What happens under the sheet metal of an electric car using the smart #1 as an example

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From the charging socket to the street


Of course, it all starts with the charging socket. The so-called Combo 2 socket contains the type 2 connector for slow charging and the CCS connector for fast charging. Cleverly combined so that you don't have to worry about what's what and what has to go where. Therefore, in general usage, it is simply the CCS socket.

In electric cars, all cables with a voltage of 60 V and more are orange. This is a good way of distinguishing between the high-voltage cables (HV cables) and the 12 V cables. The CCS socket has two cable strands. Because DC is used for fast charging, it goes directly into the battery. But because for slow charging you use AC, this cable harness must first be placed in the charger in the car. This converts the AC current into DC current, with which the battery can then be charged.


What happens under the sheet metal of an electric car using the smart #1 as an example



In the case of the smart #1, the AC line is routed to the front. There the charger is apparently integrated into the inverter. But one after the other. After the power is stored in the battery, you have to get it to the motor somehow. Since electric motors run on so-called three-phase current, you need the inverter mentioned above. The inverter turns the 400 V system voltage of the smart #1 into the three-phase current for the motor. This is then used to drive it, the term three-phase current comes directly from electric motors. In fact, three-phase motors in particular cannot work without them. Although there are also DC motors, they are less efficient and have therefore not been used in electric cars for decades.

In the electric car, the inverter determines how fast the motor should rotate. To do this, he varies the frequency of the three-phase current.


What happens under the sheet metal of an electric car using the smart #1 as an example



After the three-phase motor there is a reduction gear. This reduces the motor speed, in the case of the smart #1 up to 16500 rpm, to the speed that then arrives at the wheel. The reduction gear is usually attached directly to the electric motor and also includes the differential. The all-wheel drive variant of the smart #1 has a motor with inverter, reduction gear and differential at the front and rear. The two motors are not mechanically connected to each other. If you wanted it, the front motor could also run forwards while the rear one runs backwards and with completely different speeds between the front and rear.



Heating, air conditioning and 12 V system


The air conditioning is also operated by the HV system, in the case of the smart #1 with an integrated heat pump and, if necessary, a PTC heater. The air conditioning compressor from the air conditioning and heat pump is operated with the HV system. In most electric cars, it has an output of around 4 kW, but rarely runs at more than 0.5 to 1.5 kW in everyday operation. Again, the fan in the cooling module of the air conditioning runs with the usual 12 V, just like in the combustion engine. If the heating for the interior also includes a PTC heater, this is also controlled via HV.


What happens under the sheet metal of an electric car using the smart #1 as an example



Modern electric cars also have one or more cooling circuits for the battery, charger, motor and inverter. Especially for the battery, these not only have the option of cooling, but also of heating for the winter. Depending on the size of the vehicle, you use an auxiliary heater in the 12 V range or HV range, which then heats up the cooling water in a classic way. In the case of the smart #1, this probably runs above the HV, although it doesn't appear to be marked in the photos.



The 12 V mains, the DCDC converter and the interlock circuit


Even if an electric car has a HV system and a correspondingly large battery, it still has to have a 12 V battery with it. This is fed by the so-called DCDC converter, it is practically the alternator of the electric car. However, it is not connected to the electric motor, but works like a charger. It is called a DCDC converter because it converts the 400 V DC into 12 V DC. This is then used to feed the radio, the lights and things like the seat heating, for example. In addition, of course, all control units in the car.



The DCDC converter is permanently active during the charging process and while driving, so that the electric car could also do without the 12 V battery. The 12 V battery is important when the electric car is switched off. Because then the HV battery is also switched off and all HV cables in the vehicle are voltage-free, this is for safety. However, control units such as that of the smartphone app, the battery management system (BMS) must still function. Furthermore, the car must be able to wake up again when you unlock it. With the help of the 12 V battery, the HV battery is also started again and the DCDC converter is supplied with voltage again.


Service-Disconnect & Interlock-Kreis DE



The last important thing is the so-called interlock circuit and the service disconnect. The interlock circuit is effectively an additional electrical connection that is routed through all HV connectors, making sure everything is plugged in. An electric car could also drive without the connected air conditioning system, for example. But then there would be an unplugged plug somewhere and thus the HV network would also be open. To prevent this hazard and also to ensure that each plug is fully inserted, the car continuously checks that the interlock circuit is closed during operation.

For workshops there is also the option of using the so-called service disconnect in every electric car. This is a plug in the interlock circuit and when you pull it the circuit is open. This ensures that during repairs to the HV network and HV components, the vehicle never starts the battery and apply the 400 V.


In the smart #1, the service disconnect for the HV battery is at the front under the bonnet next to the 12 V battery. The small and inconspicuous plug with the loop.

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