Welded stud 304ah: busbar issue/question.

[nebulight]

My 304ah cells arrived the other day. The only option were the welded stud option so that's what I went with. Since this is going to be installed in to a camper van, I opted to buy an extra set of flexible bus bars that I saw on @LithiumSolar youtube channel (

) to better handle the road vibrations. Might be overkill as my last van had rigid bars, but they weren't terribly expensive. They are 3mm thick instead of 2 mm thick as I will be running 12v. Since my 3000w inverter can pull up to 260 amps I wanted to upgrade.





The bus bars aren't here yet, but as I was in the process of unboxing and inspecting the cells for damage (they were all fine) I had an "Oh crap" moment. I will be installing them in a 4p4s config, so I need to double up the busbars at the serial connection. The stock welded studs are only 10mm high so where I'm doubling up the bus bars for the serial connections they are going to be 6mm (+- a few fractions of a mm), I won't be able to get the nut all the way down on the stud. I took the stock bars to kind of simulate the new bars. I took three bars which is 6mm and put the nut down and there are about 2 of the 5 threads not catching anything:





I'm curious if this is going to matter much with a flange nut. I went overboard with the bus bars, but in reality I will never pull that much from the inverter, but since the inverter can do it, I wanted to have the system ready for any draw it can handle.

The stock bars are 40mm2 they can handle about 150 amps where as the upgraded bars are 60mm2 and should be able to handle 200amps and I planned on doubling up the serial connections between two cells. Now I'm wondering if I should just stick with the stock bars or a combination of the stock bars for the serial connections and the flexible bars for the parallel connections (3mm + 2mm of the stock bar but I can double up on them between two of the parallel cells). Here is a stack of 2 cells at 4mm. It just barely covers the stud so I figure 1 extra mm should be okay. I would cut down the length of stock bars for the serial connections and have enough were I could do a serial connection at each cell. I'm not really worried about flex at the parallel connections since the cells are narrowest at that point.





What are the forum's thoughts on this? I can do more testing once the flexible bars get here.

Thanks!

 

[time2roll]

I would be looking at a second BMS and go 2x 2p4s
Not sure I like the doubled up bars either way.

 

[nebulight]

I'm using a REC BMS, so I'm not going to split and have multiple units. The double bus bars will only be for the serial connection.

 

[WindWizard]

I would not use the busbars with the nut only catching on a few threads. I can see the stud stripping and then you will have major problems. It would be better to catch all of the threads. I am not sure how you could address it with the flexible busbars. I use 1/2" copper L pipe for all of my connections. It is not flexible but it gives a good connection for high current. Just cut it with a pipe cutter and then pound it flat with a hammer and then use a wire drill brush to make it shiny and then plate it with nickel.

 

[nebulight]

It looks like they are slightly less than I thought (or the stock bars are slightly thicker) but it appears to catch a little tighter. However still not full on tight. The bar for the serial connection may just have to be the stock bar cut down to size. Maybe I can shave off some of the serial bar for a better fit as the plan was to have two bars for each serial connection so the load would be shared by each bar.

 

[RCinFLA]

2x20x 72 mm hole spacing copper core bus bars with nickel plating are about 0.05 to 0.07 milliohms depending on nickel plating thickness. Electrical grade 99.9% pure oxygen free copper of that dimensions is theoretically 0.0303 milliohms. Don't expect that from Chinese supplied bus bars. Much more likely to receive nickel plated brass bus bars which are almost 0.2 milliohms resistance.

You will likely have more resistance in the two aluminum terminals surface to bus bar surface connections combined. An excellent terminal connection is 0.05 milliohms for each terminal compression connection.

That yields a 0.17 milliohm resistance for a good battery post-bus bar-battery post path resistance. That has 3.8 watts of heating for 150 amps. It can easily be significantly worse if aluminum terminals are poorly prepared and will cause the hot bus bars/terminals under high current draw.

Nickel has almost three times the resistance of aluminum but as surface plating it should be thin and its benefits to providing inter-metalics isolation barrier outweighs the net resistance degradation if there is reasonable control over plating thickness. Bare copper to aluminum can have inter-metalic galvanic reactions that corrodes and pits aluminum terminal surfaces over time.

Unless bus bars are really thin, less than 2 mm, you will likely increase the resistance by parallel stacking bus bars due to multiple contact surfaces.

Have not done or seen any info on the laminated flexible bus bars. Concern is the multiple laminations surface to surface contact resistance. These would have to be evaluated under a bolted down compressed pressure test or there will surely be a higher resistance just measuring unattached bars.