DE102018221234B4 - Battery unit - Google Patents

Abstract

Battery unit (1), comprising a high-voltage battery (HV) consisting of a plurality of battery cells (3), and a heating device (5), wherein the heating device (5) is designed to alternately generate a discharge and charge current at the high-voltage battery (HV), wherein the heating device (5) has at least one switching element (S) and at least one capacitor (C), wherein
the high-voltage battery (HV) is divided into n sub-batteries (TB), where n ≥ 2, each sub-battery (TB) being assigned its own heating device (5), the heating devices (5) being assigned a common control unit (7) which is designed to control the heating devices (5) at different times, characterized in that
the at least one switching element (S) and the at least one capacitor (C) of the heating device (5) are connected in parallel, wherein a parallel circuit comprising an additional inductance (L _Z ) and a diode (D) is arranged in series with the parallel circuit comprising the switching element (S) and the capacitor (C).

Description

The invention relates to a battery unit with a high-voltage battery.

High-voltage batteries typically consist of a large number of battery cells connected in series, but also partially in parallel if necessary, to provide sufficient voltage (e.g., U > 48 V) and capacity. These battery cells are designed, for example, as lithium-ion battery cells. However, other battery cell types are also in use and under development. What these battery cells have in common is that they have a limited operating temperature range. Particularly when such high-voltage batteries are used as traction batteries in a motor vehicle, it is common practice to assign them a heating device to specifically preheat them. In the simplest case, an ohmic load is connected in parallel so that a discharge current flows, generating power loss across the internal resistance of the high-voltage battery and the ohmic load. Such heating devices have several disadvantages. If the ohmic load is dimensioned too large, the majority of the power loss is lost at it and is therefore not directly available as heating power. However, if this value is chosen to be very small compared to the internal resistance of the high-voltage battery, extremely large currents flow, which could damage the battery cells.

Therefore, heating devices have been proposed that can generate alternating discharge and charge currents, so that the heat loss is generated primarily at the internal resistance. Such a heating device is known, for example, from WO 2013 / 174 268 A1 known.

From the DE 10 2010 020 683 A1 A method for controlling battery pulse heating of a traction battery of a hybrid vehicle is known. A battery temperature is detected at start-up and fuel consumption for battery heating is determined, with a decision then being made as to whether or not to perform battery pulse heating. Battery pulse heating is implemented by assisting the internal combustion engine with the electric motor for discharging while driving and applying a load point increase to the electric motor for charging. However, this deliberately generated power loss for heating the traction battery results in additional fuel consumption. The method is not suitable for purely electric vehicles or other applications.

From the DE 10 2013 226 372 A1 A battery unit is known which has at least one battery module and a heating device, wherein the heating device is designed as a flyback converter.

From the DE 10 2016 205 333 A1 A method for tempering a charged battery is known, wherein at least one discharging and charging cycle is carried out in a warm-up process to temper the battery, wherein in each discharging and charging cycle, energy is first withdrawn from the battery in a discharging process and then energy is supplied to the battery again in a charging process.

From the DE 10 2010 032 088 A1 A generic battery unit is known.

The invention is based on the technical problem of creating a battery unit with an improved heating device.

The solution to the technical problem is provided by a battery unit having the features of claim 1. Further advantageous embodiments of the invention emerge from the subclaims.

For this purpose, the battery unit comprises a high-voltage battery consisting of several battery cells. The high-voltage battery is divided into n sub-batteries, where n ≥ 2. Each sub-battery is assigned its own heating device, wherein the heating devices are designed to alternately generate a discharge and charge current at the high-voltage battery or the respective sub-battery. The heating devices each have at least one switching element and at least one capacitor. A common control unit is assigned to the heating devices, which is designed to control the heating devices at staggered times. By implementing several sub-batteries, each of which is assigned its own heating devices that are controlled at staggered times, the battery voltage is prevented from collapsing completely, which can have adverse effects on connected electronics. Even with a sub-division into two sub-batteries, the voltage drop is limited to half the battery voltage. The perceived additional effort due to the multiple heating devices is further offset by the fact that the requirements for dielectric strength are reduced, allowing the use of more cost-effective components. It should be noted that if there are several heating devices, not all of them have to be controlled at different times, but individual heating devices can also be operated simultaneously or some do not have to be controlled at all. For example, if only a subset of battery modules is modules are under-tempered, it makes sense to configure the phase control only for this subset.

The at least one switching element and the at least one capacitor of the heating device are connected in parallel, forming a series resonant circuit with the internal inductance of the high-voltage battery or sub-battery. The switching elements are designed, for example, as MOSFETs or IGBTs.

Furthermore, a parallel circuit consisting of an additional inductor and a diode is arranged in series with the parallel circuit of the switching element and capacitor. The additional inductor primarily serves to limit the discharge current, while the diode then limits the voltage during refeeding, thus preventing interference with nearby electronics such as battery management control units or battery module control units. The diode effectively short-circuits the additional inductor during refeeding and limits the voltage to the forward voltage of the diode.

In one embodiment, the high-voltage battery is composed of battery modules, each having a number of battery cells, with each battery module or battery modules grouped together being assigned a heating device. For example, battery modules connected in parallel are grouped together with a heating device. The advantage is that, in addition to the heating function, the heating device can be used for balancing. This is advantageous, for example, when replacing a battery module during servicing. In this way, a higher state of charge of the newly installed battery module compared to the existing battery modules can be adjusted by cyclical removal and reinsertion of charge by converting it into heat. If the state of charge of the new battery module is lower, the "old" battery modules can be selectively discharged in a similar way.

In another embodiment, each battery cell is assigned a heating device. This allows for cell balancing in addition to further reducing voltage drops.

In a further embodiment, the battery unit has at least one relay to galvanically isolate the heating devices from the high-voltage battery. This allows the switching elements of the heating devices to be de-energized when the heating devices are not needed. Furthermore, in the event of a fault (e.g., a short circuit in the heating device), the heating device can be switched off without any adverse effects.

In a further embodiment, each heating device has a relay.

A more preferred application of the battery unit is as a traction battery for an electric vehicle.

• 1 ein schematisches Blockschaltbild einer Batterieeinheit mit Heizeinrichtungen einer ersten Ausführungsform,
• 2 ein schematisches Blockschaltbild einer Heizeinrichtung in einer zweiten Ausführungsform,
• 3 ein schematisches Blockschaltbild einer Batterieeinheit mit einer nichterfindungsgemäßen Heizeinrichtung und
• 4a-4c Steuer-, Strom- und Spannungsverläufe einer Heizeinrichtung gemäß 3.

The invention is explained in more detail below using preferred embodiments. The figures show:

• 1 a schematic block diagram of a battery unit with heating devices of a first embodiment,
• 2 a schematic block diagram of a heating device in a second embodiment,
• 3 a schematic block diagram of a battery unit with a heating device not according to the invention and
• 4a-4c Control, current and voltage curves of a heating device according to 3 .

Before the invention is explained in more detail, it should first be explained with reference to the 3 and the 4a -c the problem underlying the invention will be explained in more detail.

The battery unit 1 comprises a plurality of battery modules 2, each of which has a plurality of battery cells 3. Each of the battery modules 2 is assigned a battery module control unit 4, which monitors the temperature and charge state of the battery cells 3. The entirety of the battery cells 3 forms the high-voltage battery HV, which has an internal resistance R _i and an internal inductance L _i . The battery unit 1 also has a heating device 5, which has a relay 6, a capacitor C, and a switching element S. The relay 6 and the switching element S are controlled by a control unit 7, with the heating device 5 and the control unit 7 forming a heating module 8. The control unit 7 does not have to be designed as a separate control unit, but can be implemented in another control unit. The battery unit 1 also has a battery management control unit 9. The battery management control unit 9 is supplied with voltage from the high-voltage battery HV via a DC/DC converter 9a. Furthermore, the battery unit 1 has fuses 10, 11 and main contactors 12, 13 for galvanic isolation of the battery unit 1 from the remaining traction network, consisting of the intermediate circuit capacitor C _Z , the pulse inverter 14 and the electric machine 15. The battery unit 1 can have further elements such as pre-charging circuits and sensors sensors that are not shown here for reasons of clarity.

If temperature sensors (not shown) of the battery unit 1 detect that the temperature of the battery cells 3 is too low, the heating device 5 is activated, whereby relay 6 is initially closed. The switch S is still open and the capacitor C is charged to a voltage corresponding to the voltage of the high-voltage battery HV. Then the control unit 7 generates a short control pulse SP of a few µs (e.g., 5-10 µs) and closes the switch S, which is preferably designed as a transistor (see 4a) This short-circuits the high-voltage battery HV and the capacitor C, causing a large discharge current to flow. This causes thermal power loss at the internal resistance R _i , which heats the battery cells 3. At the same time, energy is stored in the internal inductance L _i in the form of a magnetic field.

If the switch S is then opened, the energy stored in the internal inductance L _i drives the current further. This current charges the capacitor C to a voltage U _C = U _Batt + L _i dl/dt. At the end of this capacitor charging phase, the current from the capacitor charged to U _C > U _Batt begins to flow back into the high-voltage battery HV, whereby heating power is again generated directly in the battery cells 3 at the internal resistance R _i . A disadvantage, however, is the repeated voltage drop between HV+ and HV- in the phases with the switch S closed or the transistor switched on (see also 4c These voltage drops can have repercussions on the battery module control units 4 and, via the DC/DC converter 9a, on the battery management control unit 9. Additional problems can arise if the heating operation occurs while driving with the main contactors 12, 13 closed. In this case, repercussions on the intermediate circuit capacitor C _Z as well as the pulse-controlled inverter 14 and its power electronics are also possible. Furthermore, the very large, high-frequency charging and discharging currents lead to component stress.

These problems are now sustainably reduced by the invention, which can now be seen on the basis of 1 should be explained in more detail.

The same elements are identified by the same reference numerals as in 3 The main difference to the battery unit 1 according to 3 is that the high-voltage battery HV is divided into sub-batteries TB, where 1 a division into two sub-batteries TB1 and TB2 is shown. Each sub-battery TB1, TB2 is assigned its own heating device 5, with the heating devices 5 being controlled at different times by the control unit 7. If the upper heating device 5 is controlled first and the decay processes are waited for, with the lower heating device 5 then being controlled, the voltage drop is limited to half the battery voltage. It is understood that by dividing the high-voltage battery HV into several sub-batteries TB, the voltage drop can be further reduced. For example, a heating device 5 is assigned to each battery module 2 or to each battery cell 3, so that the heating devices 5 can also be used for module or cell balancing.

As already explained, the individual heating devices 5 can be controlled at different times so that the decay processes are completed. However, it is also possible to control the heating devices 5 in such a way that a heating device 5 is controlled when the preceding heating device 5 is in the process of overshooting the voltage (see also 4c ). This allows the voltage drop to be further reduced. The effects of the voltage drop are thus minimized, particularly at the DC/DC converter 9a and thus the battery management control unit 9 as well as the intermediate circuit capacitor C _Z. The battery module control units 4 can, if necessary, be galvanically isolated from the battery modules 2 by switching elements during heating operation.

Since the current in the discharge phase of the sub-battery TB increases very rapidly with the time constant τ = L _i /R _i (L _i and R _i of the respective sub-battery), the component load is very high. This problem can be solved by a circuit according to 2 be reduced. In this case, a parallel circuit of an additional inductance L _Z and diode D is connected in series with the relay 6 and the parallel circuit of the capacitor C and switching element S. In the discharge case, the diode D is polarized in the reverse direction and the internal inductance L _i is connected in series with the additional inductance L _Z. This increases the time constant τ and limits the current rise. This increased inductance L _i + L _Z would now also lead to a correspondingly increased voltage level at the capacitor C. This is now limited by the diode D, which limits the induced voltage at the additional inductance L _Z to the forward voltage of the diode D.

Furthermore, the "short-circuiting" of the additional inductance ensures that the feedback current from capacitor C into the sub-battery TB is only slightly limited by the additional inductance _LZ . Instead of the diode, a switching element could also be provided that shorts the additional inductance _LZ during the return flow. However, this is more complex than the simple diode D.

List of reference symbols

1

Battery unit

2

Battery module

3

Battery cell

4

Battery module control unit

5

Heating device

6

relay

7

control unit

8

Heating device

9

Battery management control unit

9a

DC/DC converter

10

Backup

11

Backup

12

Main contactor

13

Main contactor

14

Pulse inverter

15

electric machine

Ri

Internal resistance

Li

Internal inductance

CZ

DC link capacitor

S

switching element

C

capacitor

SP

control pulse

TB

Partial battery

LZ

Additional inductance

D

diode

Claims (5)

Battery unit (1), comprising a high-voltage battery (HV) consisting of a plurality of battery cells (3), and a heating device (5), wherein the heating device (5) is designed to alternately generate a discharge and charge current at the high-voltage battery (HV), wherein the heating device (5) has at least one switching element (S) and at least one capacitor (C), wherein the high-voltage battery (HV) is divided into n sub-batteries (TB), where n ≥ 2, wherein each sub-battery (TB) is assigned its own heating device (5), wherein the heating devices (5) are assigned a common control unit (7) which is designed to control the heating devices (5) with a time offset, characterized in that the at least one switching element (S) and the at least one capacitor (C) of the heating device (5) are connected in parallel, wherein a parallel circuit comprising an additional inductance (L _Z ) and a diode (D) is arranged in series with the parallel circuit comprising the switching element (S) and the capacitor (C). is. Battery unit after Claim 1 , characterized in that the high-voltage battery (HV) is composed of battery modules (2), each having a number of battery cells (3), each battery module (2) or battery modules (2) combined in groups being assigned a heating device (5). Battery unit after Claim 1 , characterized in that each battery cell (3) is assigned a heating device (5). Battery unit according to one of the preceding claims, characterized in that the battery unit (1) has at least one relay (6) in order to galvanically separate the heating devices (5) from the high-voltage battery (HV). Battery unit after Claim 4 , characterized in that each heating device (5) has a relay (6).