Current carrying capacity of Nickel Strips

[Sunder]

Hi all,

I'm building a 8S23P battery at the moment, and I'm at the stage where I need to connect the "S"s to each other. I won't mince words here, the cells are crap. They're 1C rated LiFePo4, and I think even then, the manufacturer has exaggerated. Each 5Ah cell is about 140mOhms, and after soldering 23 of them together, each 23P pack is still about 6mOhms. Not a lot of point to this, except to say that I'm not massively worried about underspecing the joints, unless it gets so hot as to melt the solder. Then I'd be worried.

I had read on here that 8mm x 0.15mm nickel strips used to spot weld 18650s and the like can carry 6A. I was hoping to do 50A sustained, with 80A bursts through this battery, so it would seem that I should solder at least 10 - 14 of these strips to the battery. Each of these strips is 5cm long, but there's about 1.5cm contact with the tabs on either side, and 2cm is actually in free air.

So, just asking the question... Should I just make a massive thick pile of these into a kind of link, or would I be better off soldering one or two on the battery, then layering maybe 2-3mm of solder on top of it to do the real conducting? Basically, I'm trying to get out of work... Been building batteries for my electric bike, my electric motorcycle, my dad's farm (this one), and even a buffer battery for an inverter in my car the last month or so, and I'm sick of soldering. Gone through about 1/2kg of rosin cored solder, and 1/2kg of plumber's stick solder in the past month.

What I'm really asking is for someone to give me permission to "cheat" and shortcut this job.

 

[kdog]

I tested some 8mmX.15 ni strip according to the amperage in nobuos repositiory (page 1)
after the amp rating is exceeded heat production goes up rapidly so I wouldn't push it much- like 1.5 times max cont. and may be 2 times peak if intermittent. of course its all about heat so if you don't mind you batteries getting hot then this maybe acceptable.
if its 23p can't you just do a strip from each cell to the next (is it a block layout that allows this). id use .2 mm nickel as well
if you cant do that, then you could solder multiple thinner copper wires for the S connections, cheap and easy, but soldering is a hassle
solder has poor conductivity so don't use that alone, and dont pile up a whole buch of nickel ontop of its self- poor current sharing between the cells .
a pic would help show the layout.
you could wait until I find you a good alternative to nickel currently trying to do so (It wont be quick)

 

[Sunder]

Thanks Kdog.

23P, 7 connections between 8S, 2 ends to each connection = 322 solder joints... It's actually what I was hoping to avoid, but what it looked like I needed to do. If each cell is 1C rated, and is 5Ah, then a 6A rated connector is probably ideal. The 322 solder joints isn't.

I would use a thick copper bar, but I understand that it's hard to solder copper to nickel.

I'll put up a photo tonight.

Thanks.

 

[redilast]

You might appreciate the testing I did on that exact size of tabbing material 0.15mm * 8mm.

All tests were done at around 25C ambient temperature in a room with little to no moving air. The test was done using an electronic load, bench power supply and fluke thermocouple for measuring temperature. Each temperature measurement was taken after 5 minutes.

5-10A continuous use is the sweet spot for this tab size. 15A+ bursts are no problem, but it is recommended to use some insulating sticker rings so if it does get hot enough to melt the wrapper its not going to short out the cell.

 

[Sunder]

Wow. That went up faster than I thought it would. Good to know.

 

[Sunder]

Well, here are the 8 "packs" of cells.



I'm now thinking maybe a strip up the top of each pack, two spread in the middle, and one at the bottom, all slathered in at least 2-3mm of solder over the whole surface area.

A bit of a compromise to allow me to do less soldering, while keeping cell voltages more even throughout the pack, and hopefully keeping heat down. I will probably try it first, put a mid level load on it for 5 mins, see how it reacts, then either solder more joiners, or increase the load.