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How close can bare copper bus bars be (12v)

My wager is the less electronics involved the better. Vibrations heat and grime will never do them any good.
 
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My wager is the less electronics involved the better. Vibrations heat and grim will never do them any good.
All the electronics involved would be in the box on the back so no grime. Heat also shouldnt really be an issue as the box will be air conditioned. Vibration is an ever present issue but I have isolated the battery and all components so it should be minimal.
 
I have been hunting around trying to find a contactor I am happy with. Have not found one yet and will keep looking but am interested in what your thoughts would be on this approach.

The Battery protect has an on/off switch. when it is closed the BP is on, open it is off. My plan was to connect this to the electrodacus and it would open the switch at LVD. The problems with this due to the type of load is discussed above and I was going to abandon using the BP.

I am now considering the following:

Generally speaking winching should be done 30 seconds on 30 seconds off as the winch motor can easily overheat if you try winching continuously. I am thinking about building an interlock to prevent the BMS from opening the BP when winching but if battery becomes low while winching, will prevent the next bout of winching protecting the battery protect from destruction and the battery from low voltage.

The + and - for the switch on the BP is connected through the BMS. in-between the BP and the BMS a small normally open relay is inserted connecting the + and - . The coil for this relay is connected to both the spool in and out power connection directly on the winch motor. When winching, the relay will close and ensure that if the BMS opens the circuit, (I can program a LVD delay to make sure this doesnt happen during inrush) the relay will remain closed, the BP switch will remain closed and the BP will not disconnect and destroy itself. When winching stops, the relay opens and if the BMS is open the BP will open when there is no current and prevents any further winching if the battery is low.

Does that sound like a good solution?
 
Got the busbars all cut. I screwed up on the one that attatches to the 2 parallel fuses and connects to the two BP220s. Fortunately it was basically exactly what I needed for the output side so it wasnt a loss.

Disconnect on the left goes to the inverter, right one to the winches and supplies "boost" power to the vehicle charging system if required. The terminal fuse just below the winch disconnect is a branch circuit to a small load fuse block.

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Also, I was a bit skeptical about trying but it all worked out. It turns out you can cut copper on a miter saw just as you can aluminum. I dont know why I was so concerned about trying it now. I dont know how it would work with a saw you can not adjust the speed on though. I turned the Kapex down to 1400 RPM which is a very close match to the surface speed on the Fein Slugger I would have used if I had an aluminum blade for it. The FB was 1/4 x 1.5" and I also cut one piece of 1/4 x 2 (The piece bolted to the left side of the parallel fuses) with no issues at all.


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Looks very robust.

Is that a shunt in the middle?
Usually hear of fuse on positive, shunt on negative.
 
I’m no electrical engineer so feel free to disregard my thoughts as they’re not based on much real world experience.

Seeing the dual fuses looks like a reliability concern to me. If one of them has a resistance a few miliohms higher than the other, you could very unequal current between them causing one to blow prematurely which would then cascade to the remaining fuse.

As for the arcing potential during a shutdown, I turn to plumbing analogies with the large inductive load leading to the equivalent of water hammer destroying your valve (or FETs). For this case could you install a zener diode to arrest the water hammer by giving the inductive current somewhere else to go before the voltage gets high enough to burn up the FETs and start arcing.
 
Looks very robust.

Is that a shunt in the middle?
Usually hear of fuse on positive, shunt on negative.
LOL, yes "robust" :)

Yes, Shunt under the right switch. Ya that was my understanding as well but the instructions for the SBMS0 called for it to be the closest component to the batteries positive terminal. I cant argue, Im just following instructions.

I’m no electrical engineer so feel free to disregard my thoughts as they’re not based on much real world experience.

Seeing the dual fuses looks like a reliability concern to me. If one of them has a resistance a few miliohms higher than the other, you could very unequal current between them causing one to blow prematurely which would then cascade to the remaining fuse.

As for the arcing potential during a shutdown, I turn to plumbing analogies with the large inductive load leading to the equivalent of water hammer destroying your valve (or FETs). For this case could you install a zener diode to arrest the water hammer by giving the inductive current somewhere else to go before the voltage gets high enough to burn up the FETs and start arcing.
I was concerned with the double fuses at first but couldnt find an 800A for a reasonable cost. I originally wanted to connect the BPs above the fuses so the path would be the same distance but packaging forced me here. Im not really worried about it but will post anything if I have issues.

For the inductive load destroying fets issue, I think I came up with a solution in post #43 above that will prevent the issue from ever occurring, but thanks non the less!
 
I was concerned with the double fuses at first but couldnt find an 800A for a reasonable cost. I originally wanted to connect the BPs above the fuses so the path would be the same distance but packaging forced me here. Im not really worried about it but will post anything if I have issues.

Do you have multiple loads?
I use two class T fuses, but not in parallel, just "Y", branching to two circuits. (each of those is two inverters, four total)
 
Do you have multiple loads?
I use two class T fuses, but not in parallel, just "Y", branching to two circuits. (each of those is two inverters, four total)
Yup, a multiplus through the single fuse and the left disconnect and 2 winches through the right switch and parallel fuses.
 
Yup, a multiplus through the single fuse and the left disconnect and 2 winches through the right switch and parallel fuses.

Do you need the current rating of two fuses in parallel for a single winch? Or only if both operated together?

Assuming one fuse is sufficient for one winch, two fuses drawing current from a shared wire through two fuses may drive sufficiently imbalanced current to blow first one fuse, then the other.

The bars are quite wide, but resistance will be slightly lower for current going through fuse closer to switch as compared to fuse further away.

More optimum balance would be with bar on left as shown, but bar on right rotated so end of bar goes to bottom fuse, middle of bar goes to top fuse, then the blue boxes are connected. It would be further balanced if both bars were identical width. Without calculating resistance of bar and comparing to resistance of fuse I don't know if that would make any noticeable change.

What would be better, if one fuse is sufficient for one winch, to wire each winch to its own dedicated fuse.

800A - what size cable(s) do you wire that with?
How long is is the circuit drawing current? How high is the current steady-state, after any starting surge?
 
No not yet. Unfortunately when I started mounting the box I found several small issues with it that I would work around but then one major issue that make it unworkable. They left a welding rod between the floor skin and the angle at the rear under the door. I am bolting the box to a steel angle that will be bolted to the truck. There was an 1/8" gap between the floor and aluminum angle and when I started bolting things together welds started breaking. Thankfully they took the box back and refunded my money without much argument. Now Im working on getting a new one built so I havent done any testing yet. Its also -30C here right now so not getting much work done on it either.

A large number of the welds were garbage too. You can see in both of the welds visable below that that they didnt even fill craters and left a keyhole.

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Do you need the current rating of two fuses in parallel for a single winch? Or only if both operated together?

Each winch draws just under 500A
Assuming one fuse is sufficient for one winch, two fuses drawing current from a shared wire through two fuses may drive sufficiently imbalanced current to blow first one fuse, then the other.

The bars are quite wide, but resistance will be slightly lower for current going through fuse closer to switch as compared to fuse further away.

Im hoping that 300A headroom will be sufficient

More optimum balance would be with bar on left as shown, but bar on right rotated so end of bar goes to bottom fuse, middle of bar goes to top fuse, then the blue boxes are connected. It would be further balanced if both bars were identical width. Without calculating resistance of bar and comparing to resistance of fuse I don't know if that would make any noticeable change.

I was planning to do that but ran into space constraints so had to go this route.

What would be better, if one fuse is sufficient for one winch, to wire each winch to its own dedicated fuse.

800A - what size cable(s) do you wire that with?
How long is is the circuit drawing current? How high is the current steady-state, after any starting surge?

just under 500A is the rated operating current for each winch. Im going to run twin 4/0 cable to each winch. You usually only use a winch for 30 seconds and let it cool for 30. How long that goes on for depends on how stuck you, how far you have to go and how many other trucks you are pulling through.
 
Each winch draws just under 500A

Im hoping that 300A headroom will be sufficient

just under 500A is the rated operating current for each winch. Im going to run twin 4/0 cable to each winch. You usually only use a winch for 30 seconds and let it cool for 30. How long that goes on for depends on how stuck you, how far you have to go and how many other trucks you are pulling through.

400A fuse run at 80% to avoid nuisance blowing, 320A
500A - 320A = 180A minimum desired through second fuse.
So 2:1 dividing of current is OK.

Length of 4/0 cable will have some resistance, assist with balancing if each has a dedicated fuse.
Consider output of each fuse separate, goes to its own battery protect, its own 4/0 cable.
The two 4/0 cables come back together at winch. (suppose other winch at other end of ?boat?, so two pairs of 4/0 cables)

I had two 60' runs of 6 awg carrying parallel current through two Sunny Island, joined through two QO270 breakers. Current imbalance was 3:1.
Replaced those breakers with 63A DIN rail Schneider breakers and it was perfectly balanced. Seems the QO vary considerably in resistance.
I would expect two identical model fuses to be better matched than breaker contact resistance.

Class T has a trip curve, unfortunately a single line not a width between "shall blow" and "shall not blow"
Nominally, 400A fuse can carry 700A for 1000 seconds.

Heating goes as amperage squared. 50% on time followed by 50% off time is half the heating, similar to current/sqrt(2)
500A/sqrt(2) = 354A equivalent heating
30 seconds is way under 1000 seconds, and 500^2/700^2= 0.51x or 51% as much heating.
"square wave" of current should have time to average out.

I think even a 400A fuse would hold up pretty well. Two in parallel will be more reliable.
As shown you probably can't fit a clamp ammeter around busbar to measure current of one fuse.
If you route separate 4/0 cable that will balance current better and you can measure it.


 
Check the manufacturer's recommendations, but good fuses can be used in parallel as long as you have the same connection to both to match the impedance of the current path through each. Tinned copper bars with the full contact surface of each fuse would be sufficient.

You should derate the fuses by 0.9 to allow for fuse variation, and be aware that by combining them in this manner you are doubling the error. If a fuse is, say, +/-10%, then the combination should be considered +/-20%.

Make sure that in the extremes the fuses will still perform to your needs and expectations.

Per this application note:


The advantages include not only increased current capacity, but also spreading the heat dissipation to a larger surface area.
 
400A fuse run at 80% to avoid nuisance blowing, 320A
500A - 320A = 180A minimum desired through second fuse.
So 2:1 dividing of current is OK.

Length of 4/0 cable will have some resistance, assist with balancing if each has a dedicated fuse.
Consider output of each fuse separate, goes to its own battery protect, its own 4/0 cable.
The two 4/0 cables come back together at winch. (suppose other winch at other end of ?boat?, so two pairs of 4/0 cables)

I had two 60' runs of 6 awg carrying parallel current through two Sunny Island, joined through two QO270 breakers. Current imbalance was 3:1.
Replaced those breakers with 63A DIN rail Schneider breakers and it was perfectly balanced. Seems the QO vary considerably in resistance.
I would expect two identical model fuses to be better matched than breaker contact resistance.

Class T has a trip curve, unfortunately a single line not a width between "shall blow" and "shall not blow"
Nominally, 400A fuse can carry 700A for 1000 seconds.

Heating goes as amperage squared. 50% on time followed by 50% off time is half the heating, similar to current/sqrt(2)
500A/sqrt(2) = 354A equivalent heating
30 seconds is way under 1000 seconds, and 500^2/700^2= 0.51x or 51% as much heating.
"square wave" of current should have time to average out.

I think even a 400A fuse would hold up pretty well. Two in parallel will be more reliable.
As shown you probably can't fit a clamp ammeter around busbar to measure current of one fuse.
If you route separate 4/0 cable that will balance current better and you can measure it.


Thank you so much for your insight! I had not considered running separate parallel strings right from the fuses. Im going to consider it and see if it works with the rest of the down stream considerations.

Now that you have given it a little context I understand those graphs and think you are probably right that a single fuse would probably work.
 
Check the manufacturer's recommendations, but good fuses can be used in parallel as long as you have the same connection to both to match the impedance of the current path through each. Tinned copper bars with the full contact surface of each fuse would be sufficient.

You should derate the fuses by 0.9 to allow for fuse variation, and be aware that by combining them in this manner you are doubling the error. If a fuse is, say, +/-10%, then the combination should be considered +/-20%.

Make sure that in the extremes the fuses will still perform to your needs and expectations.

Per this application note:


The advantages include not only increased current capacity, but also spreading the heat dissipation to a larger surface area.
Im no electrical engineer but had read through as much documentation I could find regarding paralleling fuses and was pretty comfortable it was completely acceptable. The note in you link about temperature coefficient helping with current sharing sharing in a connection that is not absolutely symmetrical along with the significantly larger than necessary bus bars was why I was comfortable with the arrangement I settled on.
 
I'm hoping this post starts a discussion, I'm not judging. Let me know if you want a new thread started instead.

I'm getting ready to put in some copper flat bar, similar to how you're doing. I thought about buying 1" wide flat bar. However, I got to thinking that if the surface contact area is no bigger than 3/4" flat bar would cover, is there any sense in going to 1". Sure 1" flat bar has a bigger cross section and can handle more amps. But if the surface contact area is so much smaller, have I wasted 1/4" of flat bar?
 
Not wasted. Ampacity of copper is far higher than any ampacity tables for wire. Not like it is going to start metal-migrating or anything. Metal migration was something to consider for thin aluminum film and array of contacts in an IC design; uneven distribution could cause them to fail one after the other. Copper and large busbars isn't anywhere near those limits.

NEC ampacity rules require derating bundles of more than 3 current carrying conductors due to greater temperature rise. But not for short lengths no longer than 2' (e.g. through a nipple between boxes) because the heat can escape lengthwise (I would say whether they are terminated in a lug, or extend a great distance in smaller bundle, should make a difference.)

The point where you bolt to a busbar may be smaller cross-section to carry current, therefore higher resistance and more heat generation per mm of length. But, it has 3-D for heat to escape, not just 2-D like for a long uniform run. After current (and heat) fans out to full width of bar, the wider bar is beneficial.

I think area of contact is almost immaterial (consider domed contacts of a relay!) (Same as I think for busbar to battery terminal). Contact resistance matters, which is related to clamping force and the number/area of tiny contact points developed.

Short busbar, total power dissipation and voltage drop probably doesn't matter much. But don't want temperature rise that affects attached fuses, breakers, or wire with particular temperature rating of insulation.
 
Ampacity of copper is far higher than any ampacity tables for wire.

I've been looking at the ampacity tables for copper flat bar, but I didn't make a direct comparison to the same cross section of copper cable. I had thought that a cross section of flat bar would have similar ampacity to the same cross section of cable, but I didn't factor in the insulation of the cable.

After current (and heat) fans out to full width of bar, the wider bar is beneficial.

I hadn't considered that.

Short busbar, total power dissipation and voltage drop probably doesn't matter much. But don't want temperature rise that affects attached fuses, breakers, or wire with particular temperature rating of insulation.

All my copper bus bars would be less than 6", likely around 3". It sounds like I'll be fine.

I was planning to put some heat shrink on the exposed parts of the flat bar. But I now wonder if the heat think would decrease the heat dissipation.
 
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