to get serious.

A wiring diagram, in particular the one showing the vehicle’s power distribution will be your mechanic’s friend for a while.  The power leaves the battery passing then to various fuses that feed power to all the items in the vehicle that require it.

Removing fuses one by one while measuring the draw at the battery will hopefully reveal the fuse that is passing the current draw.

The wiring diagrams will reveal all the circuits that get power from that fuse. The process of elimination now moves to breaking the circuits involved.

This is the tricky part because much of the wire, connectors, switches, or electric or electronic devices involved are not easily accessible.

Starting with the most accessible is prudent but after that your mechanic will have to dig deeper.

The computer network of the modern automobile presents added complexity to the diagnostic procedure.

A clear understanding of how the involved circuit works may be required but unfortunately that information is not always available.

Logic and persistence will prevail but it can be extremely time consuming and therefore expensive but very likely less expensive than guessing.

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

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