STEEL SHEET FOR TOP COVER OF BATTERY PACK AND ITS MANUFACTURING METHOD

Abstract

A top cover of a battery pack including a metallic coated steel sheet wherein the metallic coating is topped by an organic coating and wherein the organic coating has two layers, the first layer of the organic coating in contact with the metallic coating having a thickness of 2 to 25 m, and the second layer of the organic coating being based on polyester or polyurethane.

Claims

1-8. (canceled)

9. A top cover of a battery pack comprising: a press stamped metallic coated steel sheet including a steel sheet and a metallic coating, the metallic coating being topped by an organic coating having a first layer and a second layer, the first layer of the organic coating being in contact with the metallic coating and having a thickness of 2 to 25 m, and the second layer of the organic coating being based on polyester or polyurethane.

10. The top cover as recited in claim 9 wherein the metallic coating is based on zinc and optionally comprises up to 2% by weight of aluminium and unavoidable impurities.

11. The top cover as recited in claim 9 wherein the metallic coating has a thickness of 10 to 40 m on an inner side of the battery pack.

12. The top cover as recited in claim 9 wherein the metallic coated steel sheet has a coating weight of 50 to 200 g/m.sup.2 in total on both sides.

13. The top cover as recited in claim 9 wherein said first layer of the organic coating in contact with the metallic coating is based on polyester.

14. The top cover as recited in claim 9 wherein the first layer of the organic coating in contact with the metallic coating is based on polyurethane.

15. The top cover as recited in claim 9 wherein the first layer of the organic coating in contact with the metallic coating is based on epoxy.

16. A battery pack comprising the top cover as recited in claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0016] To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following figures:

[0017] FIG. 1 illustrates a battery pack and its top cover in an electric battery vehicle,

[0018] FIG. 2 illustrates a top cover according to the invention after fire exposure during 130 seconds at a temperature of 1300 C.

[0019] FIG. 3 illustrates a top cover not according to the invention after fire exposure during 130 s at 1000 C.

[0020] FIG. 4 illustrates a top cover according to the invention after fire exposure during 130 seconds at a temperature of 1000 C.

DETAILED DESCRIPTION

[0021] The invention relates to a top cover for battery pack comprising a metallic coated steel sheet wherein said metallic coating is topped by an organic coating.

[0022] For this purpose, any steel can be used in the frame of the invention. Preferably, steels having a good formability are well suited. For example, the top cover can be made of mild steel for deep drawing such as Interstitial Free steel having the following weight composition: C0.01%; Si0.3%; Mn1.0%; P0.1%; S0.025; Al0.01%; Ti0.12%; Nb0.08%; Cu0.2%.

[0023] For example, the top cover can be made of High Strength Low Alloy (HSLA) steel having the following weight composition: C0.1%; Si0.5%; Mn1.4%; P0.04%; S0.025%; Al0.01%; Ti0.15%; Nb0.09%; Cu0.2%.

[0024] The steel sheet can be obtained by hot rolling of a steel slab and subsequent cold rolling of the obtained steel coil, depending on the desired thickness, which can be for example from 0.6 to 1.0 mm.

[0025] The steel sheet is then coated with a metallic coating by any coating process. For example, the steel sheet is hot-dip coated in a molten bath and subsequently wiped by air knifes.

[0026] The molten bath can be based on zinc and comprise unavoidable impurities.

[0027] In a preferred embodiment, the bath is based on zinc and optionally contains 2% by weight of aluminum.

[0028] The metallic coating weight can be of 50 to 200 g/m.sup.2 in total on both sides or less. For example, the coating thickness on the inner side of the battery pack is 10 to 40 m.

[0029] After hot dip metallic coating, the steel sheet is painted, for example on an organic painting line. The surface can be prepared by a degreasing step and a subsequent conversion treatment applied by roll coat to ensure the grip of the 1.sup.st layer of paint.

[0030] The first layer of paint, also known as primer, can have a thickness of 2 to 25 m. The primer can be based on different resins such as polyester, polyurethane or epoxy.

[0031] The second layer of paint is also applied by roll-coat and is based on polyester or polyurethane. In a preferred embodiment, its thickness is from 2 to 40 m, preferably from 5 to 25 m.

[0032] The metallic coated steel sheet used in the invention is coated with organic paint. The organic coating used in the invention consists of two layers. The first layer of the organic coating in contact with the metallic coating having a thickness of 2 to 25 m, and the second layer of the organic coating being based on polyester or polyurethane. The organic coating is then baked in an oven.

[0033] Such a coating releases few gasses when submitted to flame temperatures. In case of fire or high temperatures, it won't increase the pressure inside the battery pack.

[0034] The metallic and organic coated steel sheet can then be cut into a blank. The blank can be formed by press stamping to the specific shape of the top cover. This specific shape is design related. The top cover being a large horizontal part, it may be subject for vibrations. To reduce these vibrations and subsequent noise, stiffeners are generally punched into the top cover during the stamping operation. Finally, the top cover is attached to the pack by any removable or non-detachable means, for example by screwing, welding or gluing.

Examples

[0035] In order to determine the resistance to fire of the top covers, several tests were performed. All tests were performed on the same test device.

[0036] The test device was adapted from the test device described in the Standard ISO 2685:1998. Both following adaptations were done: Firstly, the sample was thermally isolated from the structure of the test device by a 10 mm thick plate of calcium silicate. Secondly, the gas burner generating the flame has been calibrated to achieve the targeted temperature on the face of the sample that is exposed to the flame.

[0037] For all tests, the samples have the same dimension of 150150 mm.sup.2. Each sample is positioned in front of the gas burner to get hit by the flame. The plate between the sample and the burner has an opening area with the dimension of 9090 mm.sup.2.

[0038] Three materials were tested: [0039] material 1 is a 0.7 mm thick steel sheet. It is hot-dip galvanized. The metallic coating weight is 275 g/m.sup.2. It is also organic coated with the following layers on the face exposed to the flame: a 4 m thick first layer in contact with the metallic coating and an 8 m thick second layer based on polyester, [0040] material 2 is a 1.0 mm thick aluminum sheet of 6016 series, [0041] material 3 is a 0.8 mm galvanized steel sheet coated with epoxy-based e-coat. The hot-dip coating contains 0.2 of aluminum by weight, the remainder being zinc. The metallic coating weight is 140 g/m.sup.2. After a phosphating step, the sample was dipped in a e-coating bath. The e-coat tested is Powercron 6200 HE from supplier PPG. The dry thickness of paint after baking is 25 m on each face, [0042] material 4 is a 0.7 mm thick steel sheet. It is hot-dip galvanized. The metallic coating weight is 275 g/m.sup.2. It is also organic coated with the following layers on the face exposed to the flame: a 5 m thick first layer in contact with the metallic coating and a 20 m thick second layer based on polyester.

[0043] In the following, sample 1 is made of material 1, sample 2 is made of material 2 and sample 3 is made of material 3, sample 4 of material 4.

[0044] Two scenarios of fire exposure have been tested. In scenario A, the flame temperature is 1300 C. and the exposure time is 130 s. In scenario B, which is less severe, the flame temperature is 1000 C., and the exposure time is 130 s.

[0045] Several criteria are considered for analysis of the tests. The integrity of the sheet, i. e. whether the flame has pierced the sheet or not, the temperature of the face unexposed to the flame (back-face) at the end of the test and the presence of bubbles in the coating after the test. The presence of a bubble shows the release of gas.

TABLE-US-00001 TABLE 1 Scenarios of flame exposure Time of flame Temperature Scenario exposure of flame A 130 s 1300 C. B 130 s 1000 C.

TABLE-US-00002 TABLE 2 Scenario A: 130 s at 1300 C. Scenario Flame Temparature on Presence A Sample Piercing back-face after 130 s of bubbles 1300 C. 1* No 743 C. No 130 s 2 Yes after 45 s above 910 C. 4* No 625 C. No *according to the invention

[0046] After an exposure of 130s at 1300 C., the back-face of samples 1 and 4 made of steel remains at a temperature of less than 700 C. and doesn't show any signs of melting. On the contrary, the flame has pierced material 2 made of thicker aluminum.

[0047] Moreover, sample 1 doesn't show any bubbles as can be seen on FIG. 2, but only cracks that comes from different thermal expansion between the steel sheet the and organic coating layer. Sample 4 has a similar appearance as sample 1 and doesn't show neither any bubbles.

TABLE-US-00003 TABLE 3 Scenario B: 130 s at 1000 C. Scenario Flame Temperature on Presence B Sample Piercing back-face after 130 s of bubbles 1000 C. 1* No 655 C. No 130 s 3 No 486 C. Yes 4* No 585 C. No *according to the invention

[0048] After an exposure of 130 s at 1000 C., the back-face of sample 3 clearly shows bubbles as can be seen on FIG. 3. These open bubbles have released combustion products of the paint in form of gas.

[0049] Sample 4 doesn't show any bubbles as can be seen on FIG. 4. Sample 1 has a similar appearance as sample 4 and doesn't show neither any bubbles.