Resistance, heat, busbars for battery pack...

Batteries, Chargers, and Battery Management Systems.
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eq1
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Resistance, heat, busbars for battery pack...

Post by eq1 » Tue Nov 07, 2017 6:10 pm

Not sure if this is the right place to ask this question, but I'm mostly into batteries and I'm thinking 'battery folks' might be the ones to actually know something about this... It's mostly a theoretical/physics-type of question...

The main question is, Is the sizing of conductors, like a busbar and battery terminal, or just a wire, mostly about minimizing resistance under whatever currents you expect to see -- and then, is the resistance mostly, primarily, only about heat? For instance, if you used a small wire -- say 10 AWG for a 100 amp load, clearly an undersized choice -- yet you used some elaborate cooling mechanism, would you prevent the small wire from becoming a problem -- because, as my thinking goes, the heat is what increases resistance, so if you control the heat you control the resistance??...

Practically speaking, I'm trying to better understand the choices surrounding battery terminal size and material, connection types and materials (such as welds and fasteners and type of metal), and busbar size and material... I have batteries with aluminum terminals that have been tapped, the tap creates a big hole and reduces cross sectional area (plus as-is it has stainless steel threaded inserts and SS screws, much less conductive than aluminum), so in a conductive sense the terminal is like a bottle neck, yet I'm not sure how much that matters. I'm thinking it mainly matters because under a high load the area of the bottle neck will heat up and the resistance will increase, and voltage will sag; I'm also thinking that you can mitigate the impact if you use a large busbar and a solid connection - so the heat can easily dissipate, i.e. the bottleneck is really only a problem if/when that portion of the conductive path heats up, otherwise, it's not such a big deal... But, I don't really know...

Any insights greatly appreciated...

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spinningmagnets
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Re: Resistance, heat, busbars for battery pack...

Post by spinningmagnets » Tue Nov 07, 2017 7:51 pm

Heat is a symptom that every builder should take note of, and monitor closely. The big issue as I see it, is voltage drop. Imagine (as you have suggested) that a builder has some elaborate cooling mechanism that will be able to keep the wires and busbars cool under all circumstances. Why use a high-amp battery, controller, and motor...only to choke the flow by using smaller wire than what is optimal?

If the wire is too small for the loads, you will get voltage drop and heat, and...both are bad. As LFP often points out, wire and connectors are often the least expensive part of the system, so why not make them fat enough that they are never the bottle-neck to best-possible performance?

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Alan B
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Re: Resistance, heat, busbars for battery pack...

Post by Alan B » Tue Nov 07, 2017 10:45 pm

At the Particle Accelerators I've worked at they generally use water cooled cables. It is a tradeoff between cost, bulk and energy. If the cables were sized to not need cooling they would be much larger, more expensive, and the trays to carry them would take a lot more space. Their resistance would also vary with the environmental temperature and their heat rise and cause stability problems. At some point it is cheaper and more effective to use low conductivity temperature regulated cooling water inside the cables to manage their temperature, and to keep their resistance constant. The magnets had hollow copper conductors in most cases as well, due to the density of wires required in the magnets and the physical construction concentrating the wires in a way that would make too much heat in the magnet core. Again low conductivity water that was cooled and temperature controlled was used to provide thermal stability and cooling. The precision requirements for the magnetic fields were extremely tight, so in some sense this is not the same problem as a vehicle, but shares some of the same issues. The vehicle adds more stringent requirements for overall size and weight instead of the precision of fields.

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Re: Resistance, heat, busbars for battery pack...

Post by yewsuck » Wed Nov 08, 2017 1:39 am

Since we are on the subject of buss bars, heat, and resistance...

Here is a link to a buss bar chart. https://stormpowercomponents.com/resour ... 00-busbars I been using the area chart and circular mills portion and then referencing another chart for normal copper wire.

Is there anyone that can chime in with better charts for ampacity and such for buss bars?

-YS

eq1
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Re: Resistance, heat, busbars for battery pack...

Post by eq1 » Wed Nov 08, 2017 1:59 am

spinningmagnets wrote:....If the wire is too small for the loads, you will get voltage drop and heat, and...both are bad....
hmm... I guess at the heart of it I'm thinking that heat causes resistance and voltage drop is due to resistance. Of course, as you say, we should just use wire etc. that's big enough. But in my situation I'm somewhat stuck with the bottleneck at the terminals of each cell...

What Alan B. says all sounds very much like what I've been thinking - they control the heat to reduce/control the resistance rather than using 'big wire'. This suggests to me that my bottleneck issue IS really a heat issue and that, if I use a good sized busbar and get good heat transfer and dissipation, and/or if that bottleneck area doesn't get hot, it probably won't matter much... My back of the envelope calculations put the unmolested stock cell terminals at about 140 ampacity, but with the tapped terminal, SS screw, etc., it drops to something like 90 amps or so. I was shooting for the highest ampacity possible, but really, most likely the system won't see more than 100 amps for some seconds, and usually no more than 40 amps continuous...

So, I think I can simplify the question/issue: If what Alan B. writes is true, it seems like one could, theoretically, use a small wire and control the temperature yet get the same ampacity as a big wire, all else being equal...

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Alan B
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Re: Resistance, heat, busbars for battery pack...

Post by Alan B » Wed Nov 08, 2017 3:45 am

You can tell by measuring temperature. If it is getting hot it needs upgrading.

The heavy copper can pull heat out of the heat generating part.

Many connectors work this way, the heavy copper wire pulls heat out of the connector.

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Re: Resistance, heat, busbars for battery pack...

Post by amberwolf » Wed Nov 08, 2017 4:57 am

Regarding heat vs wire size, etc:

My phase wires are out in the open air for their long stretches. If they were not, they would probably melt their insulation off, as they get hot even like this, as they are too thin for the current they ahve to pass during repeated acceleration/braking.

So cooling too-small conductors does let them survive the abuse, but it's still better to not create the heat in the first place if you can do that.


Remember: every watt that ends up as heat is a watt that isn't moving you down the road. Enough of those and you have to add battery pack weight and volume to make up for the lost range. Doing that means you might as well have had bigger conductors/etc in the first place. :)

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Re: Resistance, heat, busbars for battery pack...

Post by eq1 » Wed Nov 08, 2017 4:40 pm

This is all very helpful. Thanks to you all... I'll think about it some more, work with things, and maybe I'll post more later, more particulars...

In the meantime, anyone ever use some kind of conductive grease, say, between the battery terminal and the busbar, to help compensate for imperfectly flat surfaces? I'm thinking something like thermal paste for CPUs vis-a-vis cooling radiators/fans, yet for electrical conductivity. I looked around quite a bit for something that fits the bill, but from what I gathered it doesn't seem like anyone makes a truly conductive - highly conductive - grease. I saw anti-oxidzers and maybe one item that claimed to increase conductivity, called 'No-ox-id A Special', but, not sure what to think of these... Here's a link to that last one: https://www.amazon.com/dp/B00HSW341A/_e ... 9BLCWZ43RA

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spinningmagnets
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Re: Resistance, heat, busbars for battery pack...

Post by spinningmagnets » Wed Nov 08, 2017 7:34 pm

"Conductivity improving grease project" (liveforphysics, 5 pages)
https://endless-sphere.com/forums/viewto ... 14&t=61542

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Re: Resistance, heat, busbars for battery pack...

Post by liveforphysics » Thu Nov 09, 2017 2:29 am

Design around uniform current density. Use extra material near any current path heat sources (fuse, contactor etc) if the bus temp has a good thermal path to the cells.

Generally over sizing the series string and end terminations tends in practice to have a pack that ages better and has fewer failure modes.
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eq1
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Re: Resistance, heat, busbars for battery pack...

Post by eq1 » Thu Nov 09, 2017 3:19 am

spinningmagnets wrote:"Conductivity improving grease project" (liveforphysics, 5 pages)
https://endless-sphere.com/forums/viewto ... 14&t=61542
Interesting read (or skim). But it kind of peters-out at the end... Fun to see others thinking about these things...

So, at the end someone says this stuff might be good: CHEMTRONICS CW7100 Silver Conductive Grease
Anyone have experience with it? It's actually available at Walmart for like $27 -- for an oz...

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