Identifix: Let’s talk about the infamous battery drain

By Brian Franta

Let’s talk about the infamous battery drain. Every shop has seen it and worked on it, but not every shop is as successful as it should be. The days of connecting the test light between the battery cable and battery post while pulling fuses are gone.

Now you have to worry about Controller Area Network (CAN) buses.

In this article, we will reference several TSBs, be sure to grab them here. There will also be a link at the end of the article.

CAN Buses 

CAN buses are data lines used to connect every computer and component in the car together. Take a dome light for an example: it’s now turned on by a computer. That computer is connected to a network that is connected to another computer set up to watch for a switch input. If any computer connected to the network decides to send information, you get a battery drain. Is the drain caused by the computer connected to the light, switch, or some other computer on the network? A little too complex for a dome light if you ask me.

Finding the Battery Drain 

Today’s complex cars have different strategies that can either help or hinder the process of tracking down a battery drain. Computer-controlled battery disconnect relays designed to disconnect fuse boxes based on inputs from battery monitoring sensors and computers can potentially hide a drain. How do you find a drain on a car that has a hidden drain? You need the right tools to dig into a battery drain with confidence. Some of those tools you may already have—others may be far from sensible to purchase. In either case, a test light is not one of them.

Some Tools You’ll Need    

Some of the tools that will help in diagnosis start with a quality Digital Volt Ohm Meter (DVOM) that reads millivolts and milliamps. Be aware that in some cases it saves time to have two. At the higher end, when there is evidence of a difficult diagnosis, you may need a compatible scan tool and oscilloscope. By far the most important tool will be access to an information system like Direct-Hit that has service bulletinswiring diagramscomponent locations, and repair information on your vehicle.

Diagnosing the Drain 

To start your diagnosis, you first need to talk to the customer and get some history: important notes like how often the battery goes dead and when. There can be special circumstances that may lead your diagnosis to a quicker repair; for instance, the battery drain only occurs after removing groceries from the trunk. That may direct you to a sticking trunk release switch or trunk ajar switch. Another step you can take to help speed up your diagnosis is to use your scan tool to perform a complete scan of the car. Look for the smoking gun such as fault codes that state switches are stuck on or that there is a faulty alarm siren.

Once you have your information, be sure to have a known good battery that maintains 12.6 volts installed. If it’s low on charge, it may complicate your diagnosis. Swap the customer’s battery with a fully charged battery if needed. Next you’ll need to set up the car for testing. This would include gaining access to all the fuse boxes before you start testing. Open all doors, hood, and trunk while closing all respective latches or manipulate ajar switches to simulate the vehicle is closed. Lock the car and wait. Wait times can differ from car to car but use the general rule of at least 45 minutes.

Manufacturer Test Procedures 

Most manufacturers’ test procedures will indicate different specifications for what they consider is an acceptable drain. We like to keep readings under .050 amps or 50 milliamps. When connecting your ammeter, we would recommend using a quality ammeter clamp that correctly reads milliamps. Without a clamp, you need to be careful not to break the circuit when installing your meter in series. This means connect your ammeter positive lead to the negative battery cable and connect the ammeter negative lead to the negative battery post. Then disconnect the battery cable from the battery. Failure to do this will blow the fuse in your meter once the car’s alarm goes off and you’ll have to wait another 45 minutes.

Choosing the Right Tools 

Now that you’ve identified a battery drain exists, it’s time to get your tools in order. On your toolbox you have Direct-Hit set up with fuse locations and wiring diagrams displayed. Next to that is your second DVOM ready to do testing. You’ve got your tools in hand, but now what? The actual amount of drain can lead you to some clues as to which direction to take. A large drain, say over 2 amps, can be caused by a load carrying component such as a light, alternator, or blower motor running. A small drain, less than 2 amps, can be caused by a CAN bus drain due to computers failing to enter sleep mode.

Using Service Bulletins 

A lot of manufacturers have published bulletins such as this one from Mercedes: S-B-54.10/162. It describes how to test the voltage drop across fuses to determine which circuit is causing the drain. Mercedes even requires this in their service information, as noted in example document AR54.10-p-1030-09GZ. Notice we did not say remove the fuses one at a time. This is not a process that works well for modern cars. There is too much going on that will lead you down a long endless diagnosis if you pull fuses out and put them back in. It’s been found that even after just one fuse is removed, you have to wait at least 5 minutes to see if it did anything. With hundreds of fuses in cars, you can see that’s a big chunk of time.

With the help of service bulletins, you can identify how much current is going through each fuse. Grab a pen and paper and record the highest voltage drop and its location. Continue testing all fuses until you find the largest one. Only after identifying what that fuse does on a wiring diagram will we remove the fuse, or the component connected to the fuse and retest for repair verification. It’s a good idea to identify the fuses that have voltage drops on them in more than one way. If you have a voltage drop on a fuse that is only supposed to be powered after the key is on, that will give you a clue to use the diagrams and find out why it has voltage when it’s not supposed to have voltage on it.

CAN Bus Drains 

When dealing with CAN bus drains, the process becomes a little more difficult. Not only will your battery drain indicate this, but you may notice during your voltage drop testing that there are a dozen or more fuses that have noticeable voltage drops on them. You’ll need access to the manufacturer’s network diagrams. These diagrams will show you how many different networks are, and they’ll also give you ideas for good locations that you can use to easily monitor the network activity. This could be at the OBD data link connector, bus comb, or an easy to access module such as a seat control unit. The idea is to identify what network is “awake.” Awake means that computers are continuing to communicate due to them not seeing a “sleep” message on the network. In these cases, every computer is pulling amps and that is why multiple fuses are showing small voltage drops.

What you will notice on your network is CAN high and CAN low wires will show voltage. This can be done with a scope that shows activity or with a DVOM that reads the voltage. A scope is quicker for identifying activity as you either see digital square waves or not. A DVOM will require you to know what the awake voltage is and what the sleep voltage is. Our example is a body network on a Mercedes. When the bus is “awake” or when there is activity that would cause a CAN bus battery drain, you would have a voltage of around 0.65 volts on the high wire and a voltage of around 4.5 volts on the low wire. If the bus is “asleep” or there is no activity, you would notice voltages of 0.1 volts at the high and around battery voltage on the low. Use your information in Direct-Hit to locate every control unit and its fuse located on the network you’re working on. One at a time, remove that control unit or its fuse, then wait 5 minutes before continuing. Do not reconnect the control unit or install the fuse. Doing so may restart your wait time of 45 minutes.

A Few Other Things to Consider

The diagnostic test steps provided are the new normal when it comes to finding battery drains. But wait, there’s more. Remember that cars have battery sensors and disconnect relays? And that having a compatible scan tool is crucial to your diagnosis? The reasoning behind that is some manufacturers build in battery drain diagnosis into their cars and scan tools. Again, let’s use Mercedes as an example. They have an excellent service bulletin LI54.10-P-051559 where Mercedes has laid out a test program for locating battery drains. In that test program, there are specific scan tool instructions used to read battery drain data along with their own flow chart.

With modern cars having several networks, you can see the complexity in locating the source of your drain along with the large amounts of time that may be required. The future of diagnosing battery drains is going to depend a lot on scan tool functions. The new factory equipment has software that turns the scan tool into a CAN bus analyzer—Mercedes calls this Keep-Alive Monitoring when using their Xentry CAN-Tool software. To summarize how that works, the scan tool will be connected to either the data link connector or to the network wiring. When connected, it records network communication in a manner that will display what control unit or control unit signal that is active in the order of first to last. This is a game changer when it comes to network battery drains. No more setting up ammeters or testing dozens of fuses one at a time.