WO2007071425A1

WO2007071425A1 - Plate or strip for producing connectors, connector and use thereof, and process for manufacturing such a strip

Info

Publication number: WO2007071425A1
Application number: PCT/EP2006/012417
Authority: WO
Inventors: Guido Plankert; Peter Arthur Boehmer; Hans-Günther STEINMANN; Tilmann Strotbek; Markus Himbert
Original assignee: Hille & Müller GMBH
Priority date: 2005-12-23
Filing date: 2006-12-21
Publication date: 2007-06-28

Abstract

The invention relates to a plate or strip used for producing connectors which connect individual battery cells or battery modules, the plate or strip comprising a first layer comprising a layer of copper or copper alloy or aluminium or aluminium alloy, and a second layer comprising a layer of carbon steel, the first and second layer being joined together.

Description

PLATE OR STRIP FOR PRODUCING CONNECTORS, CONNECTORAND USE THEREOF, AND PROCESS FORMANUFACTURING SUCH A STRIP

The invention relates to a plate or strip used for producing connectors which connect individual battery cells or battery modules. The invention also relates to the connector manufactured from such a plate or strip and the use thereof, and to a process for manufacturing such a strip.

Connectors which connect battery cells inside a battery have to have a good electrical conductivity and a good weldability. Such connectors are for instance used inside nickel-metal hydride (NiMH) batteries and Li-Ion batteries. NiMH batteries are used for portable electronic devices, such as communication equipment and laptop computers. NiHM batteries are nowadays also developed for electric vehicle applications, such as electric bicycles and electric cars, for instance hybrid electric cars, where large batteries are used. Such batteries typically consist of individual cells which are connected in series or in parallel to form a module. Several modules are connected in series or in parallel to form the entire battery. Each cell has two connectors, one positive and one negative, and a large number of such cells have to be interconnected to generate the required voltage, which can be up to 320 V high for electric vehicles. To interconnect the cells and modules a connector has to be used. hi view of the high requirements for such NiMH cells the connectors have to have a very high electrical conductivity, to be able to charge and discharge the battery fast. For a good connection the connectors also have to have a good weldability. It has been proposed to use massive nickel connectors because nickel has a good weldability, but nickel has an electrical conductivity of only 25% IACS (100% IACS is the highest electrical conductivity of pure copper). Moreover, nickel is very expensive when compared to for instance copper and steel. On the other hand it has been proposed to use a steel connector which has been plated on both sides with nickel to get a good weldability. This provides a connector material which is far less expensive, but it has a relatively low electrical conductivity of approximately 15% IACS due to the steel substrate.

It is an object of the invention to provide a plate or strip for producing connectors which connect battery cells and battery modules, which connectors have a good electrical conductivity. It is another object of the invention to provide a plate or strip for producing connectors for batteries which are cost effective.

It is still another object of the invention to provide a plate or strip for producing connectors for batteries which is relatively easy to produce. It is moreover an object of the invention to provide a connector for connecting battery cells, and the use thereof.

It is furthermore an object of the invention to provide a connector for connecting battery modules, and the use thereof.

It is a further object of the invention to provide a process for manufacturing such a plate or strip.

According to the invention one or more of these objects are reached by providing a plate or strip used for producing connectors which connect individual batteries cells or battery modules, the plate or strip comprising a first layer comprising a layer of copper or copper alloy or aluminium or aluminium alloy, and a second layer comprising a layer of carbon steel, the first and second layer being joined together.

The connectors produced from the plate or strip according to the invention have a very good electrical conductivity due to the copper or aluminium layer, and also a good weldability due to the steel layer. The steel layer also provides good mechanical properties to the connectors, such as a good formability. The connector according to the invention is thus far less costly as compared to a massive nickel conductor and at the same time has a very good conductivity.

Preferably the first layer and the second layer have been joined by cladding. Cladding of the layers provides a good contact between the first and second layer, and cladding also makes it easily possible to manufacture the plate or strip in the required thickness.

According to a preferred embodiment a nickel layer is provided on one or both sides of the plate or strip, preferably by plating, more preferably by electrolytic plating. The nickel plating is provided to enhance the corrosion resistance, which is especially important on the side of the plate or strip where the steel layer is present. Preferably the first layer of the plate or strip consists of a layer of copper or copper alloy. Of course the plate of strip is most cost effective when the first layer consists of a copper or copper alloy layer only, without any further layer present on the copper or copper alloy layer. However, it is possible to coat the copper layer with a corrosion resistant layer before cladding the copper layer onto the second layer. Instead of a copper or copper alloy layer it is also possible to use an aluminium or aluminium alloy layer, but copper has a better electrical conductivity. According to a first preferred embodiment the second layer consists of a steel layer. Thus, a very simple and effective plate or strip for producing connectors is provided, consisting only of a steel layer and a copper or copper alloy layer (or aluminium or aluminium alloy layer). Preferably, at least the steel side of the plate or strip is provided with a nickel layer, as elucidated above. According to a second preferred embodiment the second layer consists of a steel layer which has been provided with a nickel layer on one or both sides, preferably by plating, more preferably by electrolytic plating. The nickel plated steel layer is preferably diffusion annealed to increase the adhesion and formability of the nickel layer, but also to increase the corrosion resistance of the nickel layer. Subsequently a temper passing can follow to achieve the required mechanical properties. A steel strip which has been plated with nickel in both sides is commercially available and can be used easily for the plate or strip according to this invention. Moreover, the plating with nickel before the joining of the copper or copper alloy (or aluminium or aluminium alloy) layer onto the plated steel strip makes it possible to do without a further plating of the plate or strip as discussed above, since the steel layer already has a nickel layer to prevent corrosion. Of course it is still possible to plate the plate or strip according to this embodiment as elucidated above, so as to provide a nickel layer on both sides of the plate or strip.

Preferably the nickel layer has a thickness between 0,5 and 10 μm, more preferably between 1 and 3 μm. These thicknesses of the nickel layer only use relatively low amounts of nickel, so the plate or strip according to the invention is cost effective but still provides sufficient corrosion resistance.

According to a preferred embodiment the plate or strip has a thickness between 0,05 and 4 mm, the copper or copper alloy layer has a thickness between 0,03 and 3,5 mm, and the steel layer has a thickness between 0,001 and 1 mm, preferably between 0,1 and 0,5 mm. These are ranges for the thickness of the plate or strip and the layers that makes it possible to produce connectors for a variety of battery purposes, from laptop to electric cars.

More preferably, the plate or strip has a thickness between 1 and 2 mm, the copper or copper alloy layer has a thickness between 0,8 and 1,8 mm, and the steel layer has a thickness between 0,1 and 0,4 mm. These are the thicknesses that will mostly be used for connectors for NiMH batteries

Preferably the copper or copper alloy consists of pure copper having Cu-PHC quality (phosphorus deoxidised high conductivity). This grade of copper provides for the best conductivity and is still cost effective. Alternatively other copper or copper alloys having high grade electrical conductivity quality can be used.

According to a preferred embodiment the carbon steel consists of a mild steel. Mild steel is very good processable, for instance during the cladding of the strip according to the invention or during the forming of the connectors.

According to a further aspect of the invention there is provided a connector for connecting batteries cells or battery modules, which connector is manufactured from a plate or strip as described above. This connector will have a form and size depending on the use thereof for a battery.

According to a still further aspect of the invention the use of a connector as described above is provided for interconnecting NiMH battery cells or battery modules to form NiMH batteries. Such a use is advantageous in view of the very fast charging and discharging of NiMH batteries that are for instance used in electric vehicles, which have to be fully charged in less that one hour, while the energy density of the NiMH battery can be up to 250 Wh per litre.

According to a last aspect of the present invention there is provided a process for manufacturing a strip according to the first aspect of the invention, the process comprising the steps:

- providing a copper or copper alloy or aluminium or aluminium alloy strip;

- providing a carbon steel strip;

- cladding the steel strip with the copper or copper alloy or aluminium or aluminium alloy strip with a rolling reduction of at least 30%.

This process is a very elegant route to manufacture a strip according to the invention from which optionally plates can be cut, and from the strip or plates can connectors be punched or otherwise produced, which connectors can be formed to fit within the batteries, especially to connect battery cells or modules in a NiMH battery.

Preferably the strip is annealed after the cladding step. The annealing provides a better bonding between the clad layers. According to a preferred embodiment the plate or strip is plated with a layer of nickel on both sides of the plate or strip after the cladding and optionally annealing. The plating is performed to provide a sufficient corrosion resistance to the strip.

According to an embodiment the carbon steel strip is plated with a layer of nickel on both sides of the steel strip before the steel strip is clad onto the copper or copper alloy (or aluminium or aluminium alloy) strip. The nickel plated steel strip is preferably diffusion annealed to increase the adhesion and formability of the nickel layer, but also to increase the corrosion resistance of the nickel layer. Subsequently a temper passing can follow to achieve the required mechanical properties. This makes it possible to use commercially available nickel plated steel strip in the process, and to do without the plating with as discussed above. However, that plating can still be performed so as to get a corrosion resistant layer of both sides of the strip.

Preferably the strip is rolled to a thickness of 0,05 to 4 mm, more preferably to a thickness of 1 to 2 mm after the cladding and optionally annealing. This is the thickness needed for connectors inside batteries, especially in NiMH batteries. Fig.1 schematically shows part of a battery type in which an embodiment of the connector according to the invention is used.

Fig. 2 schematically shows part of another battery type in which an embodiment of the connector according to the invention is used.

Fig. 3 shows a first preferred embodiment of the connector according to the invention.

Fig. 4 shows a second preferred embodiment of the connector according to the invention.

Fig. 5 shows a third preferred embodiment of the connector according to the invention. Fig. 1 in a very schematic way shows how the connectors 11, 12 according to the invention can be used to connect circle-cylindrical battery cells 13 to form a battery module consisting of four battery cells 13. The connectors 11 on both sides are present to externally connect the module as shown in Fig. 1 with other battery modules. The connectors 12 are present to internally connect the battery cells 13 with each other. These connectors 11 and 12 are welded to the battery cells 13 to form welding joints 14. The connector according to the invention is especially suited to connect NiMH battery cells, which each generate a voltage of 1,2 V. The module as shown in Fig. 1 thus generates a voltage of 4,8 V.

Fig. 2 shows another type of construction of a battery module. External battery connectors 21 according to the invention are present to interconnect battery modules as shown in Fig. 2. Each individual battery cell 23 has a plastic casing 22, and the prismatic battery cells 23 having a square or rectangular cross-section are interconnected by welding joints 24. Also the connectors 21 are connected to the battery module by such welding joints 24. This type of battery module provides a very compact battery and is very suitable for NiMH batteries to be used in hybride electrical vehicles (HEVs). Fig. 3 shows one of the embodiments of the connector according to the invention.

The connector consists of a steel layer 31 which has been provided on both sides with a plated nickel layer 32. This nickel plated steel layer has been diffusion annealed. Onto the nickel plated steel layer a copper layer 33 has been clad. This clad copper-nickel- plated steel layer has been nickel plated again to provide the connector as shown in Fig. 3, having a plated nickel layer 34 on both sides.

Fig. 4 shows a second embodiment of the connector according to the invention. The connector consists of a steel layer 41 which has been clad onto a copper layer 42. This clad layer has subsequently been plated with a nickel layer 43 on both sides.

Fig. 5 shows a third embodiment of the connector according to the invention. Essentially it is the same as the connector as shown in Fig. 4, but after the cladding of a steel layer 51 onto a copper layer 52 and the plating of this clad product with nickel layers 53 on both sides, the nickel plated product has been diffusion annealed. The connector thus has a Ni/Cu-diffusion annealed layer on the copper side and a Ni/Fe- diffusion annealed layer on the steel side. To confirm the good weldability of the Ni plated steel material used for such connectors, welding trials have been performed with steel strip samples having a gauge of 0,25 mm. The test material was without a Cu clad layer since the welding will always be made only between the Ni plated steel layer of the material and not with the Cu clad layer. For all specimen a 2 μm Ni plating layer was plated on top of the steel surface. Material A was just Ni plated in accordance with the embodiment of Fig. 4 (but without the copper layer being present), whereas the other material B was subsequent diffusion annealed in accordance with the embodiment of Fig. 5 (without copper layer). After degreasing and activation of the specimen the Ni plating was performed in a Watts Ni electrolyte at a current density of 10 A/dm², a temperature of 60° C and at pH of 2,5. Subsequent diffusion annealing was performed batchwise in an H₂-atmosphere at 630°.

Both type of materials were spot welded with a spot welding machine at identical welding parameters (identical current, contact pressure, contact area and welding time). The specimen were afterwards tested in a tensile test according to European standard EN 10002-1 to determine the yield and tensile strength which can be achieved with both types of Ni surface treatments. The tension calculated by the tensile test machine is based on the cross section area of the specimen (gauge time width of the standard flat specimen) and not on the contact area of the spot weld.

Tablel : Tensile test results according to EN 10002-1

Table 1 clearly shows that the different types of Ni surface treatment show comparable results.

For the steel layer a mild carbon steel can be chosen, to provide a connector having a very good formability. For the copper layer a PHC quality or other copper or copper alloys having high grade electrical conductivity quality can be chosen, to provide a very good conductivity. A high conductivity is required for NiMH batteries which have to be able to charge and discharge very fast. However, instead of copper or copper alloys in the above embodiments also aluminium or aluminium alloys can be used when a slightly lower conductivity could be used. The nickel plating layer is present to provide a good weldability and a good corrosion resistance and is usually performed as electrolytic plating.

^• The plating of the nickel layers, the cladding, the rolling and the diffusion annealing is performed in the usual way. Skin passing can be performed as a last step. Rolling of the nickel layers provides a reduced porosity of the nickel layer. If required, temper rolling is performed. For the skilled person it will be clear that other layers might be present between the ones mentioned above; however, such layers do not add to the functionality of the connector and only add to the cost of the strip from which the connectors are made.

The person skilled in the art will understand that the products and processes according to the invention can be varied in several ways, but that it is essential that the connector and the strip or plate from which it is made comprise a copper or aluminium layer and a steel layer.

Claims

1. Plate or strip used for producing connectors which connect individual battery cells or battery modules, the plate or strip comprising a first layer comprising a layer of copper or copper alloy or aluminium or aluminium alloy, and a second layer comprising a layer of carbon steel, the first and second layer being joined together.

2. Plate or strip according to claim 1, wherein the first layer and the second layer have been j oined by cladding.

3. Plate or strip according to claim 1 or 2, wherein a nickel layer is provided on one or both sides of the plate or strip, preferably by plating, more preferably by electrolytic plating.

4. Plate or strip according to any one of the claims 1 - 3, wherein the first layer consists of a layer of copper or copper alloy or aluminium or aluminium alloy.

5. Plate or strip according to any one of the claims 1 - 4, wherein the second layer consists of a steel layer.

6. Plate or strip according to any one of the claims 1 - 4, wherein the second layer consists of a steel layer which has been provided with a nickel layer on one or both sides, preferably by plating, more preferably by electrolytic plating.

7. Plate or strip according to any one of claims 3- 6, wherein the nickel layer has a thickness between 0,5 and 10 μm, preferably between 1 and 3 μm.

8. Plate or strip according to claim 1 - 5, wherein the plate or strip has a thickness between 0,05 and 4 mm, the copper or copper alloy layer has a thickness between

0,03 and 3,5 mm, and the steel layer has a thickness between 0,001 and 1 mm, preferably between 0,1 and 0,5 mm.

9. Plate or strip according to claim 8, wherein the plate or strip has a thickness between 1 and 2 mm, the copper or copper alloy layer has a thickness between 0,8 and 1,8 mm, and the steel layer has a thickness between 0,1 and 0,4 mm,.

10. Plate or strip according to claim 1 - 9, wherein the copper or copper alloy consists of pure copper having Cu-PHC quality or a quality giving a higher electrical conductivity.

11. Plate or strip according to claim 1 - 8, wherein the carbon steel consists of a mild steel.

12. Connector for connecting battery cells or battery modules, which connector is manufactured from a plate or strip according to any one of the claims 1 - 1 1.

13. Use of a connector according to claim 12 for interconnecting NiMH battery cells or battery modules to form NiMH batteries.

14. Process for manufacturing a strip according to any one of the claims 1 - 11, the process comprising the steps:

- providing a copper or copper alloy or aluminium or aluminium alloy strip;

- providing a carbon steel strip;

15. Process according to claim 14, wherein the strip is annealed after the cladding step.

16. Process according to claim 14 or 15, wherein the plate or strip is plated with a layer of nickel on both sides of the plate or strip after the cladding and optionally annealing.

17. Process according to claim 14, 15 or 16, wherein the carbon steel strip is plated with a layer of nickel on both sides of the steel strip before the steel strip is clad onto the copper or copper alloy or aluminium or aluminium strip.

18. Process according to any one of claims 14 - 17, wherein the strip is rolled to a thickness of 0,05 to 4 mm, preferably to a thickness of 1 to 2 mm after the cladding and optionally annealing.