In new energy PACK, we often consider the use of copper bar to carry positive and negative power current input or output, and copper bar design is basically around to meet the energy input and output of PACK under the previous stage.

In junior high school physics, we have studied Ohm's law, from this point of view, any conductor in the load current, must face the resistance of the conductor itself and the interaction of current temperature problem, temperature may be very high, hundreds of degrees Celsius, but in such a high-temperature environment, the entire system insulation, temperature resistance design can meet the needs of it? Is it going to work? Promise is certainly not, because the system of all materials are high-temperature materials, it is not realistic, even if designed, it is also high-cost, not in line with the law of the market.

In that case, the first thing we need to figure out about the design of our copper bar is the overall power carrying capacity. Pack in the charge and discharge parameters, there are two parameters we have to consider, the first is the continuous charge and discharge of the maximum current, the second parameter is the charge and discharge peak current size and duration.

This problem is obvious. We definitely designed to meet the functional parameters of the appeal for design calculation, but if we do not meet these two parameters, we can safely say that we have solved it, what we need to think about is what the environment is like, and whether it has an impact on each other, in terms of this functional parameter.

Next, take a simple example, for you to carry on the conceptual introduction. Suppose our battery pack is continuously charged and discharged with a maximum current of 200A and a peak current of 600A and 15s.

According to the recommended table in Table 11 of GB7251.1, the cross-sectional area is 95mm2 and the rated current carrying capacity is 200-225A. With reference to this standard, the cross-sectional area of the copper bar should be larger than 95mm2. Therefore, the copper bar is designed to be 40 * 2.5, that is, 100mm2. The specification of the copper bar selected from the rated current case is then verified by the peak current case:

According to the formula in GB3906-1991, Appendix F: s = (I/A)(T/△)1/2 to determine the minimum cross section of the bus bar. S = I/a * √(T/△) = 600/13 * √(15/180) = 13.3 MM2I is the rated short-term withstand current; a is the material factor, copper is 13, aluminum is 8.5; t is the rated short-circuit duration; △ is the calculation of the temperature rise (k) appeal, we can conclude that, when the rated current is satisfied, the peak current can completely meet the requirements.

At this point, I think it's time to end, because these theoretical calculations, they look pretty good, but they're still a little bit shaky, because you don't know what the actual temperature is? We don't know if it's what we want. One way to find out is to experiment, the other is to simulate. Now we still carry on a preliminary simulation analysis.

We set the length, width and thickness of the copper bar as 250 * 40 * 2.5(mm) . From the Formula R = l/s, we calculate the resistance r = 1.7510-8x0.25/(100X10-6) = 0.00004375 and the loss power P = I r = 1.75 w from the rated current 200A. Using the fluid simulation software, we input the rated current power into the simulation software, where the fluid is a static closed 25 °C air, we get the surface temperature of the copper bar about 32.55 °C, as shown in figure 1 below; In addition, the surrounding air will be heated by radiation, as shown in figure 2 below. By the temperature of the copper bar itself and the surrounding radiation temperature, we can further analyze the influence of the surrounding devices. If a temperature-sensitive device is installed near the copper bar, further analysis of its location is required.

After the three-dimensional simulation of the fluid, the system parameters are simulated again, and the following curves are obtained.

From the curve of change, compared to the fluid analysis, one temperature is 32.55 °c, the other is 37.69 °C. The analysis and boundary between different software may lead to a certain error in temperature, which we can not consider, because this is the result of simulation, and this result needs to be compared and verified with the test, in order to achieve the combination of theory and practice, through a test correction simulation model, to achieve the follow-up of similar cases, thus reducing the test cost and cycle, of course, other economic benefits not to say, is Not the main research direction of this paper.

The temperature variation of the surface of the copper bar at rated current is described above, but the simulation of the peak current is not mentioned. Appeal peak current boundary condition is 600A, 15S, this is actually not complete, the lack of an interval time, the interval time and the control of the PACK discharge parameters, let us say 1min. Get the input boundary curve below.

In the system parameter simulation, I set up according to this parameter, get the temperature curve of the surface of the copper bar as follows.

According to the temperature curve of the surface of the copper bar, We know the general trend of the surface temperature change. The maximum temperature of the temperature equilibrium is 59.1 °C. This result appears to be quite different from the cross-sectional area previously calculated for peak current. That is because the boundary input is not consistent, we actually peak current is in the operating mode, we should be more based on the limit operating mode analysis and design, to meet the peak and rated current case, more consideration should be given to the effect of accumulated temperature under operating conditions. The analysis of the design of the copper bar has reached the end, so to speak. There seems to be nothing to say, or simply to say that the design of the copper bar on the lap contact resistance has been ignored, this issue is still a bit complicated, follow-up opportunities to share. Also, the most important thing is, PACK inside the temperature design analysis is very important, long-term high-temperature environment on parts of the device and battery life, so the design of temperature analysis should be cautious. 

Source: The struggle of the lazy you don't know the public number