TECHNICAL FIELD

Embodiments of the present invention relate to an energy system for an electrically-driven vehicle and to methods for operating the energy system. The energy system has a low-voltage charging function and an active load-balancing system for traction batteries.

DESCRIPTION OF THE RELATED ART

Today's electric vehicles have a high-voltage vehicle electrical system to which various consumers are connected, e.g., a charger (on-board charger, OBC), DC voltage converters for converting high voltage into low voltage (HV-LV DC/DC converters), traction converters, heaters, air-conditioning compressors, etc. At present, electric and hybrid vehicles are mainly charged using the alternating current network (AC charging infrastructure). However, investigations show that, in the future, charging in the private as well as in the public area will take place via direct current charging (DC charging).

Currently, two expensive HV control units are installed for charging the HV battery and for supplying the 12 V vehicle electrical system, wherein the OBC is active only when the vehicle is at a standstill and the HV-LV DC/DC converter is active only when driving. In addition, only a passive load balancing currently takes place between the individual battery cells (passive cell balancing) in the HV energy store (HV battery). In order to keep the individual cells at the same voltage level, excess energy of the cells is converted into heat (power loss) and dissipated.

DE 10 2009 033 185 A1 discloses a charging system and charging methods for charging a battery of a motor vehicle. Instead of an on-board charger, the charging system comprises an additional converter which performs the function of the OBC. The integration of the second converter is implemented by a double converter. The second converter converts the AC mains voltage into a DC charging voltage suitable for the HV battery. In order to charge the 12 V battery when driving, an HV-LV DC/DC converter is required.

WO 2018 107 065 A1 relates to an intelligent energy storage system, e.g., for an electric vehicle, which has a series of energy storage modules, which in turn have a series of module cells, each equipped with a bi-directional DC voltage converter and serving for cell current balancing.

BRIEF SUMMARY

Against the background outlined above, embodiments of the invention are based upon the aim of providing an energy system for an electric vehicle which has neither an on-board charger nor an HV-LV DC/DC converter, and also methods for operating the energy system.

The aim is achieved by a device with the features described herein and by methods with the features described herein.

The device according to embodiments of the invention manages completely without an OBC and without an HV-LV DC/DC converter. Since, in the future (after 2025), vehicles will be charged 99% via DC charging, the AC charger can be omitted. For the remaining 1%, the new architecture provides low-voltage DC charging, e.g., of 12 V.

Embodiments of the invention relate to an energy system for an electrically-driven vehicle. The energy system comprises a traction circuit having at least one HV energy store (HV battery, traction battery) that comprises several storage cells connected in series, and a traction inverter that supplies alternating voltage to at least one electric machine of the vehicle drive.

Each of the storage cells of the at least one HV energy store is assigned a bi-directional DC voltage converter with galvanic isolation, the inputs of which are connected to the poles of the storage cell and the outputs of which are connected to a low-voltage vehicle electrical system. In one embodiment, a bi-directional DC/DC converter is integrated into each battery cell. Since the DC/DC converters are bi-directional, energy flow can occur in both directions.

The energy system enables a vehicle architecture without an on-board charger (OBC) and without an HV-LV DC/DC converter. The functionalities of the two omitted HV control units are realized at the cell level of the at least one HV energy store by means of small, bi-directional DC/DC converters with galvanic isolation. The energy system manages with only one bi-directional DC/DC converter stage per cell. As a result, it is also possible to ensure energy exchange between any two cells of the at least one HV energy store, which is necessary for complete cell balancing.

The functionality of the HV-LV DC/DC converter is replaced in that, when driving the vehicle, the DC/DC converters convert the energy from HV to LV and thus charge the low-voltage vehicle electrical system (for example, a 12 V vehicle electrical system).

The functionality of the on-board charger (OBC) is replaced in that, during charging, the DC/DC converters convert the energy from LV to HV and thus charge the traction battery. In this case, the charging voltage is, conceptually, a protective low voltage (U<60 V), e.g., 12 V, which has no particular requirements as to contact protection and insulation.

In one embodiment of the energy system, the low-voltage vehicle electrical system is operated at a voltage of less than 60 V, e.g., at 12 V.

In one embodiment, the low-voltage vehicle electrical system has at least one low-voltage energy store (LV battery).

In one embodiment, the low-voltage vehicle electrical system has an interface for connecting a low-voltage DC charger (low-voltage charger).

In one embodiment, the traction circuit of the energy system has an interface for connecting a high-voltage DC charger (e.g., a charging station, charging column, or DC wallbox).

In one embodiment of the energy system, the at least one HV energy store has an output voltage in the range of 300 to 1,200 V, e.g., 400 V or 800 V.

The subject-matter described herein also includes methods for operating the energy system.

In one embodiment, the method includes charging the at least one HV energy store by connecting a low-voltage DC charger to the low-voltage vehicle electrical system and transferring electrical energy from the low-voltage DC charger to the storage cells of the at least one HV energy store via the low-voltage vehicle electrical system and the bi-directional DC voltage converters with galvanic isolation.

The charging operation takes place during a standstill of the vehicle. The HV charging is effected via the low-voltage vehicle electrical system, which, in one embodiment, is a 12 V vehicle electrical system. For charging, a 12 V charger can simply be connected to the jump-start connection point in the car, for example. The DC/DC converters convert 12 V to the necessary cell charging voltage and charge the HV energy store.

This low-voltage DC charging (charging with a protective low voltage, i.e., a voltage <60 V, LV charging) creates the prerequisites for automated charging since no special requirements as to insulation and contact protection have to be met.

In a further embodiment, the method comprises supplying the low-voltage vehicle electrical system and, optionally, charging the at least low-voltage energy store by transferring electrical energy from the at least one HV energy store, via the bi-directional DC voltage converters with galvanic isolation, to the low-voltage vehicle electrical system.

In a further embodiment, the method comprises a load balancing (“active cell balancing”) between the storage cells of the at least one HV energy store, in which electrical energy from at least one storage cell whose output voltage is higher than the average output voltage of all storage cells is transferred, via the bi-directional DC voltage converter with galvanic isolation and the low-voltage vehicle electrical system, to at least one storage cell whose output voltage is lower than the average output voltage of all storage cells.

Active cell balancing can increase the service life of the cells, increase efficiency, and identify and support weak cells. Active cell balancing also leads to an increase in energy efficiency, i.e., an increase in the range of the vehicle.

The advantages of the energy system also include the ability to dispense with an on-board charger together with an HV control unit, cooling tubing, mounting, and HV wiring, as well as an HV-LV DC/DC converter, including cooling tubing, mounting, and HV wiring. As a result, the total weight of the vehicle is also reduced, thereby increasing the range of the vehicle.

The energy system can be operated with HV energy stores of different output voltages. For example, the traction network can be operated at a voltage of 400 V or even 800 V.

The HV energy store of the energy system can be charged with a low-voltage energy source, i.e., with a voltage of less than 60 V. At such a low charging voltage, there are no special requirements as to insulation and contact protection. Endangerment of human life is ruled out. The safe voltage level (U<60 V) therefore even makes automating the charging process (automatic LV charging) possible.

Additional advantages and embodiments result from the description and the accompanying drawings.

It goes without saying that the features mentioned above and the features yet to be explained below can be used not only in the combination specified in each case but also in other combinations or alone.

