Sleep is Important for Humans and Cars

Tyler Betthauser
Mar 15
12 min read

Power-Moding in Vehicles

Turns out that poor quality sleep isn't just a bane to human health, but also your vehicle as well. In some respects, vehicles operate on the same principles that drive our own biology. One of those principles is that both of our systems need to be capable of sleeping. Obviously a mechanical system can perform its functions much longer without a lot of perceived wear and they are not biological. But, even the front line IT technician often asks: "when is the last time you restarted your machine?" Humans also have only so much uptime before things start to break from stress. Both automotive and biological systems need to have the opportunity for cleaning up processes that might have failed and data that might be corrupted. Some drivers might be wondering why their vehicles have seemingly random software instability or dead battery. Sometimes, it is as simple as the vehicle is not going to sleep enough!

Domain Centralized Architectures (Pre-2024 vehicles)

The Power-Moding system is usually comprised of an electronic control unit (ECU) and software which commands other modules on the CAN BUS (Bus) network when to start up and shut down. In many cases the command unit is the Body Control Module (BCM) or some equivalent of it. The Domain Centralized Architectures and Zonal Architectures vary somewhat, but the mechanics are largely the same. Each ECU on the Bus implements their own logic to recognize and begin their processes to shut off or go into a standby mode. The BCM also manages the flow of electrons from the battery to the other modules in the network.

Diagram of Domain Centralized Power Architecture showing connections: Battery to BMS, Gateway, and ECU with labels like 12V Main and CAN Bus. — Domain Centralized Power Architecture showing connections: Battery to BMS, Gateway, and ECU with labels like 12V Main and CAN Bus.

Starting Up and Shutting Down in Domain Centralized Architectures

Starting up and shutting down the vehicle are the two basic use cases which are most important for the power management system of the vehicle. Starting up: so you can do work with the vehicle. Shut down: to conserve power, stop the systems, and even do basic cleanup. We'll cover the processes with a visual called a sequence diagram:

Flowchart showing domain architecture for a vehicle's full power cycle. Includes processes like "Startup" and "Shutdown," with various CAN messages and triggers. — Domain architecture for a vehicle's full power cycle. Includes processes like "Startup" and "Shutdown," with various CAN messages and triggers.

Step	Description
1	Everyone typically starts their car with a start button or brake (EVs). You could also start the same system remotely, but through another avenue that we will not go through here.
2	An electrical impulse is sent to the BCM (or whatever the body control master is for the vehicle).
3	The BCM itself wakes up
4	An initial wakeup message is sent via the Bus or a physical coper wire is sent to each ECU to provide power for that ECU to be capable of receiving a message
3	The BCM (hopefully) sees that the button has been pressed and then starts some processes to verify the key presence and brake pedal position. One of the areas to look for issues with no-crank-no-start conditions would be between the start button and the BCM. Probe the pins at the BCM and button. Then verify the can signal is being set properly using a scan tool.
4-5	BCM then sets another can signal value for waking up, service mode, run, etc.
6-7	The bootloader (or lowest level software inside the 'box') powers up the software so it can be capable of receiving additional CAN signal messages
8	BCM then tells the Battery Management System (BMS) to close the contactors and verify the voltage. A scan tool can be used to make sure these signals are being sent as well. It would be useful to know whether these signals are being set properly if there are modules not powering up properly or even race conditions. Ford recently had a recall related to a similar condition which we cover in a recent article: The Digital Ignition Switch Debacle: Why the Ford ITM Race Condition Recall Is a Warning Shot for the Industry
9	BMS sends a message back that the battery is ready
10	BCM updates everyone on the network the power state to the Central Gateway
11	The CAN Bus ECU then notifies the network that it is ready (sometimes this is called a heart beat)
12	All of the applications on various ECUs are supposed to be starting.
13	A button press by the driver cascades into a shut down process
14	A signal is sent over the wire to the BCM
15	BCM instructs everyone on the network to shut down
16	All of the ECUs on the network save data in non-volatile memory (if required). This is an important detail because if power is suddenly removed during shutdown, the risk of corruption can go up!
17	Devices on Ethernet shutdown
18	BCM asks the CGM if the system is ready to shut off
19	An acknowledgement is returned
20	BCM requests the contactors open from the BMS
21	BMS responds that the contactors have opened properly and shut down.
22-23	The final shutdown occurs and broadcasted over the copper line or CAN Bus.

Why a Vehicle Might not go to Sleep

There are times where a vehicle might not have good sleep hygiene and it behooves drivers to pay attention to it closely. When a shutdown process fails, it is usually a battle between a module's internal state and the vehicle's main power logic. In most power-moding architectures, the BCM is trying to be polite, but sometimes the guests refuse to leave the party.

A common reason a module keeps the bus awake is a pending background task. For example, a Telematics module might be in the middle of a large over-the-air (OTA) update or a map sync. Because you don't want to corrupt data with an interruption in power, many times the BCM will keep the 12V power on in hopes the process finishes. As long as the BCM sees traffic on the bus, it often keeps the Retained Accessory Power (RAP) active, assuming the user might still need certain features (like finishing a phone call over Bluetooth).

Other times, ECUs can enter a state where it is sending nonsense data on the CAN Bus. This happens when a hardware fault or a loop causes a module to broadcast junk data continuously. In many architectures, the BCM or Gateway is programmed to stay awake as long as any valid traffic is detected. If one ECU gets stuck in a boot loop or an in-between state where it’s just sending empty frames, the entire vehicle stays awake. This is why many manufacturers implement a sleep timer—after a fixed period (e.g., 20 or 30 minutes), the BCM will ignore the bus traffic and physically drop the 12V wake up line to save the battery. Many times these safety procedures work; however, there have been many cases where remote wakeups to collect data or perform an OTA will override the BCMs logic and drain the 12V battery completely.

Sometimes a gap occurs when there is a mismatch in ECUs acknowledging their sleep. If the BCM sends a command across the network asking ECUs to shutdown, but the Infotainment system is waiting for the Gateway to acknowledge a message, you can get a logical deadlock. The Gateway is waiting for the BCM to go to sleep, the BCM is waiting for the Infotainment, and the Infotainment is waiting for the Gateway.

Mechanical issues are sometimes the root cause of strange wake ups or shutdown failures. In a BCM is making logic decisions based on the electrical signals it sees on its pins. If those signals are noisy due to physical degradation, the software can get trapped in a loop.

There is a growing occurrence of intermittent bad grounds (ground offset). This is perhaps the most difficult issue to diagnose. In a vehicle, the chassis serves as the common ground. If the ground strap for a specific module, like the Gateway, is loose or corroded, the module’s 0V reference might actually sit at 1.5V or 2V (a ground offset). This can cause very voltage sensitive software to become a bit erratic. Drivers might notice seemingly unrelated issues like a light on the cluster and different ECUs falling off the Bus with communication codes. As this condition pertains to shutting down, when the BCM drops the wake up line to 0V to signal sleep, the module with the bad ground might still detect a potential difference between its floating ground and the signal line. This can prevent the internal transceiver from shutting down. Ground offsets will typically cause noise (extra messaging) on the CAN bus. The BCM might see this noise as valid traffic (a start of frame bit, for example) and reset its sleep timer, thinking a module is trying to talk.

High frequency vibrations at the connector pins can cause fretting corrosion, which adds resistance. If there is high resistance on the discrete wake up line, the 12V signal might arrive at the ECU as only 7V or 8V. Some PMICs have a grey zone where they aren't fully on but aren't off either. This can lead to a state where the processor is constantly rebooting, never reaching the stage where it can acknowledge a sleep command. High resistance on a power feed can cause a voltage drop when a module attempts a task requiring higher current (like writing to NVM during shutdown). This causes the module to crash mid operation, leading to the data corruption mentioned earlier. The next start will likely result in a failure where the ECU fails to communicate, DTCs are set, and performance degraded.

If water or road salt gets into a connector (especially in the BCM or an exterior mounted ECU), it creates a high resistance connection between pins. Moisture can bridge the constant 12V pin to the discrete wake up pin. Even when the BCM tries to pull that line to ground, the current from the 12V constant pin keeps the voltage high enough to keep the module awake. This is a common cause of batteries dying only when it rains and flood damaged vehicles.

Is your 12V Battery always dying?

It could be that the issues lie in your vehicles sleep schedule. The Car Conservatory will perform a sleep study to prescribe a fix!

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Impacts on Vehicles Unable to Sleep

A vehicle that can't sleep is at a higher risk for issues that have little to do with the hardware exactly. Drivers should periodically pay attention to vehicles that appear to be on but for seemingly no reason. Many newer vehicles start up and shut down more often because of the proximity to the key fob. If the garage or driveway is too close to where keys are hung or are moving in and out of the range often, it can inadvertently wake up the car often. As long as there are no anomalies during these sleep and wake cycles then the risk is low for an issue; however, if there are mechanical issues then software can misbehave and corrupt itself.

Here are just a few of the consequences that Drivers need to be aware of:

Dead Batteries

If the Bus is kept awake for too long, the 12V battery will eventually be depleted and may not be rechargeable. Batteries aren't cheap so it is important to be able to recognize when a vehicle might be struggling to power down properly. In the context of EVs, a dead 12V battery can lead to disaster. Because of a very tight coupling between the high voltage battery and 12V there is software which will command charging of the 12V to maintain whatever process is keeping the Bus awake. It will do this to its detriment until dead. A dead high voltage battery is typically not recoverable and necessitates replacement. For both EVs and ICE vehicles it is a good idea to keep it on a charger if not being driven often.

Propensity for Software Anomalies

Most ECUs write critical state data (odometer readings, diagnostic trouble codes, or user settings) to Non-Volatile Memory (NVM) during the shutdown phase. If power is cut mid write operation, the file can be partially written or the file system can become corrupted. On the next startup, the module may fail to boot because its configuration file is unreadable. Many, if not all, mitigate this condition with atomic writes so if the write operation is not complete then nothing is written at all. This protects the memory, but can cause some strange behavior if there are lots of cuts to 12V power. An example might be a drop in voltage while saving a memory seat setting. The next ignition cycle a seat might just return to the default position.

Modern ECUs use complex Power Management ICs (PMICs) to ramp voltages down in a specific order. A sudden loss in the 12V power can cause internal voltage spikes or leave capacitors partially charged. This can lead to a logic lock up condition where the processor behaves erratically on the next startup, often requiring a physical battery disconnect (the reset) to clear the state.

If a module like the Electronic Stability Control (ESC) is shut down ungracefully, it might not have time to set its sensors or calibrate its zero point. On the next wake up, it may detect an inconsistency and throw a communication fault or a DTC related to the chassis system, disabling safety features like traction control until the next clean drive cycle.

Sometimes, too much up time is a bad thing! A memory leak occurs when a software application requests a portion of RAM but fails to release it back to the system after the task is finished. Over days or weeks of uptime, these tiny leaks accumulate. In a vehicle, this is frequently seen in the Infotainment and Telematics domains where third-party apps (like streaming services or navigation) are running. An Infotainment system that has been suspended and resumed for a month might become sluggish or crash when the driver tries to open the backup camera. This happens because the leaked memory from a music app has left no room for the camera’s video buffer. Only a clean restart flushes the RAM entirely and returns the system to its baseline performance. A single event such as ionizing radiation (like cosmic rays) or electromagnetic interference (EMI) physically flips a bit in a memory cell or a processor register. If this flip occurs in a status register—such as a bit that tracks whether a door is open or closed—the software can become stuck in a failed state. If a bit flips in the BCM register that monitors the start button, the vehicle might refuse to crank because it thinks the button is already being held down. a vehicle that hasn't slept would preserve this corrupted register state. A full power cycle forces the hardware to reload its initial, verified configuration from the read only flash memory, clearing the fault. Most ECUs communicate with smaller internal peripherals (like radio tuners, GPS chips, or camera sensors) using protocols like I2C or SPI. Occasionally, a voltage spike or timing jitter can cause the peripheral to hang. The main application might still be running, but it can no longer communicate with the hardware it controls. For example, the telematics control module might lose its connection to the cellular network. Even if the application software is suspended and resumed, the internal hardware registers of the cellular chip remain in a standby state. A clean shutdown usually includes a reset signal that physically toggles a reset pin on these subcomponents, forcing the hardware to reinitialize its connection to the network. Modern vehicles rely heavily on SQLite databases to manage everything from your contacts list to your navigation history. When a system is always suspended, these databases never have the opportunity to run cleanup operations. Furthermore, system logs can grow so large that they begin to fill the available storage space on the module. A navigation system might become increasingly slow at calculating routes. This is often due to a fragmented database that has grown too large over hundreds of drive cycles. A graceful shutdown sequence often triggers garbage collection routines that prune these logs and optimize the database before the power is cut, ensuring the next boot is lean and fast.

Keeping your 12V system and Software Healthy

Try to drive your vehicle often enough to keep the battery charge up.
Don't drive through very deep puddles often. Moisture intrusion can cause issues which go unnoticed for a long time until it is too late.
Try to make an effort to make sure vehicles are turning off after 20 min or so and remain off.
If you are having intermittent problems try removing the negative battery cable from the 12V battery for 20 min to let the entire system discharge. Put everything back together and retry.

Ultimately, a vehicle’s ability to sleep is as vital to its longevity as the fuel it burns or the electricity it stores. Whether governed by a BCM in a domain architecture or a more decentralized zonal setup, the graceful transition from active processing to deep standby ensures that critical data is preserved and the 12V battery remains healthy. When this cycle is interrupted—whether by a stubborn OTA update, a software deadlock, or ground offsets and fretting corrosion—the result is more than just a dead battery. It is a recipe for memory corruption, diagnostic codes, and systemic instability that can leave even the most advanced machines in a sorry state.

For drivers and technicians alike, maintaining this sleep hygiene means looking beyond the code and into the physical reality of the vehicle. It requires an awareness of the environment, from avoiding moisture intrusion in connectors to ensuring the integrity of ground straps. By respecting the vehicle's need for a quiet shutdown and a clean restart, we prevent the types of power-moding failures that lead to costly high voltage battery replacements and unpredictable software anomalies. In the end, a car that sleeps well is a car that performs well, reminding us that even in a world of silicon, sometimes the most important task is knowing when to turn off.