Automotive electricity is often the site of breakdowns, which are not always easy to spot. It may very well happen that an element short-circuited and constantly pulls on the battery, which results in a rapid discharge of the battery. You have to find the source of this leakage current. The research is carried out with a digital multimeter connected to the terminal and the negative cable of the battery. Next, you have to go through all the circuits to find the one on which there is a problem and finally, find on this line the short-circuited element. It's not very complicated, but a bit long.
Part 1 of 2: spot a leakage current with a multimeter
Step 1. Turn off anything that uses electricity
Turn off the car radio, turn off the heating or air conditioning, turn off the headlights, turn signals, windshield wipers … even the glove box must be securely closed! Apply the handbrake, stop the engine and remove the ignition key. Make sure that the doors (passenger compartment and trunk) are properly closed so that no lamp (or warning light) is lit because of them.
Likewise, unplug any external power cable, such as the one for the GPS or cell phone
as you are going to disconnect the battery, when you replace it and if your vehicle is equipped with it, you will have to enter the security code which protects you against theft: the code is written on a sticker in the car or in the manufacturer's booklet.
Step 2. Fully charge the battery
Since there is a suspicion of a current leak, you should start your investigation by fully charging your battery. Open the hood and locate the battery. For recharging you need a charger.
- At the end of charging, check with a multimeter that you have a voltage at the terminals of 12 to 12.5 V, this will be proof that the battery is well charged.
- If your battery has been used a lot, it may not be holding a charge. In this case, if you want to find the origin of the leak, you will first need to replace it with a new one.
Step 3. Undo the black cable from the negative battery post
This cable is easy to spot, it is all black, sometimes with a black cabochon on the “-” terminal of the battery. Move the protective cap, if it exists, and using a 10 mm wrench, loosen the nut on the cable lug. Remove the cable from the terminal.
- Leakage current is detected on the negative terminal. The positive terminal is easy to avoid: it is red. And then you avoid a possible short circuit!
- If the terminal does not come off, insert the tip of a screwdriver between the two parts of the terminal to push them apart, the terminal should come off easily.
Step 4. Set your digital multimeter
Every multimeter comes with a red cable and a black cable that you plug into different terminal blocks. Insert the black cable into the terminal block COM, as for red, it will be plugged into the 20 A terminal. Then set the button on the central dial to the same intensity.
Choose a digital multimeter with current values ranging from 200mA to 20A
Step 5. Position the keys of the multimeter correctly
The tip of the multimeter's red wire should be in contact with the terminal you just removed, and the tip of the black wire touches the negative terminal, the one just released. Contacts must be frank.
If you have them, you can insert the metal tips into alligator clips, leaving your hands free to work
Step 6. Be aware that an amperage greater than 50 mA indicates a leakage current
In fact, you will never have a reading of zero, as some devices run continuously, like the board clock. It is therefore normal to have a current measurement between 20 and 50 mA. On the other hand, any reading above 50 mA indicates a loss of current, something is constantly pulling on the battery, when it should not be.
Part 2 of 2: Checking a Car's Fuses
Step 1. Check all fuses one by one
Under the hood, locate the fuse box. Remove them by hand or with an extractor, going from the lowest intensity to the highest. You've taken the multimeter reading, look what happened after the fuse was removed. If the measurement has not changed, replace the fuse and go to the next one in order.
- If you have not noticed anything on this fuse box, go to check the one that is under the dashboard, usually on the driver's side. For these tests, it is better to be two, one in the cabin removing the fuses, the other nose in the hood checking the multimeter. If you are alone, try mounting the multimeter against the windshield, so that you can read the measurement without having to go out all the time.
before testing the passenger compartment fuses, unplug your multimeter, wedge the internal lighting contact button with a piece of wood, which is located on the front part of the door contact area. Reconnect the multimeter.
Step 2. Stop when the intensity drops significantly
Removing a fuse only causes a very slight drop in current, in the order of a few milliamps, which is normal. On the other hand, if the drop is spectacular, you go from 3 A to 0.03 A, you can be sure to hold the circuit on which there is a problem!
Step 3. Find out what components are mounted on this circuit
Several elements are mounted on each of the electrical circuits. Now you just have to find out which one of them is causing the problem. To do this, take a look in the booklet provided by the manufacturer or on the Internet.
Some fuse boxes have pictograms on their lids which symbolize these elements
Step 4. Check each component of this circuit
This is the most complex part, because you have to undo the connector (especially find and access it!) Of each of the components of the circuit, until you find the one that is faulty. Of course, the operation is a bit long, because you have to check the behavior of the multimeter each time, but with rigor, it is doable, especially if there are few elements on the line.
Suppose you have located the leakage current on the line that feeds the antenna and the car radio. Unplug the car stereo and see what the multimeter displays. If there is nothing wrong, then the antenna is the source of the problem
Step 5. Repair or replace the failed component or device
Of course, if you don't know anything about mechanics, leave this job to a professional. Check that everything is working fine after reconnecting the battery correctly. If you are the one doing the repair, after checking that it is working properly and before bending everything, check it again with the multimeter and if all is well (<50 mA), reconnect the loose cable and you will be done.
This is the simplest case, but it can also happen that it is not a faulty component or device. These removed, you will have to conclude that it is a cable problem and there the situation becomes very complicated, because you have to check them one by one
- This 50 mA bar is very relevant. Any measurement greater than this value, while everything is off in the vehicle, always indicates a loss of current.
- With this kind of problem, we always think of a burnt component, but we often forget that a small external element (a small piece of metal, for example) could have fallen on a contact of a device or a component, thus creating a short circuit… and a discharge of the battery. This happens in all the hollow parts in a car: cigarette lighter, jack plug …
- Even if the current is not very high, it is best to take all precautions, especially putting on gloves and protective goggles.
- To avoid a general short circuit, it is better to undo the negative terminal, so you will work in safety.