Mild Hybrid Electric Vehicle (MHEV) – electrical architecture

In this article (tutorial) we are going to discuss about the electrical architecture of a mild hybrid electric vehicle (MHEV). By electrical architecture (also called electrical topology) we understand the high level overview of the electrical components and the connections between them.

Before going through this article, for a better understanding of hybrid electric vehicles (HEVs) in general and MHEVs in particular, read the following articles:

12V electric system architecture

The electrical architecture of a classic 12V (non hybrid) road vehicle consists mainly of:

  • a source of energy (battery)
  • a starter
  • a generator
  • several electrical loads

The battery needs to supply all the required electrical energy during the engine start, which includes the starter, ignition system, fuel injection system, various electronic control units, safety and comfort electronic devices, lighting, and any other electrical equipment.

12V electric system architecture - engine start

Image: 12V electric system architecture – engine start (example)

The starter can be also considered as a load, becasue it’s only consuming electrical energy during engine start-up phases.

After the engine has been started and it runs autonomous, the generator needs to supply all the electrical energy to the loads and also charge the 12V battery.

Depending on the operating conditions of the vehicle and engine, the amount of electrical current drawn from the battery varies, as described in the table below.

Vehicle stateCurrent drawn from the battery [A]
Stationary with the engine off0.01 … 0.05 (e.g. for clock, the antitheft alarm system or the remote-controlled central locking system)
Stationary with engine idle (or slowly driven)20 … 70
Engine start300 (for 0.3 to 3 s) with peaks of 1000 A

Source: Bosch

The lower the engine temperature, during starting, the higher the friction torque, the higher the current drawn from the battery. At low temperature, the starting current can be around 600 A with peaks of 1000 A.

12V electric system architecture - engine running

Image: 12V electric system architecture – engine running (example)

For example, let’s assume that during engine start the battery voltage U is 12 V and the current I is 410 A. The electrical power P [W] required will be:

\[P = U \cdot I = 12 \cdot 400 = 4920 \text{ W} = 4.92 \text{ kW}\]

12-48 V MHEV electric system architecture

If we had to provide the same electrical power 4.92 kW (during engine start), from a 48 V electrical network, the required battery current would be:

\[I = \frac{P}{U} = \frac{4920}{48} = 102.5 \text{ A}\]

As you can see, a 48 V electrical network can provide the same electrical power but with a lower current. The advantage is that the conductors (wires) will require a smaller diameter (lower mass) and the losses (joule effect) will be reduced as well.

It’s obvious that, the higher the voltage of an electrical system, the higher the efficiency and performance. On the other hand, higher voltages could trigger electric shocks for the human user. Voltages under 50 V are considered safe from the electric shock point of view and do not require special treatment compared with 12 V systems.

European Union has adopted the Low Voltage Directive (LVD) 2014/35/EU, which ensures that electrical equipment within certain voltage limits, provides a high level of protection for European citizens, and benefits fully from the Single Market. Electrical equipment under the LVD covers a wide range of consumer and professional products e.g. household appliances, cables, power supply units, laser equipment and some components such as fuses.

The LVD covers all health and safety risks of electrical equipment operating with a voltage between 50 and 1000 V for alternating current and between 75 and 1500 V for direct current. These voltage ratings refer to the voltage of the electrical input or output, not to voltages that may appear inside the equipment.


Electric systems operating at 48 V will not add additional health and risk requirements for the vehicle manufacturers and, in the same time, will provide clear benefits in terms of maximum available electrical power, high efficiency and lower mass.

The German Association of the Automotive Industry (VDA – Verband der Automobilindustrie) has defined a standard (VDA Recommendation 320) which covers the electric and electronic systems in motor vehicle with 48 V power supply.

The document defines the functional requirements for the electric system as well as the test scenarios and tests performed on electric, electronic and mechatronic components and systems for use in road vehicles with a 48 V on-board power supply. The standard is also defining the voltage ranges for 48 V electric systems.

Definitions of voltage ranges for 48V systems acording to VDA 320

Image: Definitions of voltage ranges for 48V systems according to VDA 320
Credit: VDA

The voltage limits are explained in the table below:

Voltage limitsDescription
U48r – U48r,dynVoltage tolerance (2 V)
U48max,unlimited – U48max,high.limitedUpper limited operation range (intended for calibrating the storage medium and for the uptake of recovered energy)
U48min,unlimited – U48max,unlimitedUnlimited operation range (allows the components to operate without restriction)
U48min,low,limited – U48min,unlimitedLower limited operation range (the system may operate only temporarily in this range; countermeasures should be taken to return to the unlimited operation voltage range)
below U48min,low,limitedUndervoltage
below U48stopprotectStorage protection voltage

Source: ZVEI – German Electrical and Electronic Manufacturer’s Association

The maximum voltage (60 V) is the maximum permissible contact voltage safe for human operators therefore the system is not classified as “high voltage” with the risk of electrical shock. Standard 48 V electrical systems used in MHEVs are considered to be reasonably safe for most people under normal conditions.

A mild hybrid electric vehicle (MHEV) has a dual electrical architecture, which consists of a 12 V network connected through a DCDC to a 48 V network.

MHEV 12-48V electric system architecture

Image: MHEV 12-48V electric system architecture

The main difference is that the 12 V generator doesn’t exist anymore since its function is taken over by the 48 V electric machine. All the energy required by the 12 V network to operate is supplied from the 48 V network through the DCDC converter.

Most of the 48 V MHEV still use a 12 V starter, even if the 48 V electric machine can provide faster engine starts. There are two reasons behind this. First, during engine start, at low temperature, the belt of the 48 V BiSG might slip, therefore cold engine starts are performed with 12 V starter. Second, in case of a failure of the 48 V, the engine stop & start sequences are going to be fulfilled by the 12 V system.

The 48 V electric machine requires a 3-phase alternating current (AC), therefore an inverter is integrated with the electric machine.

Electrical architecture of a dual voltage board net with a 48 V system

Image: Electrical architecture of a dual voltage board net with a 48 V system
Credit: Continental


AC – Alternatinc Current
DC – Direct Current
IBat [A] – battery current
Iph [A] – phase current
L1, L2, L3 – phases
PDC [W] – inverter input power
Pel [W] – electrical power of the motor
Pme [W] – mechanical power of the motor
R1 [Ω] – battery internal resistance
RL0 [Ω] – resistance of the wiring harness
RL1, RL2, RL3 [Ω] – resistance of the phases
U0 [V] – battery internal voltage
UBat [V] – battery output voltage
UL-L [V] – voltage drop between two phases

The inverter input power is equal with the battery power minus the DCDC input power.

\[P_{DC}=P_{Bat}-P_{DCDCin}=U_{Bat} \cdot I_{Bat} – \eta_{DCDC} \cdot P_{DCDCout} \tag{1}\]


PDCDCin [W] – DCDC input power
PDCDCout [W] – DCDC output power
ηDCDC [-] – DCDC efficiency

The electrical power of the motor is equal with:

\[P_{el}= P_{DC} \cdot \eta_{inv} \tag{2}\]


ηinv [-] – inverter efficiency

The mechanical power of the motor is equal to:

\[P_{me} = P_{el} \cdot \eta_{em} \tag{3}\]


ηem [-] – electric machine efficiency

The BiSG power that is supplied to the crankshaft is equal to:

\[P_{BiSG} = P_{me} \cdot \eta_{belt} \tag{4}\]


ηbelt [-] – efficiency of the belt and pulley mechanism

Replacing (1) in (2) in (3) in (4) gives the expression of the BiSG power at the crankshaft function of battery power and components efficiency:

\[\bbox[#FFFF9D]{P_{el}= \eta_{belt} \cdot \eta_{em} \cdot \eta_{inv} \cdot \left ( U_{Bat} \cdot I_{Bat} – \eta_{DCDC} \cdot P_{DCDCout} \right )}\]

The electric system of a hybrid electric vehicle (HEV) has to perform the following main functions:

  • output electric energy to the powertrain during motoring phases
  • store the electrical energy during reccuperation/regeneration phases
  • supply electrical energy to the consumers (loads), both 12 V and 48 V

The high voltage electrical system has a high voltage battery with a precharging circuit, an electric machine with an integrated inverter and a DCDC converter with a DC link capacitor.

Electrical system topology (architecture) of a MHEV

Image: Electrical system topology (architecture) of a MHEV


  1. 48 V electric machine (motor-generator)
  2. inverter (power electronics)
  3. DC link capacitor
  4. pre-charging capacitor
  5. main contactors
  6. pre-charging resistor
  7. high voltage battery (48 V)
  8. 12 V starter (motor)
  9. electrical loads
  10. low voltage battery (12 V)
  11. DCDC converter

The DC link capacitor is used to stabilize the DC voltage in case of sudden load variations (caused by the inverter).

When connecting a high voltage battery to a load with capacitive input, there is an inrush of current as the load capacitance is charged up to the battery voltage. With large batteries (with a low source resistance) and powerful loads (with large capacitors across the input), the inrush current can easily peak 1000 A.


In order to avoid a surge of the battery current when the main contactors (5) are closed, the battery has a pre-charging circuit. When transitioning from an OFF to an ON state of the high voltage battery, the operations are as follows:

High voltage battery stateDescription
OFFAll contactors are open
Pre-chargeThe low side of the main contactor (5) is closed and the pre-charge contactor (4). Due to the pre-charging resistor (6), the inrush current (from the battery) is limited. Without this resistor, closing the contactors would generate a large amount of inrush current causing the contacts to arc.
ONAfter the battery current has stabilized, the upper side of the main contactor (5) is closed and the pre-charge contactor (4) is open

Since using an electrical network with higher voltage is more efficient than low voltage (due to lower electrical currents), more and more vehicle systems and components are being migrated to the high voltage (48 V) system.

Delphi - 48V MHEV electrical architecture

Image: 48V MHEV electrical architecture
Credit: Delphi

Current generations of 48 V mild hybrid electric vehicles (MHEV) have already air conditioning compressors, electric superchargers, heating elements, etc. connected to the high voltage bus.

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