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Please critique my configuration

Piper

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Apr 18, 2021
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I bought two case of the Navitas 25 A/h batteries when they popped up. They have been resting comfortably at half charge but I am now ready, copper bus bars in hand, to assemble them into three packs. I plan on two batteries of 24 cells (150A/h) and one of 12 cells. The big ones are for the house (sailboat) bank and the smaller as the starter battery for the two outboards. This layout is ideal for the space I have available, though if I am way of base I can possibly make two long skinny packs work.

Below is my proposed configuration for the large packs. Any suggestions, thoughts or comments welcome.

I tested the voltage and resistance of these when I got them and 29 of each box of 30 was at identical voltage and resistance. One cell in each box was fractionally different. And please excuse the primitive drawing, I could not find the original drawing I did. The blue scribbles on the bottom right are leftover from something else and are irrelevant to the pack. I plan on compressing these between fiberglass/foam composite board using stainless all-thread, then mounting them in closed boxes of the same foam/glass composite board.

IMG_20210429_205214998.jpg
 
Looks fine. This is called 3P8S configuration (or 3S8P, depending on who is writing the comment). Just balance their state of charge first. I would also use a really fat bus bar or cable on the one leg in the middle on the right, where (+) loops back to (-) in the middle of the pack.
 
Thanks Paul, I appreciate the input. I have a mix of 1/8 x 1/2 and 1/4 x 1/2 inch bus bar. I was thinking of using the heavier bar, rated around 220A for the entire right side. Do you think this'll be adequate? I will be using 2/0 cable with fuses for connecting the packs to the system. And I don't anticipate the current draw will ever be close to the capacity of the batteries. Incidentally, Electrodacus and 900w of solar through 4 MPPT's with the control cables to the BMS for charging and management. I should be able to keep them cycling gently and hopefully they'll last for years...
 
That’s probably fine. I think most people using copper stock for their bus bar use 1” wide and not 1/2” wide though. For comparison my bus bars that came with my cells are nickel plated copper and are roughly 3/16” x 3/4”. I can go out and measure one to double check if you’d like a comparison point. I think 2/0 is probably overkill since most of the loss is in the connectors and not the wire itself, but we all like to overkill the design in some places.
 
That’s probably fine. I think most people using copper stock for their bus bar use 1” wide and not 1/2” wide though. For comparison my bus bars that came with my cells are nickel plated copper and are roughly 3/16” x 3/4”. I can go out and measure one to double check if you’d like a comparison point. I think 2/0 is probably overkill since most of the loss is in the connectors and not the wire itself, but we all like to overkill the design in some places.
Thanks again. If you have a chance the measurements for your busbars would be an interesting comparison but I don't anticipate drawing anywhere near the capacity of the bank at any one time. If I turn everything electrical on the boat on simultaneously, including all the emergency bilge pumps it won't add up to 75A and the fuses are sized for the appropriate load rather than the wire size. My whole system is probably overkill for my anticipated electrical use but if I decide I need an electric anchor windlass rather than the manual one or an electric outboard for the dinghy, it won't present any problems finding the capacity. Now I just have to go and run a thousand feet or so of various gauge wires and hook it all together....
 
I decided to measure anyway for reference. My bus bars are: 2mm thick x 20mm wide x 90mm long (metric because they're from China)

So that's actually not 3/16" like I said above, it's a bit thicker than 1/16". In imperial units it is:
5/64" thick x a bit over 3/4" wide x a bit over 3.5" long

Yours need to be a bit longer (approx 4") because of the direction you're going between cells. But that should be ok since you're making them custom. The diameter of the EVE terminals is roughly 15mm or 0.59", so what you lose with 1/2" bus bars is full contact with the terminals...you might want to go to at least 5/8" wide to get full contact, or ideally 3/4" like the ones that come with the cells.
 
Paul,

Thanks for measuring your bus bars. It reassures me that mine are okay. I checked the ampacity and for the 1/4 inch bar it's rated a 238A. The batteries I have are the lime green Navitas 25A units, so very different from the EVE cells. The terminals are 6mm and because of the small battery size are very close together. In retrospect I should probably have used fewer high capacity cells to get my two 150 A/H packs as this build is quite complex with lots and lots of connections but I already have these. They were pretty reasonably priced for what will give me 375A/H at 12v, quite a bit cheaper than the larger cells. They seem to be really good quality and should work well for my application as long as I can keep them safely constrained in my battery boxes. I read a number of threads about bad Chinese cells and looooong delays in shipping so I was a bit reluctant to buy the larger cells on Alibaba.

I appreciate your input. Thanks.
 
I have are the lime green Navitas 25A units, so very different from the EVE cells. The terminals are 6mm and because of the small battery size are very close together.
Ok great! So your busbars are wide enough then, if they cover the entire terminal. I was obviously assuming the 272/280/310Ah cells most people are using on this forum.
 
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