Home made bus bar specs?

tacothat

New Member
I recently bought 16x280ah prismatic LifePo4 cells. The included terminal bus bars are not great. More importantly I only got 20 bus bars, so I can't top balance my cells. I looked for terminal bus bars on aliexpress - they were expensive, and shipping was slow. So I figured I could make my own bus bars, I looked into buying a copper bar and machining it into busbars, but I don't have the tools. Then I figured I could just use a battery cable with hydraulic crimped ring terminals. I like this idea because its easy to source the cable and terminals and I have access to a hydraulic crimper. What size cable should I use? based on an ampacity chart, I should use at least 1 AWG to get a 100 amps reliably to match the Ali Express bus bars. At a 1C discharge rate, I would have to use a 350 cable, and that just seems overkill. My main circuit has an 80 amp fuse, and I could probably program the BMS to close the discharge/charge relay at 80 amps too. In that case I could probably get away with 2 AWG bus bars.

Any recommendations?
 

smoothJoey

Get offa my dock
350 mcm?
That's very very big!
For batteries it's typical to size the wires and ocpd on the max expected current plus a margin to avoid nuisance trips.
How much current do you plan for?
What system voltage?
 

Opie91

Solar Enthusiast
Do you have a vice or clamp, hacksaw and a drill? Maybe a center punch if you want to be fancy. If you get a unibit too then your golden. Thats about all you need to make bus bars. It will be way easier and probably cheaper than crimping all of those connectors.

Order the copper from McMaster-Carr.
 

smoothJoey

Get offa my dock
For 280 amps 3/0 awg pure copper 105 Celsius wire or equivalent busbar.
Transitions add resistance. Bus bars half the number of transitions.
 

MisterSandals

Participation Medalist
Some folks flatten copper tubing/pipe to make bus bars for top balancing. Some even say they aren't as classy as we think they are. Electrons like them a LOT.

Fun fact:
Screen Shot 2020-08-21 at 8.54.52 PM.png
 

ianganderton

Covid free in NZ
I really like the idea of braided busbars

1598068590947.png

This image is from this post https://diysolarforum.com/threads/12v-560ah-build-for-overland-van.9164/post-116399

Braided and tinned earth strap with copper squares soldered on each side at attachment points. I've also seen copper pipe squashed in a vice at the teminal locations then drilled

Couple of advantages
  • Easy to manuafacture, the braid can apparently be cut with scissors
  • Flexible so any points dont need to be super accurate
  • Flexible so less stress put on teminals if there is any movement (swelling)
  • Easy to add multiple attachment points to one busbar
What I'm not sure of is the different specifications and whether it comes 'big' enough for the planned current. But seeing as its ground/earth strapping my guess is it should
 

nosys70

Solar Addict
it is a nice idea, but i would not solder the copper to the strap, because
1) the solder joint will be weak.
2) while soldering, all the solder will go first into the strap by capillarity making it very rigid.

i would either simply make a piece of copper U shaped and slip the stip inside. then with a press you just smash the coppper.
anyway when you screw the terminals, the screw will go through the strip and will press on the copper, so even without solder you can expect
a good contact.
I would also reduce de length of strip to minimum making copper longer, unless you really need that.
 

tacothat

New Member
Some more details: It's a 48v system, I don't expect to use more than 80 amps at any given time, the system is designed for long periods of low draw.

I got a deal on some 2/0 AWG welding wire, and quality tinned copper lugs. So that is probably what I will go with since 2/0 AWG gives me >150 Amps at the expected operating temps. I'll just be careful to make sure I program the BMS and inverter charger to only work at <80 amps and add some 80 amp fuses to the circuit.

I'm beginning to like the idea of a flexible busbar. This system mostly be in a moving vehicle and flexible cable busbars should be more resistant to vibration and movement. I've also read that LifePo4 cells expand and contract based on heat/charge, I'm not sure how much of an issue that is but flexible busbars should withstand it better. I'm almost glad I got stiffed on those dingy busbars.
 

smoothJoey

Get offa my dock
I'll just be careful to make sure I program the BMS and inverter charger to only work at <80 amps and add some 80 amp fuses to the circuit.

Don't forget the nuisance trip headroom.
Typically 1.25x max expected current.
I use 1.5x.
I take it you are using nec 90 celcius ampacity tables?
 

Haugen

Tron God
Some more details: It's a 48v system, I don't expect to use more than 80 amps at any given time, the system is designed for long periods of low draw.

I got a deal on some 2/0 AWG welding wire, and quality tinned copper lugs. So that is probably what I will go with since 2/0 AWG gives me >150 Amps at the expected operating temps. I'll just be careful to make sure I program the BMS and inverter charger to only work at <80 amps and add some 80 amp fuses to the circuit.

I'm beginning to like the idea of a flexible busbar. This system mostly be in a moving vehicle and flexible cable busbars should be more resistant to vibration and movement. I've also read that LifePo4 cells expand and contract based on heat/charge, I'm not sure how much of an issue that is but flexible busbars should withstand it better. I'm almost glad I got stiffed on those dingy busbars.
I'm building the same 48V system with 280Ah cells. I thought I would offer a little of my insight to this thread.

I don't see that anyone considered the corrosion aspect of the bus bars.
Copper connected to the aluminum terminals, especially in an environment where humidity or other moisture can be present will have corrosion and the threads of the aluminum terminals will be attacked first.

Due to this, I made my bus bars out of aluminum bar stock.
Attach them with 6061 screws and washers and you have a worry-free system.
20210211_191230.jpg
 

fhorst

making mistakes so you don't have to...
Andy from @Off-Grid-Garage shows his latest bus-bar on YouTube, aluminium :)

They look great.
It will solve the problem with dissimilar metal (galvanic corrosion)
You do need anti-oxidation compound like ox-gard as aluminium oxidised fast, really fast to a level where the oxidation is a potential problem.

Oxidation remover, or fine grid sandpaper/metal sponge to make clean, alcohol or acetone to remove grease, and quickly apply protection layer.

One challenge might be to find aluminium studs/headless bolts and aluminium nuts..
 

Dhasper

Solar Addict
Andy from @Off-Grid-Garage shows his latest bus-bar on YouTube, aluminium :)

They look great.
It will solve the problem with dissimilar metal (galvanic corrosion)
You do need anti-oxidation compound like ox-gard as aluminium oxidised fast, really fast to a level where the oxidation is a potential problem.

Oxidation remover, or fine grid sandpaper/metal sponge to make clean, alcohol or acetone to remove grease, and quickly apply protection layer.

One challenge might be to find aluminium studs/headless bolts and aluminium nuts..
I am still trying to get over how they pronounce aluminum in Australia.
 
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