How do I size bus bars for my LiFePO4 280Ah cells?

PeteW

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I've received my order of (16) 280 Ah LiFePO4 cells, but the bus bars washers and nuts I expected to be with the cells were not. Now I need to order some bus bars and I need to determine the correct size. Naturally, they need to be long enough to fit cell stud to cell stud, but what about thickness and width? How do I determine they can handle charging and discharging efficiently without over heating?

As for the washers and nuts, is zinc coated okay or should I be thinking stainless? I notice that my studs are probably only a zinc coated steel. I say this because of their color and the fact that they are magnetic. The batteries will be in a travel trailer and protected from weather.
 

FilterGuy

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Naturally, they need to be long enough to fit cell stud to cell stud, but what about thickness and width? How do I determine they can handle charging and discharging efficiently without over heating?
The primary determination will be current.
Current will be determined by 3 things
1) What max load will be on them.
2) What the configuration of the batteries is
4) What is the max charge that will be on them. (This is usually *not* the driving factor)

Once you determine the current, you can use this resource to determine sizing.

Note: I generally try to go select bus bars sized for at *least* 50% more than the current.
 

PeteW

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The primary determination will be current.
Current will be determined by 3 things
1) What max load will be on them.
2) What the configuration of the batteries is
4) What is the max charge that will be on them. (This is usually *not* the driving factor)
Thanks for pointing me to this resource. You're probably getting tired of constantly bailing me out. 😄

My plan is to configure the cells into a 24V 560Ah battery. I'm planning on using a hybrid 3000 W inverter. My trailer will only be wired for 120V, 30A, on the AC side. I guess I still need to figure out exactly what that means in terms of load on the batteries, etc. Solar panel wise I will be at 800 W (8, 100 W panels), which I think I'll wire in sets of 4 in series and then parallel the two sets. I'm planning on mostly charging the batteries that way, but of course they will also occasionally be charged form shore power or a generator. I don't think I'll try and set up a way to charge form the tow vehicle.

Honestly, I still haven't spent enough time on sizing the MPPT charge controller, but I will before I get much further. I've been dealing with multiple obstacles. I've been over coming installation issues with mounting my panels, which I've mostly figured out, as well as my over all learning curve. to that end, I've been reading a lot about electricity and electronics thanks to your material as well as others. It's a bit of a rabbit hole and my A.D.D. does get the best of me quite often sending me far afield. I do intend to put together a detailed diagram and post that for perusal and comments before I buy stuff like the charge controller and the inverter, etc. and begin physical assembly. At this point I can't even top balance or test my cells properly without bus bars.
 

FilterGuy

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My plan is to configure the cells into a 24V 560Ah battery. I'm planning on using a hybrid 3000 W inverter.

A 3000W 24V inverter @24V.... Lets do some math.

Assume a 90% efficiency. That means to pump out 3000W of AC it will need to draw 3000/.9=3333W from the battery. If the battery is low the voltage could be as low as 24volts, so the current would be 3333/24=139A. If we use my 50% extra we get 139A x 1.5 = 208A.

Looking at the chart, 1/8 x 3/4 would work.... or if you want to be extra cautious 1/8x1.
 

chrisski

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Looking at the chart, 1/8 x 3/4 would work.... or if you want to be extra cautious 1/8x1.
I got between 205 amps and 209 amps when I looked 1/8” X 3/4” copper.

I guess the question I have is, do you size the busbars no wider than the terminals? My 25 ah cells have terminals that are 3/4” wide, so that is perfect for them. if I put a wider busbar, it would overhang the flat area the busbar is supposed to attach to.

I do not know how wide the contact surface for the 280s are. Mine just came in today (excited!!), but it will be until Monday before I can unpack them.
 

PeteW

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A 3000W 24V inverter @24V.... Lets do some math.

Assume a 90% efficiency. That means to pump out 3000W of AC it will need to draw 3000/.9=3333W from the battery. If the battery is low the voltage could be as low as 24volts, so the current would be 3333/24=139A. If we use my 50% extra we get 139A x 1.5 = 208A.

Looking at the chart, 1/8 x 3/4 would work.... or if you want to be extra cautious 1/8x1.
Super helpful. Thank you! May I ask about your "50% extra rule"? Is it for surges? How did you arrive at that? Do you recommend any good sources for bus bars? Do you feel that it's best if they are plated?
 

FilterGuy

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Super helpful. Thank you! May I ask about your "50% extra rule"? Is it for surges? How did you arrive at that?

To be honest. I do not have a good reason for it. I just figure it is an area that there is no significant downside to go big on. Even the cost is not going to be that big.

Do you recommend any good sources for bus bars?
No.... sorry. I make my own. I buy the copper from onlinemetals.com

Do you feel that it's best if they are plated?
Yes
 

PeteW

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To be honest. I do not have a good reason for it. I just figure it is an area that there is no significant downside to go big on. Even the cost is not going to be that big.
Totally makes sense.
No.... sorry. I make my own. I buy the copper from onlinemetals.com
Thanks, I was looking at this myself, but wasn't sure about the plating. Thanks for clearing that up as well.
Yes
Thank you.

Well, back to my cave! 😅
 

PeteW

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Okay,so I guess I still have a question or two. Since you have done this, do you have a preferred method? Also, do you just drill holes in your bus bars or do you oval them a bit.
 

chrisski

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I nickel plate using that method. I tried tinning my Bars with a MAF torch, and did not work for me. The MAF torch came out sloppy.
 

PeteW

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I nickel plate using that method. I tried tinning my Bars with a MAF torch, and did not work for me. The MAF torch came out sloppy.
Thank you. It looks doable. Based on your experience about how long does it normally have to be the solution?
 

FilterGuy

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Also, do you just drill holes in your bus bars or do you oval them a bit.
I do try to oval them a bit..... But it is difficult to do without a mill. One technique a poster said recently but I have not tried yet is to use an end mill in a drill press to do a plunge cut to make the hole oval.
 

chrisski

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how long does it normally have to be the solution?
Took four hours. I used a power supply, 30 volt, 10 amp, and ran it between 10 and 15 volts and between ,3 amps and 1 amp. Could make a difference if you used a AA battery or old computer power supply, but people have used those in videos.

With nickel, I don’t think timing is all that important. It’s fairly cheap, and if you go to long all that will happen is the nickel on one lead will disappear into the solution and on the other lead with a bunch of “salts”. I just filtered these out with a paper towel.

There’s a whole art behind plating, but I’m not going for the shiniest finish or putting additives in it also.

As far as plating there’s YouTube videos, there’s some good ones for the basics like in the resources. There’s not much else available I could find, but its not that hard.
 

PeteW

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Took four hours. I used a power supply, 30 volt, 10 amp, and ran it between 10 and 15 volts and between ,3 amps and 1 amp. Could make a difference if you used a AA battery or old computer power supply, but people have used those in videos.
Is this 4 hours to electroplate a single bus bar? A bit more detail please. I would hope theres a way to do several at once.
 

PeteW

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If you have any tips...... I am all ears.
At this point I do not, but I'd love to return the favor of information as you have provided me with so much. I don't have any copper at this point but I do have some 1/8" aluminum to do some testing on. It won't be right away but I will do some testing and let you know what I can do. It's been a while since I used it and as I remember it was clumsy to work with,and I was also doing something different with it. Been so long ago I don't really remember what I was doing. :rolleyes:
 
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