Bus bar thickness?


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rogales141

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Feb 18, 2018
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Guys,

What do you think should be the minimum bus bar thickness for 24V- 7s80p system?
 

ajw22

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What do you think should be the minimum bus bar thickness for 24V- 7s80p system?

It totally depends on how many Amps the bus bar has to conduct, ie the power of the charger/load, ie. the charge/discharge rate.
Eg.
If you charge it with only a 10W PV panel, and powering a 10W LED lamp, then the current would be approx 10W/24V = 0.4Amps.
For that, anything that could be called a "bar" is overkill.
If you instead had 3kW PV panels and/or similarly sized load attached, the current would be 125A, requiring some serious bus bars.
 

Korishan

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It's not really the thickness directly, it's the square inches/mm that makes the difference. You can have a 0.1mm thick bar, but be 10mm wide, and it'll be able to carry the same current as a 0.5mm thick by 2mm wide.

If you are meaning 'bus bar' in the sense of just a heavy gauge wire that connects all the cells together, then generally speaking, just get the size wire that you'd expect to carry the max current that the battery will ever see. So being the pack is 80p, then each cell is probably capable of being able to do 1A, so 80A max for the pack. You would need an appropriately sized wire that can handle that current, which would be about 10 awg.
You can combine 2x 16 awg wires together, twisting them together, to achieve the same rating (actually, 2x 16 would be more inline with 8awg total, but be more flexible)
 

floydR

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https://www.electrical4u.net/calculator/busbar-current-calculator-online/ I have been told this calculater is conservative but the formula used is much the same thumb rule that the aluminium carries 0.8 times of the area, copper will be 1.2 times, silver will be 1.6 times, iron and GI will be 0.6 times.
THUMB Rule for Busbar :
For Aluminium : 0.7 Amps / 1 Sq.mm of Bar.
For Copper : 1.2 Amps / 1 Sq.mm of Copper.
later floyd
 

Roland W

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For Aluminium : 0.7 Amps / 1 Sq.mm of Bar.
For Copper : 1.2 Amps / 1 Sq.mm of Copper.
later floyd
Its more like 3,5A/mm2 for Al and 5A/mm2 for Cu.

Would really be tragic is a 50mm2 Cu conductor can only carry 60A ;)
 

Korishan

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Just for added reference, a 10mm2, or 8awg, wire can do up to about 150A (this is <4Ft/1M in length)
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1654340558817.png
 

rogales141

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Thank you all for your very educational answer. I'm looking for a wire (bus bar) thickness which will connect all 18650 cells. I think I will assume max 100 Amp for the whole battery -7p. Hopefully, 10 or 8 AWG should be enough.

Thank you
 

Korishan

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I think I will assume max 100 Amp for the whole battery -7p
I'm hoping this is a typo. Did you mean 7s instead?? Because 7p would mean each cell is having to output 14A to meet the 100A peak.
Going off your original post of 80p, then each cell would be 1.25A, which is far within specs of most cells.
So we'll just assume you meant 7s and/or 80p :p
 

Korishan

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I was thinking about the characteristics of DC vs AC and wire carrying current.

According to this calculator: https://www.omnicalculator.com/physics/dc-wire-size
8 gauge should be plenty. Even 10 gauge would be good. With a minimum voltage of 23V (3.4V/cell discharged), then 10 gauge would be the smallest to go with.

One thing to note is the wire length. The shorter the wire, the more current it can handle before heating up. This is because there's less resistance in the wire.
 

Redpacket

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The shorter the wire, the more current it can handle before heating up. This is because there's less resistance in the wire.
Not really - a given wire size can handle xxx amps for yyy temp rise & this doesn't really change with length.
It can change if the cable is in a bundle of other cables, in conduit, underground, surrounded by insulation ie any factor that means it can't loose heat generated as fast as "in free air".
Cables are also rated by the type of insulation on them, eg some PVC is only good to 75C, most cables 90C (here in Australia anyway, might be different in N. America...), XLPE higher temps.
Cables installed in roof cavities here have to allow for reduced current due to already hot ceiling air vs insulation max temp.
It's true that with length, the resistance & hence voltage drop for the same current increases. Eg if you get 0.1V drop for a 1m run, you would have 0.2V drop for 2m run.
Also info found related to cables for cars & motor homes industries may be underrated.
 
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Redpacket

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I tried that calculator & I think it has a bug or two!
If you put in some numbers & change the cable max temp, the size result does not change correctly.
If I increase the cable max temp, the cable size should drop (thinner cable gets hotter), lower the temp size should increase (bigger cable stays cooler)....

This one might be better (Australian oriented):
 

Korishan

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Not really - a given wire size can handle xxx amps for yyy temp rise & this doesn't really change with length.
The longer the wire, the more the resistance. This is why length matters. All wire-size/amp charts include length as a measurement.
 

Redpacket

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The longer the wire, the more the resistance. This is why length matters. All wire-size/amp charts include length as a measurement.
Yes that's true but it doesn't change the current carrying capacity rating.
Capacity ratings just look at safe temp rise in xyz situation.
If you were just focused on cable capacity limits, you could design a system to have the cables running hotter up near the insulation ratings.
But like you say, voltage drop is important so you design for lower drop = fatter cables (& cooler running too).
 
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