Upping the voltage

Upping the voltage

There is more to hybrid technology than simply having two forms of propulsion. With this in mind, Autodata tracks the progression of the 48V mild hybrid system, as well as the factors affecting how the technology is set to develop in the future.

Unlike the true hybrid configurations that use the engine and/or electric motor to propel the vehicle, the 48V mild hybrid system is employed in a typical stop-start vehicle with the integration of a 48V electric motor/generator assembly to supplement the engine, improving acceleration and enhancing fuel economy.

In addition, electrifying components such as the air conditioning compressor, power steering pump and engine oil pump further reduces the engine load and increases fuel efficiency.

Electrifying vehicles with a voltage higher than 12V is not new; the automotive industry contemplated using a 42V system in the 1990s, although this idea was later dismissed due to concerns about the cost, as well as practical matters such as switches and relays prematurely failing. However, driven by environmental concerns, modern electronics make use of transistors, diodes and microswitches, which are more robust, and thus make this a more viable option.

So, why stop at 48V? Current regulations state that anything over 60V officially becomes ‘high voltage’.

This adds extra cost due to the addition of expensive shielding, connectors and conduits like the orange ones commonly seen on many hybrid and electric vehicles. Yet, this doesn’t mean the electrical architecture of the entire vehicle will move to 48V. The conventional 12V supply is still used to power many of the standard circuits such as lights, door locks, electric windows and an infotainment system.

The common 48V mild hybrid consists of only a small number of additional components: electric motor/generator assembly, AC/DC inverter, DC/DC converter, a 48V battery and an e-charger.

Electric motor/generator assembly

The water cooled, belt driven electric motor/generator replaces the regular alternator and functions to restart the engine after a stop-start event, while the conventional 12V starter motor is used for normal starting via the ignition key.

At times, the electric motor/generator can support the engine to improve acceleration and reduce the load strategically to maximise fuel economy. In addition, the electric motor/generator, in generator mode, recharges both batteries, similar to a conventional alternator, but also when the vehicle is coasting or braking.

AC/DC inverter

The AC/DC inverter can either be integrated or non-integrated into the 48V electric motor/generator and exists to perform two functions. Firstly, it converts the direct current (DC) from the 48V battery to alternating current (AC), which then powers the electric motor/generator in motor mode.

Secondly, it converts the AC generated by the electric motor/generator whilst in generator mode to DC, which recharges the 12 and 48V batteries as a result.

DC/DC converter

As this vehicle encompasses both 12 and 48V systems, a DC/DC converter is installed to reduce the electrical voltage from 48 to 12V.

48V battery

The lithium-ion 48V battery is generally located in the rear of the vehicle, and just like the electric motor/generator, it can use the cooling system to dissipate heat.


The conventional turbocharger is superseded with an electrified version, which is better known as an e-charger. Instead of waiting for the exhaust gases to spin the impeller up to speed, an electric motor is used to drive the impeller, instantly providing the necessary boost without the familiar delay that is usually experienced with turbocharged engines. Alternatively, superchargers can also be electrified to provide the same results as an electric motor driven turbocharger.

Upping the voltage
1. Electric motor/generator assembly, 2. AC/DC inverter, 3. 48V battery, 4. DC/DC converter, 5. 48V power distribution unit, 6. 12V battery, 7. 12V power distribution unit, 8. E-charger

The 48V mild hybrid system is always evolving to fulfil exhaust emission regulations, improve fuel economy and increase acceleration. For this reason, vehicle manufacturers are already developing other intelligent  enhancements to complement the 48V system, with the following just some of the examples:

  • Dynamic skip fire (DSF) technology:

Integrates cylinder deactivation with the 48V mild hybrid system. The DSF system isolates a cylinder by disconnecting the camshaft followers, which locks the inlet and exhaust valves in the closed position when less power is required, resulting in better fuel economy

  • Extended stop-start technology:

Unlike the conventional stop-start technology of switching the engine off when the vehicle comes to a complete stop, the extended stop-start system will also switch off the engine when approaching a stop or while the vehicle is cruising at a constant speed.

  • Electrically heated catalytic converter:

In order to reduce the amount of harmful emissions, the catalytic converter must reach operating temperature as quickly as possible. Hybrid systems exacerbate this due to frequent stop-start events or coasting with the engine off. However, this can be easily solved by heating the catalytic converter electrically using the 48V system.

  • Electrifying engine driven ancillaries:

The water pump and air conditioning compressor are two examples of components that can be electrified. This reduces parasitic engine drag and more notably, allows the components to set their own duty cycle based on vehicle and driver demands.

So, whilst the current 12V system struggles, compared with the other more expensive true hybrids, the 48V mild hybrid technology offers a cost-effective solution in satisfying emission regulations and future increases in energy hungry electrical components.

The use of 48V technology is set to grow even further, especially as stricter COemission targets are being set and the decline of the internal combustion engine continues.

With the growth in popularity and demand for HEVs and EVs, Autodata is rolling out drive system diagrams for electric and hybrid vehicles.

For more information, click here.

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