Michael Ross wrote:
creepage... and clearance... on the Thundersky cells I have is a couple inches and the voltage under 4V - I think that will be quite OK.
Ah, but the problem is that EVs will string a hundred cells in series to make a high voltage pack. Let's say they are in a metal box (a common situation). Now you have a creepage path from the most positive cell to the box, and then from the box to the most negative cell. Several hundred volts is driving the leakage current.
This leakage current is also DC. This accelerates corrosion on the surfaces it passes over.
EVs are outdoors, and subject to plenty of "environment". They get dirty and wet. Bugs and rodents will crawl around. Batteries need ventilation and cooling, so there are always ways for things to get inside.
The result is that when you look at old battery boxes, there is *always* evidence of leakage and corrosion. Often, the worst corrosion is on the terminals at the farthest ends of the pack, and closest to whatever metal structures happen to be the closest.
This is speculation: I bet I could paint a track of conductive grease between the poles of a LiFePO4 battery and the current would be negligible. 4V is just not much pressure.
Correct... for one cell. Now try it with 100 cells. You get 100 times the leakage current.
The leakage current also corrodes the terminals, which creates additional ions to enhance conduction.
I am not too worried about dry joints in my environment, but I would go with a conductive grease before a dielectric. If I see a problem then I will make a correction. A sealed box might be in order in some environments.
"Conductive" is just marketing with these products. Measure their actual resistivity, and it is many orders of magnitude worse than metals.
My best and cheapest pack has all the cells welded together.
Yes! This has proven to be the only way to make reliable connections. Pretty much all commercial packs are welded.
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