Parasitic battery drains

May 2, 2016
The first step to diagnosing any parasitic draw starts with what’s being drawn down to begin with — the battery. I think more than any other component, the battery is the most overlooked item when it comes to testing for a parasitic drain.

For the most part, it will almost always be a long process to track down the cause of a parasitic battery drain in a late-model vehicle, as there are so many modules, accessories and networks involved. In any case, there are a few things that I have found that will help more quickly narrow down the possibilities and also prevent overlooking some basics that cause severe headaches for some technicians.

Vehicle: 2008 Chevrolet equipped with 4.8 liter engine and automatic transmission
Odometer: 55, 291 miles
Concern: Vehicle will not start after sitting for a few days

The first step to diagnosing any parasitic draw starts with what’s being drawn down to begin with — the battery. I think more than any other component, the battery is the most overlooked item when it comes to testing for a parasitic drain. Where I live, most summer days are 100 degrees F or higher, which definitely takes its toll on the life of a battery. In fact, a large portion of batteries that I find worn out are only 2-3 years old. So before any testing can even start, we need to make sure the battery is sufficiently charged. Unless the open circuit voltage of the battery is 12.45V or higher, the battery needs to be charged before testing. While the battery is being recharged, this is also a perfect time to multitask and check for TSBs, hotline archives and groups like iATN for similar vehicles that have experienced the same problem. Many times techs will try to diagnose a system draw with a battery that will barely crank the vehicle’s engine over to start it; this can cause inaccurate test results.  Some signs of battery problems are obvious, like visible leakage, especially across the top of the battery, which will cause case drain. The leaking acid creates a conductive path that can easily drain a fully charged battery overnight. Then there is a problem you can’t see, at least with the naked eye, which is a shorted battery. When measuring the open circuit voltage of the battery and the DMM reads in the area of 10.5V, you have a bad or shorted cell and it must be replaced. 

One of the most important but overlooked steps in diagnosing a battery drain problem is verifying the vehicle has a good battery. If recharging is needed before testing can begin, it is a good idea to use the time to multitask and research the problem looking for TSB, hotline archives and technician sites like iATN.
One thing to look for is modified items such as this battery post adapter. This one is dropping nearly 700mV with no load. This was enough to cause the vehicle to have a no-start condition when a high load of the starter was placed on it, making the customer think that the battery had been drained down. This would also have an effect on the charging system performance as well.

If the battery checks out OK, the next check to make is for codes. While not ignoring powertrain codes, look for modules that have stored UXXXX (communication) codes. Why? While most modules on the bus power down as they are supposed to at key off or a specified length of time thereafter, there may be a single module that never goes to sleep, AKA a babbling node. This module continues to send and/or looks for state of health (SOH) or other data from the remaining modules that have gone to sleep. When those messages are not received or responded to, that module sets a Loss of Communication code. If this is the case, you have significantly reduced the field of possible causes; however, don’t automatically condemn the module as there may be inputs that are causing that module to stay awake. There is also a possibility of the one module waking up another module on the bus, creating an even larger current draw.  

While sometimes tricky to attach the ammeter inline, this does provide a more accurate and stable measurement of lower amperage draw than a clamp-on ammeter. Here is the original current draw on the 2011 Dodge Challenger.
The four-digit Julian Code on the door latch is located just under the foam insert. The first three digits indicate the day of the year and the last number indicates the last digit of the year. The number on this latch is 0761A, which means that it was produced on the 76th day of 2011 or March 17th, 2011. Since this was between the Julian Date of 3200 (Nov. 20, 2010) and 0841 (March 25, 2011), it was affected by the TSB.

Some tips before you begin to test for a draw:

•          Open the hood, doors and trunk and flip the latch to the closed position. More than likely you will need to access components in these areas and if you open one of them you will wake up a module(s) and then have to wait for them to go back to sleep.

•          On vehicles that have a push-button type of door or hood ajar switch, if you are certain that it is not the cause of the draw, remove the connector from the switch and jumper the connection so it appears closed to the vehicle.

•          Make sure the key is turned to the locked position and removed from the ignition cylinder, as this will help power down the modules more quickly than if the key was simply turned to the off position.

•          In the case of proximity keys, make sure the key is out of range of the vehicle or some modules may not enter sleep mode. Some vehicles will remain in an Off-Awake mode when a proximity key is in range, causing a parasitic drain on the battery.

•          Not that aftermarket accessories are bad (radio, alarms, DVD players, etc.), but they can be an unintended load on a circuit if they stay powered on when they should not. If easily accessible, these are the first items I try to eliminate and have had a fairly good success rate having them be the cause of the battery drain.

Using the Class 2 Message Monitor on the GM Tech 2 scan tool, it is easy to see that the Digital Radio Receiver (DRR) is continuously waking up, causing a draw on the battery.
At this point, the even numbers are showing the modules are asleep, but since the number next to the module increases each time a module wakes up again when it goes to sleep, the 13 indicates the Digital Radio Receiver has woken up and gone back to sleep six times more than the other modules.

Real-world cases

My first case study is a 2011 Dodge Challenger with a draw of nearly 2 amps. I inherited this from another technician who had already changed out the battery because it would not hold a charge, thinking that the Totally Integrated Power Module (TIPM) was the cause of the draw. Well, not that a TIPM failure on a Chrysler is rare, but it does control a lot of circuits and components. Looking at the wiring diagrams and trying to narrow down a fault in a module like that is an arduous process at best. Since the battery was just replaced, I could not retrieve any codes from the vehicle. I checked for draw on the battery and of course, it was now at 0.026A, not the original 1.92A that was measured while the other technician was working on the vehicle with the key off. One problem that can happen quite often is when the battery is disconnected, an electrical problem may mysteriously disappear for a time; this is also something to try to avoid when connecting an ammeter inline with the negative battery cable and post. Next I checked for TSBs and I found one that could save me a lot of diagnostic work and waiting for the problem to reappear. TSB 23-039-11 states that the smart glass feature cycles on its own with the key in the off position, draining the battery. The problem was the windows were not acting up now, so I asked the tech who was previously working on the vehicle if he had noticed the windows cycling. He did notice the driver window moving slightly up and down when he closed the door, but thought it was due to the low state of charge of the battery. Since the problem was not present and the TSB stated that it was an intermittent problem, I decided to check the Julian dates on the door latches. A simple procedure to perform, I had to move a small piece of foam at the striker area of the latch to view the code. Both the driver and passenger door latches had number 0761A, indicating there were produced on the 76th day of 2011, which was in the affected range, so these could very well be the cause of the parasitic draw. The customer stated the problem had happened for the last couple days and after the vehicle sat overnight, the battery needed to be jumped in the morning. Since the problem was not present, I could not confirm this was the cause, but it seemed very likely that the latches were going to be the problem. Since it was Friday afternoon and time was short, I decided to unplug the latches and let the vehicle sit over the weekend. When I came back, the vehicle started immediately on Monday morning. It even sat a couple more days until we received the updated door latches without any further battery draw concern. We even checked with the customer a couple of weeks later and they stated no further problems with the battery were experienced.

Measuring voltage drop across a fuse is one of the best methods to narrow down a circuit with a parasitic drain. Here the voltage drop of the radio fuse is showing 4.8V. Using Ohm's Law, Voltage ÷ Resistance = Amperage. So 0.0048mV ÷ 0.00458Ω = 1.048 amps being drawn on the radio circuit with the vehicle off.

Next was a 2008 Chevrolet Tahoe that was also given to me by another tech who did some research on a drain, but was unable to narrow down the cause. He had hooked up an ammeter in series with the negative battery cable and stated that the amperage draw was over 1 amp. A lot of times, this large of a draw points to a problem with a module staying awake. The battery had already been replaced, so the next step was to check for codes. In this case, there were some codes in the BCM for headlamp concerns, but no communications codes. I noted the codes and moved on, since they may be a clue to the parasitic battery drain, but wanted to do a little more testing before looking into them. Next I used a great feature of the GM Tech 2 scan tool to watch the states of the modules by using the Class 2 Message Monitor. This feature allows you to see what modules are active and which are asleep or inactive. The number next to each module increases each time the module state changes. For example, when the module is awakened, the number changes to 1; when the module goes to sleep, the number increments to 2. If the module is active again, the number will increment to 3 and so on. At first, when the key was turned off and the doors locked, everything seemed fine as the modules started changing their state from 1 to 2, indicating the modules on the bus were going to sleep mode. Then I noticed the Digital Radio Receiver and BCM turned back on and then went back to sleep. They turned on again, and this continuously reoccurred. So was it a module itself or was something waking up the module. First, I needed to decide which module was waking up first. With the DRR being directly behind the passenger foot panel, this is the one I decided to disconnect. My logic is that the BCM has multiple inputs such as door locks, latches, lights and other accessories that could trigger it to wake up, but the DRR is only used for receiving Sirius radio. Luckily, when this module was disconnected, the bus no longer woke up. Just to be sure, I let the vehicle sit overnight with the DRR disconnected, and the battery remained fully charged the next morning. I think that the codes that were stored in the BCM were related to either the module being woken up and not having other modules online or a result of false codes that were caused by a weak battery.  

Even though some manufacturers specify a current draw of 50 milliamps or less, this is the actual reading after the new radio was installed and programmed in the 2010 GMC Yukon.

My last case study is another GM vehicle, a 2010 GMC Yukon that a customer had brought in to have a battery replaced. They stated that they had to jump start their vehicle in the morning and also noticed corrosion on the top of the battery posts when attaching the jumper cables. Well, the battery was only a year old, but there was some corrosion on the top of the battery that could be the cause of the battery draining overnight. So after charging the battery and cleaning the terminals, it was tested and passed. Although there was corrosion on the posts, I did not measure any case drain across the top of the battery. So after cleaning the terminal, but before reconnecting the negative side, I installed an ammeter in line with the battery negative terminal and post. After almost 30 minutes, the battery draw was still around 1.05 amps. Looking inside the vehicle, everything was turned off and no aftermarket accessories appeared to have been installed. Since I already had an ammeter installed on the battery, I decided to check the voltage drop across the fuses in the underhood fuse block. One of the benefits was it was easily accessible, then I could just use Ohms Law to figure out if the draw from one fuse was equal to the amount of current draw I was reading with my ammeter. The theory is that if a fuse has a voltage drop, it must have current flowing through it. The cold resistance of the fuse, which is measured in milliohms or thousands of an ohm, is used in the Ohm’s Law equation to determine the amount of current flowing in the fuse’s circuit. Keep in mind that each type of fuse has a different cold resistance value. For example, a 20-amp rated Mini Fuse has a cold resistance of 3.21 m and a standard 20 Amp ATO Blade fuse has a cold resistance of 3.38 m. A 20-amp Maxi Fuse has a cold resistance of 3.10 m, while a 20 Amp J-Case fuse has a cold resistance value of 4.29 m. I have created a chart of the cold resistances from a popular fuse manufacturer and included it in this article.

Use the milliohm rating next to the fuse that voltage drop is being measured on. For example, a voltage drop of 2.8mV across a 15-amp mini fuse would be 0.0028V ÷ 0.00458Ω = 0.611 amps, or 611mA. To make things a little easier to read, move the decimal three places to the right (2.8mV ÷ 4.58mΩ = 611mA).NOTE: Depending on the fuse manufacturer, there will be some variances in the cold resistance values of their fuses, which will change the results slightly.

After measuring for voltage drop on fuses in the underhood fuse box, I found a voltage drop of 4.8mV on fuse 42, which is labeled Radio. Using Ohms Law and dividing volts by ohms, or millivolts by milliohms in this case, we have: 0.0048V / 0.00458=1.048 amps (or to make things easier, 4.8mV ÷ 4.58mΩ = 1.048 amps) being drawn on that single fuse with everything turned off. Well that looked right, so checking out a wiring diagram shows the fuse powers the radio, the rear-seat radio controls and the digital radio receiver. Well the easier one to disconnect is the controls for the rear at the back of the center console. Popping the cover off and disconnecting the controls shows that there is no difference in the amount of current draw. Next is the radio itself. When the power to the back of the radio is removed, the amperage draw drops to 13mA. So it looks like this is the cause of the draw.

A replacement radio was installed and programmed and as any of you who have installed a radio in a newer GM vehicle know, there are an incredible amount of options to choose from. I find it easier to take a picture of the Regular Production Option (RPO) codes with my cell phone and have them in front of me to save time.

Hopefully these examples will help save you some frustration the next time you have to diagnose a parasitic battery drain on a newer vehicle.

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