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Car manufacturers have been dynamically developing the segment of hybrid vehicles for several years, introducing a number of different solutions and configurations. More and more vehicles are available in the so-called micro-hybrid, mild-hybrid or full hybrid version. Among these groups of vehicles, the mild-hybrid technology, based on a 48V system, is developing dynamically.The dynamic development of vehicles with a 48V system results from the fact that the use of this technology does not require major structural changes in the vehicle, and allows achieving satisfactory results from the point of view of environmental protection and performance. The use of a 48V system allows for a number of additional functions, e.g. extended function of the start-stop system, e-boost function, i.e. support of the internal combustion engine by an electric motor, or recuperation function, that is switching the electric motor to generator operation mode, in order to power the 48V system battery. In addition, thanks to this solution, the so-called “sailing” function can be implemented, i.e. switching off the internal combustion engine when the brake and accelerator pedals are released. 
System construction
Please note that the 48V system is an additional system and does not replace the 12V system, but works closely with it. The configuration of the 48V electrical system includes a 48V battery, an electric motor (the so-called alternator-starter), a DC/DC converter and auxiliary installation cables. More and more often, especially in highly equipped vehicles, the auxiliary system is used to power additional loads that require high power, including: electromechanical stabiliser motors (eAWS system) or electric charging air compressors. The 48V system battery does not replace the 12V battery, it is an additional battery. It is made in lithium-ion technology. The use of this technology forces the use of elements that monitor the voltage on the cells of this battery and manage the temperature. This is the responsibility of the 48V battery controller, which is located in the housing of this battery. Inside the auxiliary battery or by the battery itself there is a fan for its cooling and an internal power contactor connecting the battery with the loads of the auxiliary system. The auxiliary battery is usually located inside the vehicle, in the luggage compartment or under the passenger seat. 
Rys.1 Akumulator instalacji 48 woltowej [materiał własny]
Fig. 1 48V system battery [own material] 
The DC\DC converter is an element that connects the 12V system with the 48V system. It takes over the role of the alternator, as it is responsible for charging the 12V battery by taking energy from the 48V system. The converter, like the battery, is also equipped with a cooling fan. It is usually located inside the vehicle or in the engine compartment. The electric motor, the so-called alternator-starter, is installed in the engine compartment, connected to the crankshaft pulley by means of an accessory belt. The electric motor is responsible for charging the 48V battery, starting/restarting the internal combustion engine, and assisting the internal combustion engine during acceleration. The electric motor works as a generator, powering the 48V battery, similar to an alternator, and can run in engine mode when starting and supporting the internal combustion engine, where power is taken from the secondary battery. A large angle of wrap of the accessory belt of the alternator/starter pulley is required for the indicated functions to be performed. For this purpose, two tensioners are used, which ensure proper belt tension regardless of the operating phase. In selected solutions, the electric motor is cooled with liquid from the cooling system. The elements of the 48V system are connected by characteristic bright purple wires.
Service problems
When servicing vehicles with a 48V system, you should pay special attention to occupational safety. Additional electrical licences are not required to operate this type of system, but you should follow appropriate safety procedures to eliminate the risk of accidents. On select vehicle brands, it is a basic safety procedure to deactivate the 48V system before performing specific operations. The deactivation procedure is required, among others, when replacing auxiliary system components. To carry out the disconnection procedure, use a suitable diagnostic device with this function. You should follow additional maintenance procedures for vehicles with a damaged system or 48V battery, such as in an accident.Service challenges in 48V systems that can occur in accident vehicles relate not only to mechanical damage, but also to blocking of the 48V system for safety reasons. As an example, we will use a Volvo vehicle, the 2021 model XC 60, with a 2.0-litre diesel engine delivering 145 kW. In this vehicle, when an event impulse is detected by the airbag controller, the system operation algorithm may decide to disconnect the auxiliary battery from the 48V system. The algorithm makes this decision regardless of whether the pyrotechnic elements for the protection of the driver and passengers, i.e. seat belts or airbags, have been activated. After disconnecting the auxiliary system as a result of detecting, for example, a collision, an appropriate error code is saved in the memory of the controller that supports the 48V system battery. Example below:

5. Memory error

B16527F Crash detected

Component disconnected

Fig. 2 Error code of high-voltage disconnection as a result of a collision read in the 48V system battery controller [BOSCH KTS diagnostic tool]
The disconnection of the system is carried out by the disconnecting relay, which is located inside the auxiliary battery, and its status can be verified by analysing the actual parameters of the controller. Reactivation of the relay is impossible while the error code B16527F is stored in the controller’s memory. Usually, it is impossible to remove the defect in question; when it is registered, in accordance with the manufacturer’s recommendation, you should replace the 48V system battery controller. On this particular vehicle, the battery controller is located inside the auxiliary battery and is replaceable with the entire battery.

48V Battery, Disconnect Relay: switch status

Status: component open

Fig. 3 Status of the 48V system disconnecting relay read out in the battery controller [BOSCH KTS diagnostic tool]
Another service problem may be excessive discharge of the 48V battery. Insufficient battery charge can be caused by, among other things: operational aspects, e.g. frequent starting/stopping of the combustion engine, frequent manoeuvering and use over short distances and in low temperatures. In these situations, the driver is informed of the problem by the appropriate battery indicator lights (yellow or red) and an error message. When the charge level of the auxiliary battery drops below 30%, a message will appear in the instrument cluster. In this case, stop the vehicle in a safe place and let it idle until the light and message go off. Another cause of abnormal auxiliary battery charge may be a problem with the electric motor or its accessories. When supporting an internal combustion engine, electric motors generate power at the level of 10 kW and more, which puts a lot of strain on the accessory belt and rollers, tensioners, and thus their accelerated wear. Accessory belts used in vehicles with a 48V system are additionally reinforced with aramid fibres, due to the loads they carry, and must be installed with appropriate procedures, i.e. they cannot be bent or twisted during assembly. At the moment, electric motors, in accordance with the manufacturer’s recommendations, are not subject to repair, but to replacement.  Note that faults in the 48V system directly affect the 12V system, and can result, for example, in a lack of charging of the main electrical system. From previous experience, this type of fault is most often caused by the failure of the DC/DC converter, which is responsible for charging the 12V battery from the 48V system. Similarly to electric motors, DC/DC converters, according to the manufacturer’s technology, are not repairable, they are subject to replacement.
Vehicles with modern power systems are becoming increasingly popular on the market. Every repair workshop that wants to be competitive and sees the need for development absolutely must be prepared to service vehicles of this type, from 48V systems to high-voltage systems. To meet these challenges, this topic will be developed in subsequent series of articles, where real problems and service cases as well as diagnostic and repair procedures for this type of vehicles will be presented and described.