If you are not a specialist auto-electrician, you may be surprised to learn that among the many advanced control mechanisms that manage the electrical systems on modern vehicles today, there is one particular mechanism that can initiate a process of systematically shutting down non-essential electrical systems under certain circumstances. This process is commonly known as “load shedding”, and in this article, we will take a closer look at why it is needed and how it works, starting with this question-
In simple terms, "load shedding" is the process whereby a control module (typically a body control module), begins to "shed" electrical loads when the total demand for electrical power exceeds the available supply. There are many possible reasons why this could happen, but before we get to specifics, let us first provide some context or background for this article at the hand of a practical example that rolled into this writer's workshop on the back of a recovery truck a while ago.
This example involves a 3-year-old Ford Focus and its distressed owner who was something of a DIY mechanic that had a somewhat disturbing propensity for experimenting with his vehicles' programming. Long story short though; this customer had just acquired the Ford Focus, and since (as he claimed) the car felt a bit "sluggish", he thought he would reprogram the ECU to restore the cars' performance.
Since he owned a high-end, Ford-specific scan tool and bought the relevant software from an official Ford source he set to but before the reprogramming could complete, his scan tool (as he put it) went crazy and started displaying messages about electrical sub-systems either shutting down or being altered. He also managed to record a total of 22 trouble codes, including 9 UXXXX codes before the electrical system shut down completely, hence the need for a recovery truck to deliver the car to us.
We no longer have the complete list of trouble codes so we can't reproduce it here, but suffice it to say that almost all of the non-UXXXX codes related to low system voltages, or body control functions like seat adjustments, power window operation, navigation, audio, and/or infotainment system failures. All the UXXXX codes involved communication failures between various control modules, and/or failures of one or more CAN bus systems.
Fortunately for this car’s owner, this writer had seen this type of issue before, so the first order of business was to test the battery, which showed a state of charge of just more than 10 volts. The terminals and cable ends were clean, tight, and free of corrosion, but to be on the safe side, we replaced the battery and since the system had shut down in the middle of an ECU reprogramming event, we hooked up a clean power supply and reprogrammed the ECU from scratch. We then reset the charging system to recognise the new battery, and the Focus fired up immediately.
It is perhaps worth noting that when we scanned the Focus again after performing some relearning procedures and a drive cycle, all but 3 of the UXXXX codes had resolved themselves.
Image source: https://www.samarins.com/glossary/images/battery-current-sensor.jpg
No? Well, the battery current sensor (arrowed) on the negative battery cable sensed that the battery was discharging to below a critical level, and started shutting down non-essential electrical consumers one by one to reduce the load on the battery in a load shedding process. In this particular case though, the battery turned out to be defective, and the load caused by the programming event drained it to the point where everything stopped working.
However, while load shedding system parameters vary somewhat between vehicle makes and models, shutting down completely like the Ford Focus in our example is not what typically happens during load shedding on a modern vehicle.*
* Note that while this article is intended to provide an overview of electrical load shedding on Ford products, the technical details of how the system works, and the order in which some sub-systems are shut down largely depends on both the model and the equipment on that model. Also note that since electrical load shedding systems on other vehicle makes are largely similar to the system used by Ford, the symptoms of load shedding on other makes largely resemble those that occur on Ford products.
Depending on the vehicle and how it is equipped, the system will predictably shed electrical loads under certain conditions, which begs this question-
On all applications, load shedding is initiated by the ECU, but actual control and management of the process is typically performed by the body control module. In practice, the system monitors several parameters that typically include the best rate of charge, which is (among other parameters) based on the age of the battery, total electrical load, engine speed and load under KOER conditions, the ambient temperature, and the temperature of the battery.
Based on this cocktail of parameters, the system will a), monitor all electrical consumers continuously, and b) determine if the current power supply can meet the current demand for power. If the system determines that the supply cannot meet the demand under both KOEO and KOER conditions, it will initiate one of four possible load shedding strategies, these strategies being-
Primary Load Shedding
This strategy is typically initiated if the alternator is operating at full capacity, but the battery’s state of charge is at or below 11.5 volts*. Depending on the vehicle make and model, the first systems/consumers that will be shut down include heaters in the seats and/or the steering wheel, followed by other systems/components that do not affect the safe operation of the vehicle. Systems and/or components that are shut down during this process will typically be reactivated automatically when, and if, the battery voltage returns to normal- i.e., to above 12 volts.
* This value may be different for different vehicle makes and battery types.
Transient Load Shedding
On Ford products, transient load shedding typically occurs when the electrically operated power steering system requires power while the battery voltage is at or below 11 volts. Which system(s) are shut down, and in which order, depends on the model and the equipment fitted to that model. On other vehicle makes without electric power steering, several vehicle-specific criteria may be used to initiate transient load shedding.
Continuous Load Shedding
On most modern vehicles, continuous load shedding will typically be initiated when a period of transient load shedding exceeds about 20 seconds. Note though that as with transient load shedding, which systems are shut down, and in which order, during continuous load shedding depends on the vehicle make and model.
KOEO Load Shedding
In most cases, KOEO load shedding will occur when a body control module detects a battery state of charge that is below about 50%, or, when the ignition is in either the "ACC" or "RUN" positions for longer than about 45 minutes while the engine is not running. On Ford products with navigation systems, this system will be shut down first, followed by infotainment and other non-essential consumers to conserve battery power, the aim being to retain enough battery power to start the vehicle.
The above are necessarily brief descriptions of complex electrical system management protocols, and while there is nothing much that we as technicians can do to prevent the first three load shedding strategies, the fourth, i.e., KOEO load shedding, often happens in our bays without us even being aware of it.
Although many high-end, non-vehicle specific scan tools have control module programming capabilities, the problem is that few non-vehicle specific san tools can access the parts of body control modules that control/manage electrical load shedding functions directly. What this means in practice that several systems can become unavailable during extended reprogramming procedures, but the first time we become aware of this is often only when multiple symptoms and/or trouble codes appear after we have completed a programming procedure, which begs the question of-
If you don't do much ECU programming, you may not realise that some programming procedures are extremely energy-intensive, and in some cases, a major programming event can drain even a fully charged battery before the procedure can be completed. In such cases, you have to recharge the battery and start over, but there are alternative ways of completing reprogramming events without draining a vehicles' battery. Here are some of those alternative ways-
Use an additional, clean power source
The term "clean power" refers to DC currents that are a), essentially free of AC ripple currents and b), stabilised to 14.5 volts. We need not delve into the complexities of clean power supplies here, but suffice it to say that most battery chargers deliver a), DC currents that contain significant AC components, and, b) are not voltage stabilised, since they are designed to vary their output to suit a specific battery type’s charging profile.
Note the word "essentially" in the paragraph above because in practice, it is very difficult, if not impossible to remove all AC ripple current from DC currents. However, the electronic systems on most modern vehicles are designed to cope with between 50mV and about 100mV of AC ripple without suffering adverse effects like overheating and the subsequent failure of, particularly capacitors, around which most electronic systems are built.
So what is the point of all this, you may ask? You may well ask, and the answer is that you need a clean, stable power supply (to replace the alternator's output) when you are placing large loads on a vehicle's battery during extensive programming or diagnostic events. Note that using a clean power supply during programming events under KOEO conditions is a requirement stated in the service information of almost all modern vehicles, and in this writer's experience, the only allowable exceptions to this rule are the following-
Anything beyond the two points above requires the use of a clean, stabilised power supply that delivers DC currents that contain an AC component of less than 0.5% of the float voltage value per cell for that specific battery type. AC ripple currents that exceed this value have the potential not only to overheat both the battery and components in electronic circuits, but also to cause processors in ECU's to reject a programming attempt.
More importantly, though, insufficient battery power during a KOEO programming event could initiate one or more load shedding events, which you may not notice unless you have a vehicle-specific scan tool. There is however one more aspect of clean power supplies we need to discuss, which is-
We are all used to connecting both leads of battery chargers directly to battery posts, but this is not recommended for clean power supplies.
The problem is that contrary to popular belief, electrical current does not leave a battery through the positive pole; electrical current leaves the battery through the negative pole and flows back to the positive pole through the electrical system. Therefore, since the battery sensor is located on the negative side of the battery, connecting the power supply's negative lead to the battery post or terminal has the effect of making the battery monitoring system think that a new battery had been connected.
Therefore, best practice is to avoid upsetting or even wiping battery charging/monitoring data by connecting the power supply's positive lead directly to the battery and connecting the negative lead to a ground point that is not connected to the battery directly, to ensure that the load shedding system will continue to work as intended.
Note though that while connecting a clean power supply’s negative lead directly to the battery will typically not damage anything, the battery monitoring system will need at least eight hours to reset itself, but only if the vehicle is allowed to remain in sleep mode for the entire time it takes for the system to reset automatically, which leaves us with this-
While this article focused mainly on Ford products, all, or most other late-model vehicles from other manufacturers use similar load shedding systems/strategies, and although there are certainly some differences between the parameters that will trigger a load shedding event on different vehicle makes, all load shedding systems have a common goal.
This goal is not only about preserving battery power under certain conditions; it is also about ensuring that sufficient battery power is available under both KOER and KOEO conditions to ensure that critical system control functions are not interrupted unexpectedly by a discharged battery.
Having said that though, it must also be said that load shedding systems on cars are a relatively new development, so if you recently came across electrical systems on a late-model vehicle shutting down without any apparent reason, chances are that the shutdowns were the result of a load shedding event on that vehicle.
We do realise though that service/ information on battery monitoring systems is hard, or expensive to come by, so the best advice this writer can offer to prevent being caught unawares by a load shedding event on a vehicle is to a), always connect a clean power supply during programming events, and b), to check that a battery/charging system reset had been performed on every vehicle you work on.
One other piece of advice would be to learn as much about battery monitoring and load shedding systems as you can, simply because none of us can afford to wait eight or nine hours for a battery monitoring system to reset itself.
