orangezero
10 µW
I just registered here and read through 12 pages of people discussing copper/nickel spot weld sandwiches.
I am in the process of upgrading a 8s40p 32650 lifepo4 24v lifepo4 battery pack test run into something much larger. I have 1200-1400 new 32640 lifepo4 cells, some 0.15mm x 10mm nickel strips and a malectrics spot welder. I was considering 0.10mm or 0.15mm copper to connect the cells in series. This build is not for a vehicle but will be for a stationary off-grid solar system, currently using a 4000w inverter and lots of smaller DC loads. The draw on these cells, per cell, should end up being quite low. The max my solar could theoretically charge is 120a at 30v, but I only have 5.5kw (3kw in use) of solar right now. I would see using maybe 200-300amps at some point in the future if I upgraded my inverter.
My super simple plan was to have a rectangle/square of cells, negative on bottom, stacked in series above. Eight layers of cells, 8s120p. It would basically be 120 cells parallel, with another 120 cells stacked on top, with another 120 cells stacked on top in a very boring fashion. I am not sure if 0.15mm x 10mm nickel strip spot welded to each cell is enough for parallel connections, which is why I was reading about copper strips. I've read over and over that this parallel connection is basically to keep each cell connected in parallel at the same voltage, and this wouldn't need much even as it is drawing 100a from the battery pack.
I found a similar setup with battery born 12v batteries (and a few on here today), and it looks as if the full nickel sheet of one positive end is literally just laying against the nickel sheet from another group of cells, which is then compressed. So, in my case, would gravity almost be enough to provide a connection, similar to putting multiple AA or D batteries in a remote? Oddly enough, I went with cylindrical cells so I would avoid having to compress them, but I could also see how running some insulated threaded rods top to bottom wouldn't be difficult and keep things more stable.
I did some mass charging of cells, 8 in series and didn't bother connecting them in parallel at all, and was amazed how well they all fit together. No heat increase at the interconnections of the cells based on a thermal camera. Long-term I'd definitely want parallel connections... just not sure on a proper/efficient way to do it. In some weird thought experiment spending a lot more on 1/4in copper plates to go between each group in series doesn't seem like it would have the benefits I was originally envisioning.
I could see the bottom negative and the top positive would likely extend an inch or two into a busbar of sorts, allowing other connections to my inverter/fuses/solar.
Also, with my suspected use, it still seems I should be putting the cells pretty close together rather than leaving more space between them for airflow/temperature control.
And if anyone read this far down, feel free to throw out comments about arranging my cells 10x12 per layer vs something more rectangular like 6x20 or 3x40. Lots of powerwall diy I see they do 4x40 and hang them on a wall. I realize this will be 250-300lbs of just cells, and my concrete floor seems the best place to take that weight.
Final thought is if I need to purchase the specific positive end paper covers, or if I could laser cut some kaptom or other material for a similar result.
Appreciate any thoughts.
I am in the process of upgrading a 8s40p 32650 lifepo4 24v lifepo4 battery pack test run into something much larger. I have 1200-1400 new 32640 lifepo4 cells, some 0.15mm x 10mm nickel strips and a malectrics spot welder. I was considering 0.10mm or 0.15mm copper to connect the cells in series. This build is not for a vehicle but will be for a stationary off-grid solar system, currently using a 4000w inverter and lots of smaller DC loads. The draw on these cells, per cell, should end up being quite low. The max my solar could theoretically charge is 120a at 30v, but I only have 5.5kw (3kw in use) of solar right now. I would see using maybe 200-300amps at some point in the future if I upgraded my inverter.
My super simple plan was to have a rectangle/square of cells, negative on bottom, stacked in series above. Eight layers of cells, 8s120p. It would basically be 120 cells parallel, with another 120 cells stacked on top, with another 120 cells stacked on top in a very boring fashion. I am not sure if 0.15mm x 10mm nickel strip spot welded to each cell is enough for parallel connections, which is why I was reading about copper strips. I've read over and over that this parallel connection is basically to keep each cell connected in parallel at the same voltage, and this wouldn't need much even as it is drawing 100a from the battery pack.
I found a similar setup with battery born 12v batteries (and a few on here today), and it looks as if the full nickel sheet of one positive end is literally just laying against the nickel sheet from another group of cells, which is then compressed. So, in my case, would gravity almost be enough to provide a connection, similar to putting multiple AA or D batteries in a remote? Oddly enough, I went with cylindrical cells so I would avoid having to compress them, but I could also see how running some insulated threaded rods top to bottom wouldn't be difficult and keep things more stable.
I did some mass charging of cells, 8 in series and didn't bother connecting them in parallel at all, and was amazed how well they all fit together. No heat increase at the interconnections of the cells based on a thermal camera. Long-term I'd definitely want parallel connections... just not sure on a proper/efficient way to do it. In some weird thought experiment spending a lot more on 1/4in copper plates to go between each group in series doesn't seem like it would have the benefits I was originally envisioning.
I could see the bottom negative and the top positive would likely extend an inch or two into a busbar of sorts, allowing other connections to my inverter/fuses/solar.
Also, with my suspected use, it still seems I should be putting the cells pretty close together rather than leaving more space between them for airflow/temperature control.
And if anyone read this far down, feel free to throw out comments about arranging my cells 10x12 per layer vs something more rectangular like 6x20 or 3x40. Lots of powerwall diy I see they do 4x40 and hang them on a wall. I realize this will be 250-300lbs of just cells, and my concrete floor seems the best place to take that weight.
Final thought is if I need to purchase the specific positive end paper covers, or if I could laser cut some kaptom or other material for a similar result.
Appreciate any thoughts.