100a help.. made a few posts on the wrong side of the forums!

[weegee32]

https://endless-sphere.com/forums/viewtopic.php?f=10&t=112357

I made a few posts on a build thread a few weeks ago. I wasn’t aware we had a battery tech. thread, my apologies for polluting the pages up!

Anyways, I’ve run into some trouble while connecting 24 LifePo4 cells in series. They discharge at 100a continuous and they do not have screw tabs. They have some thick, metal that nickel can easily spot weld to.

I purchased 0.15mm (thick) x 9.5mm (wide) copper strips from eBay, cuttable lengths. I looked at copper strip ampacity tables and found I need more volume of Cu than what I have already begun to mount on my tabs. I originally referenced a source that made me believe what I was mounting would be sufficient. Currently I have 4 of my strips (mentioned above) stacked tall. Each strip is roughly 50mm in length.

Given what I just mentioned, the cross-sectional area of the copper is 9.5mm x 0.6mm = 5.7mm^2 at 50mm length. From what I referenced, this is not nearly enough to handle 100a and maintain a safe temperature for the batteries. Am I correct? I have struggled with this for a little bit now and became impatient when spot welding a few of these bars on the tabs.

I have only made 6 connections with this copper volume and believe I need to up it to at least 15mm^2 of cross-sectional area. I have also done the mounting unorthodox and possibly unsafe. What I did was I laid the bars flat, took nickel strips over the top and spot welded either side to the tab. It seems very, very sturdy yet the copper is not welded itself to the tab.

Can someone help me out. I can provide pictures to what I have done thus far.


Joe

 

[goatman]

what batteries are those?
can you unscrew that terminal your spotwelding to from the battery?

 

[weegee32]

what batteries are those?
can you unscrew that terminal your spotwelding to from the battery?

“LiitoKala 3.2v 50Ah lifepo4 cells 3.2v lifepo4 lithium batteries for electric bike battery pack solar energy system” is how you can find them. These are from AliExpress. No, the tabs came laser welded onto the battery.

I feel as if the method I provided could work if I simply stacked enough of the copper strips onto each other and had them truly securely mounted. Otherwise I feel as if I have to solder onto these tabs.

 

[Hillhater]

the cross-sectional area of the copper is 9.5mm x 0.6mm = 5.7mm^2 at 50mm length. From what I referenced, this is not nearly enough to handle 100a and maintain a safe temperature for the batteries. .......

FYI... The thickness/number of the copper strip will not change the temperature of the battery cells.
Cell temp is purely determined by its IR and the current flowing.

 

[Chalo]

FYI... The thickness/number of the copper strip will not change the temperature of the battery cells.
Cell temp is purely determined by its IR and the current flowing.

AND the heat conducted back into it by hot interconnections.

 

[goatman]

looking at this picture, id drill the four corners and rivet the bus bars onto each terminal.
you would need different copper. id almost just go and get a foot or two 1/2" soft copper pipe , slit it down the middle and open it up flat giving you a 1.5 inch wide bus bar that about 1/16" thick

 

[weegee32]

the cross-sectional area of the copper is 9.5mm x 0.6mm = 5.7mm^2 at 50mm length. From what I referenced, this is not nearly enough to handle 100a and maintain a safe temperature for the batteries. .......

FYI... The thickness/number of the copper strip will not change the temperature of the battery cells.
Cell temp is purely determined by its IR and the current flowing.

The current that is able to flow is resisted by less copper strips though, no?

 

[weegee32]

FYI... The thickness/number of the copper strip will not change the temperature of the battery cells.
Cell temp is purely determined by its IR and the current flowing.

AND the heat conducted back into it by hot interconnections.

Why would there be hot interconnections? Sorry but I am still relatively new to this.

 

[Chalo]

FYI... The thickness/number of the copper strip will not change the temperature of the battery cells.
Cell temp is purely determined by its IR and the current flowing.

AND the heat conducted back into it by hot interconnections.

Why would there be hot interconnections? Sorry but I am still relatively new to this.

P = I² R

Heating in the cell connections is a function of their resistance and the square of the current passing through them.

 

[larsb]

i think some solder between the copper would get this in the right resistance for 100A, what’s the RI2 loss if you calculate it in theory?

 

[Chalo]

This problem is kid stuff if you use cells with threaded posts. Welded terminals are unsuited to DIY use except in low power applications.

 

[fatty]

I wouldn't trust that attachment method regardless of current/copper thickness, though it'll be worse with resistive heating since the different metals will expand at different rates, potentially loosening, reducing contact, and further increasing resistance.

Hindsight is 20/20, but the poor/limited attachment methods are reason enough not to use that type of cell.

Looks like the posts could be drilled and tapped for small machine screws though. That, or solder.