Five kinds of electrified vehicle
“Electric vehicle” covers very different electrical architectures. What they share is a traction system that is separate from the low-voltage network on which the vehicle's ECUs, lights and comfort systems run, and a DC-DC converter that links the two. What differs is the traction voltage, the size of the battery and how often the vehicle wakes up while parked.
A 48 V system belongs to voltage class A (up to 60 V DC), so by the electrical-safety definition it is not high voltage. It is still a high-current system: a short circuit at 48 V can draw an arc and cause burns and fire. Treat it with the respect its energy deserves.
OFF, ACC, ON, READY: the new meaning of ignition
In a combustion vehicle, terminal 15 meant two things at once: the electronics are on, and the engine can run. Electrified vehicles separate these. ON means the low-voltage electronics are awake and the high-voltage system may still be disconnected. READY means the high-voltage contactors are closed, the inverter is enabled and the vehicle can move, silently, at the touch of the accelerator. With keyless entry and start buttons, a power-mode controller manages these states and distributes them over CAN, so terminal 15 is often a network state rather than a wire.
- Body · CANAsleep
- Infotainment · CANAsleep
- Chassis · CANAsleep
- Powertrain · CANAsleep
- Battery management · CANAsleep
Vehicle locked: the networks sleep and only a small quiescent current flows.
Typical example. Wake-up behaviour differs between manufacturers.
What happens when the vehicle goes READY
- 01Wake and self-check
The driver presses the brake and the start button. The battery management system (BMS) wakes, checks cell voltages, temperatures and the insulation resistance between the high-voltage system and the chassis.
- 02Close the negative contactor
One side of the traction battery is connected to the high-voltage bus.
- 03Precharge
A precharge contactor connects the positive side through a resistor, charging the large DC-link capacitance of the inverter and DC-DC converter with a limited current instead of an inrush that would weld the contacts.
- 04Close the main positive contactor
When the DC-link voltage is close to the battery voltage, the main contactor closes and the precharge path opens.
- 05Enable the drive
The DC-DC converter starts, the low-voltage rail rises, the READY indicator appears and the inverter accepts torque requests. The whole sequence usually takes from a fraction of a second to a few seconds.
Terminal 15 in an electrified vehicle
Many electrified vehicles still provide switched supplies labelled 15 or “ignition” in their fuse boxes. Those supplies may follow ON rather than READY, may switch on during charging or remote preconditioning while nobody is in the car, and on newer platforms may be switched by electronic fuses according to the power mode. A switched fuse is therefore no reliable indication of “vehicle in use”. Use the supply that the vehicle documentation designates for the purpose, and see Installation best practices for connection practice.
The low-voltage battery: small, but critical
In an electrified vehicle the low-voltage battery powers the controllers that close the high-voltage contactors. If it is flat, the traction battery cannot be connected and the car will not go READY, even with a fully charged traction battery. Because it never cranks an engine, the low-voltage battery is often smaller than the starter battery of a comparable combustion car. Some manufacturers have moved to lithium-ion low-voltage batteries, and not all of them use a nominal 12 V.
The DC-DC converter replaces the alternator. It charges the low-voltage battery from the traction battery, but only while the high-voltage system is connected: in READY, during charging, or during a scheduled top-up on vehicles that support it. Parked and asleep, every milliampere comes out of the small low-voltage battery.
Network management: how a vehicle falls asleep
ECUs agree to sleep through network management (NM). In the AUTOSAR CAN NM scheme used by most current vehicles, every ECU that needs the network sends periodic NM messages. When an ECU no longer needs it, it stops sending them but keeps listening. Only when no NM message has been seen for the NM timeout does the whole cluster move to prepare bus-sleep and then to bus-sleep. Any single participant can keep the network awake, and any wake-up event brings it back.
Wake-up comes either from the bus, when a transceiver in low-power mode sees a valid wake-up pattern as defined in ISO 11898-2, or locally: a door handle, the key, a charging connector, a timer. Partial networking, standardised as selective wake-up in ISO 11898-2:2016 and carried into the 2024 edition, lets transceivers stay asleep while other parts of the network communicate, waking only on specific wake-up frames. It lowers consumption during long charging sessions, and it means that a segment can be partly asleep while traffic flows on it.
Wait until the networks are asleep before reading the quiescent current.
Why an electrified vehicle wakes while parked
- Start and end of a charging session, and charging timers.
- Cabin and battery preconditioning before a planned departure.
- Remote requests from the owner's app through the telematics unit.
- Topping up the low-voltage battery from the traction battery.
- Battery thermal management and cell balancing.
- Alarm and security sensors, keyless-entry proximity and door handle approach.
“Parked” therefore does not mean “asleep”. An electrified vehicle may run through many sleep and wake cycles in a day. Anything connected to its networks has to follow every one of them, including waking cleanly and falling silent again.
Sleep current: the budget every device must respect
Sleep current, or quiescent current, is what the vehicle draws from the low-voltage battery when all networks are asleep. Manufacturers allocate this budget carefully, ECU by ECU. Aftermarket equipment draws from the same battery but sits outside that budget. A simple formula shows what is at stake:
parking time (h) ≈ usable capacity (Ah) ÷ total quiescent current (A)- 01Set the usable capacity
Assume, for this example, a 45 Ah low-voltage battery of which about half can be used before starting becomes unreliable: 22.5 Ah.
- 02Vehicle alone
With an assumed vehicle quiescent current of 20 mA: 22.5 Ah ÷ 0.020 A = 1,125 h, about 47 days.
- 03Add a well-behaved device
A device drawing 5 mA while asleep brings the total to 25 mA: 900 h, about 37.5 days.
- 04Add a careless device
A device drawing 30 mA brings the total to 50 mA: 450 h, about 19 days. The parking range has dropped by more than half.
- 05Keep the network awake
If a device prevents bus-sleep and the vehicle draws, say, 1.5 A with its networks awake, the same 22.5 Ah last about 15 hours. One weekend at the airport is enough to strand the car.
Measuring sleep current correctly
- 01Prepare the vehicle
Close all doors, the boot and the bonnet (or set the bonnet switch as the OEM procedure describes), switch off all consumers, disconnect any charging cable and keep the key well out of range.
- 02Connect without interrupting the supply
Use a DC clamp meter with milliampere resolution on the battery cable, or an ammeter in series with a bypass switch so that the circuit is never opened. Breaking the supply wakes ECUs and restarts their timers.
- 03Lock and wait
Lock the vehicle and wait for bus-sleep. Depending on make and equipment this takes from a few minutes to more than half an hour. Watch the current step down and confirm that the CAN lines have gone quiet.
- 04Read the settled value
Note the stable current and any periodic peaks, which are often scheduled wake-ups for battery top-up, telematics or thermal management.
- 05Compare with the device installed
Repeat with the aftermarket device connected. The difference is the device's share, and the network must still reach bus-sleep in the same time as before.
High-voltage boundaries for installers
ISO 6469-3 and UNECE Regulation No. 100 define voltage class B, the high-voltage class, as above 60 V and up to 1,500 V DC, or above 30 V and up to 1,000 V AC rms. Passenger car traction batteries today belong to the 400 V or 800 V class. Class B cables and harnesses outside enclosures must have an orange outer covering, and high-voltage components carry warning labels. The high-voltage system is isolated from the chassis; R100 requires at least 100 Ω per volt of working voltage for DC circuits and 500 Ω per volt for AC circuits, and the vehicle monitors this insulation continuously.
Routing, drilling and qualification
Traction batteries usually fill the floor, and high-voltage cables run along sills, tunnels and subframes, so drilling or fixing anything in these areas needs the vehicle documentation first. Route CAN twisted pairs away from high-voltage cables and inverters, whose fast switching produces strong electromagnetic noise, and keep the twist intact up to the connection point. Never create any electrical connection between the high-voltage system and the chassis or the low-voltage network. Work on the high-voltage system itself requires a specific qualification under national rules; Germany's DGUV Information 209-093, for example, defines graded levels from basic awareness up to work on live high-voltage systems. Installers who only work on the low-voltage side still need to know where the boundaries are and to follow the manufacturer's and their employer's rules.
CAN inside the electric drivetrain
The electric drivetrain has its own networks, often called EV CAN or hybrid CAN, linking the vehicle control unit, the BMS, the inverter, the DC-DC converter and the on-board charger, increasingly as CAN FD on newer platforms. Inside the battery pack, cell monitoring units often communicate with the BMS over isolated daisy-chain links rather than CAN. The link between vehicle and DC charger is separate again, and depends on the charging system:
These charging links are point-to-point connections between the charger and the vehicle's charging controller, and they are separated from the vehicle's internal networks. The J1939 heritage of the Chinese standard is explained in SAE J1939, and the place of the drivetrain networks in the overall architecture in Vehicle network architecture.
Why does an electric car's 12 V battery go flat when the traction battery is full?
The DC-DC converter only charges the low-voltage battery while the high-voltage system is connected. Parked and asleep, everything runs from the small low-voltage battery, and if it is drained the contactors cannot close.
Can I take an ignition signal from a fuse marked 15 in an electric car?
Not as a reliable indication of use. Such supplies may follow ON rather than READY and may switch during charging or preconditioning. Use the supply the vehicle documentation designates for your purpose.
Is a 48 V mild hybrid a high-voltage vehicle?
No. 48 V belongs to voltage class A, up to 60 V DC. It does, however, carry high currents, so short circuits can cause arcing, burns and fire.
How long does it take for an electric car's networks to fall asleep?
It depends on make, equipment and what the vehicle is doing. Expect anything from a few minutes to more than half an hour after locking, and longer if charging, preconditioning or thermal management is active.
Do I need a high-voltage qualification to install a 12 V device in an electric car?
Work on high-voltage components requires one. Low-voltage-only work generally does not, but you must know where the high-voltage boundaries are, follow the manufacturer's procedures and comply with national and employer rules.
