I guess I need to make 72v 35ah battery pack

This may be hard to visualize, but this is what first came to mind. This would be 1 parallel, of 7.
20 cells welded in series, folded over at the halfway point along the doted line. That makes it 4 rows of 5 cells stacked 2 cells high, and 2 cells wide. I could easily get 7 in my frame.celllayout.jpg
 
harrisonpatm said:
This depends on your pack layout. Perhaps you can draw it, or at least describe it? I just skimmed through the thread quickly, so perhaps you've stated it and I just missed it, but it seems like you're planning a 100a capable pack. Meaning your total series connections need to be able to handle 100a. How many strips of nickel will be in the parallel connections? I couldn't tell for sure from your descriptions in the above posts whether you're planning 7p or 10p.

endless-sphere.com

Nickel Strip ratings

Hi All, I am just wondering if someone can point me to a chart or resource that will show me the current ratings for this: https://eu.nkon.nl/accessories/battery-solder-strip/nikkel-batterijpack-verbinding-soldeerstrip-27mm.html nickel strip 27mm x 0.15m. I have asked Nkon and they don't...
endless-sphere.com endless-sphere.com

If you follow this chart, you'll see that .15x8mm nickel is only good for about 5 amps. To safely carry 100a, you'd need 20. If you're planning 10p, and there's 10 series connection points between all your parallel groups, then it's "possible" to get what you need by stacking 2ea .15x8mm strips per nickel connection. If you're doing 7p, I wouldn't recommend stacking 3 strips.
Click to expand...
I see what you mean. According to the chart 0.15 x 8 is only equivalent to 7g wire. 0.3 is like 14g. 14g seems small for 20 amps? Maybe use 0.3 with 0.15 welded on top?

My welder came today. I'm not sure how thick of metal it can weld. I didn't realize I had to put it all together. It's like a test, if you can't figure it out put your hands up, and back away from the battery box.
 

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I'm now thinking of 2 ways of building a 72v pack that would fit my space.

Plan a
7 parallels of groups of 20 cells in series = 72v, 140 cells
or
Plan B
groups of 7 parallel cells, in 20 series, = 72v 140 cells

I'm guessing either method would end up the same, but I'm not sure. Would one way be better then the other?
 
I believe that the ideal layout for a battery to stack each group in one column. Then place the groups end to end, so that the series current flows directly from one cell to the next. Here is how it would look for a 13S-7P. Each horizontal section of nickel needs to be thick enough to carry 1/7 of the battery current, The vertical connects are only for small parallel currents, so don't have to be very heavy, At the positive and negative ends of the array, you have to connect the power cables so the current flows evenly out of all 7 cells.
13S-7P.jpeg

I do not like stacking strips, I think they can be problematic when you do more than two. However, you have a big welder so it shouldn't be a problem.
 
docw009 said:
I believe that the ideal layout for a battery to stack each group in one column. Then place the groups end to end, so that the series current flows directly from one cell to the next. Here is how it would look for a 13S-7P. Each horizontal section of nickel needs to be thick enough to carry 1/7 of the battery current, The vertical connects are only for small parallel currents, so don't have to be very heavy, At the positive and negative ends of the array, you have to connect the power cables so the current flows evenly out of all 7 cells.
View attachment 351829

I do not like stacking strips, I think they can be problematic when you do more than two. However, you have a big welder so it shouldn't be a problem.
Click to expand...
I'm finally starting to understand how, and why, to build a pack with more parallel cell groups as opposed to all series cell groups. Apparently building with parallel cell groups in series, instead of single cell groups in parallel you get less sag. I hadn't thought about that before, but it makes sense to me. If I'm wrong about this feel free to say so. Nothing is set in stone.

I'm now thinking in terms of 20 groups of 7 parallel cells each. That still ends up 72v and 140 cells.
 
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I ordered this BMS. Looks intimidating. If anyone knows anything about this brand let me know. There's still plenty of time to order a different one.

BMS wiring diagram.JPG
BMS.JPG
 
Ok moving slow, but I finally got to the point where I can calibrate my KWelder.
First problem, I can's seem to get it too calibrate. When I move through the menu everything seems good. When I get to the point were it says to touch the probes together, and it the button resistance is in spec 2.89 MR but it still doesn't calibrate.

I'm wondering it the used car battery I'm using is the culprit. It was given to me by a friend after he bought a new one for his can. When I first checked i's voltage it was only about 9v, but after charging it for a while it holding 12.8v for over a half hour.

What makes me suspicious of the battery is when I get to the last calibration step, I short the probes, hit the button, and the first thing on the display is, under current, then it scrolls through 4 items. R 2.89, STD 46.0, I 496a, T 28.9c
 
Theodore Voltaire said:
What makes me suspicious of the battery is when I get to the last calibration step, I short the probes, hit the button, and the first thing on the display is, under current, then it scrolls through 4 items. R 2.89, STD 46.0, I 496a, T 28.9c
Click to expand...
The bare minimum is 800 A for proper functioning. 1200 to 2000 A is the recommended range. 496 A is inadequate. That battery is very tired.
 
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I bought a new battery at Walmart, and that solved the calibration problem. Now I'm going to practice. After a little trial and err l decided 130j worked pretty good on 0.15
 
Since my first design was a flop, this is my latest new and improved design. I'm hoping this one won't be quite as embarrassing. The reason it's like this is because of space limitations. 20s 7p. In the end it will be 5 cells wide, and 2 cells tall, and 14 cells long, 140 cells in all. The red box is the shape of the battery compartment. I'm thinking I should fold the battery pack in half along the red line making it 5 cells wide, unless anyone knows a better way. I'm not exactly sure how to connect the 10th series to the 11th. I'm thinking maybe 3 8g wires soldered like in my diagram. I want to get this right so I don't mind constructive criticism. Fire away. 20s 7p.jpg
 
I'm ready to order the battery cells. I'm planning to use Samsung 50s x 160, that will give me 20 extra cells. This is the first time I've ordered battery cells. When I search them the first 2 online stores are 18650batterystore, and IMR Batteries. Does anyone know a better place to order from?
 
Last night after getting my welder to calibrate my good friend and I started testing it's out. If you've ever worked with an engineer then you know what a pain in the ass they can be to work with. He's a retired double E from Texas Instruments. Even though he has no experience building battery packs he loves to remind me that he knows more then I do. He doesn't know about you guys standing behind me though. He conveniently forgets that when we first failed to get the welder to calibrate, I was the first one to guess it might be a worn out battery.


first test welder.jpg
 
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