In the last post I described the reason for connecting the busbars to the cells via wires that serve as fuses, like in the picture above.
The "fuses" are made out of copper wire, to match the copper material used by the battery posts on the Leaf modules. By selecting a specific gauge (diameter), we can choose the maximum current they'll carry safely, while providing protection by melting at much higher currents.
In this fusing test picture, taken from the video further below, the spark preceded by a fraction of second the melting and opening of the wire (sorry for the probe in the foreground...):
So, how to size the wires ? First, the expected max current that the application will see in normal conditions must be determined.
In my case it is limited by the inverter's DC specs. The GS8048A inverter's continuous power is 8KW, with ~92% nominal efficiency (actually, likely lower at max output power...). The Solar panel array's max output is below 7KW, i.e. lower than the inverter's max drawn power. So the maximum continuous DC power will be ~8700W.
The maximum DC current seen by the battery pack will be at the minimum 40V pack voltage, i.e. 8700/40= 218A. Since my pack is set up with groups of 8 parallel modules, 8 fuse wires will be used per group. Each must therefore withstand 218/8= 27A.
However, this does not take into account possibly failed cells toward the end of life of the pack, nor does it include any inverter surge margin, Assuming a goal of a working pack with 2 failed cells / opened fuses, an add'l 8/6= 1.33X continuous factor should be taken into account, i.e. 27x1.33= 36A max.
As for the surge power, the inverter is capable of 18KW for 100ms, so an 18/8= 2.25X surge factor should theoretically be considered. Giving a total of 27x2.25x1.33= 81A. In my case, though, since the inverter's AC side is only feeding the grid, and is not connected to any backup load, the grid takes care of any demand surge from the house. There should be no condition leading to such an extreme 18KW surge.
Conclusion: wires that can safely support 36A should be enough. To ensure enough margin and low stress on the wires, I selected gauge 12 (AWG). Per the Handbook of Electronic Tables and
Formulas for American Wire Gauge, AWG 12 is recommended for up to 41A continuous (AC or DC?, need to check) in chassis wiring applications. The fusing current is 235A (free air, temp dependent), per Powerstream table copies
To double check that AWG 12 is appropriate, I did some rough real world testing. Ok, maybe it was primarily an excuse to blow up copper wires 😇
The video below shows 3 tests of 5" copper wires, with a 12V lead battery (225 CCA capable):
- AWG 14 @ 63A thanks to a 0.17ohm resistor array in series --> held up
- AWG 14 in short-circuit --> blew up in a second, and... yay sparks !
- AWG 12 in short-circuit --> melted open in ~3.5 seconds
Since AWG 14 sustained 63A without problem, AWG 12 will have plenty margin to carry the expected max 36A of my application. Should the current spike much higher (~200A) it will melt and open like a fuse.
Although not shown in the video, this was also tested directly on a Leaf cell @ 4V. The AWG 12 wire in short-circuit blew up almost immediately.
Well, that was fun. Gave me an itch to blow up other stuff too, and maybe build a crude spot welder... Anyway, AWG 12 it is. Boop !
Next time we'll cover how to successfully solder 96 of those babies on big busbars despite the large delta in thermal masses. Hint: requires a lot of patience... 😣
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