Effect on torque on bus bars to current
- Thread starter Jennifer
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That was a great video. Really shows how important tight busbar connects are.
BUT - he only hooked two cells in parallel to demonstrate a point, that for the most part there will be no over-current damage to the other cell. Ie, one cell fully charged, and the other nearly fully discharged. Ok cool. Great if you are only putting a 2-cell bank together. No worries. But what about more cells?
I'd like to see the same experiment, but this time, with say 7 cells already connected in parallel, and seeing just how a nearly discharged cell reacts to that when connected. Overcurrent event?
This would emulate someone who is putting together a common 8-cell system for example on his bench in preparation for for a parallel top balance. Even if those previous cells are not fully charged, by the time you put a bunch in parallel, the overall parallel capacity might have detrimental effects to that poor lone cell that was well discharged when that initial connection is made going past it's rated current limit.
BUT - he only hooked two cells in parallel to demonstrate a point, that for the most part there will be no over-current damage to the other cell. Ie, one cell fully charged, and the other nearly fully discharged. Ok cool. Great if you are only putting a 2-cell bank together. No worries. But what about more cells?
I'd like to see the same experiment, but this time, with say 7 cells already connected in parallel, and seeing just how a nearly discharged cell reacts to that when connected. Overcurrent event?
This would emulate someone who is putting together a common 8-cell system for example on his bench in preparation for for a parallel top balance. Even if those previous cells are not fully charged, by the time you put a bunch in parallel, the overall parallel capacity might have detrimental effects to that poor lone cell that was well discharged when that initial connection is made going past it's rated current limit.
What I find interesting at about 16:00 he gets 160+ amps with a finger tight connection. Does not seem to have excessive heating at that connection. I am thinking a bit less torque on these terminals would be just fine. Certainly more than finger tight but maybe 4 Nm is more than necessary.
Boondock Saint
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I think clean and smooth firstly, then from moderately tight on is a marginal gain. Too tight is too great a risk in the screw terminal or weld joint. Just my 2 cent and hunch.
It was fascinating to see how we get away with paralleling a bunch of cells at first with just finger-tightening without any fireworks due to the really poor initial contact. No drama! Good on him for demonstrating that.
The point I was making that I've run into this over-current problem in the past when I was in a hurry and can be overlooked if all you are doing is testing two cells. Things change when you are doing more than 4.
Constructing an 8-cell paralleled battery for initial top balance? Let's say you have eight, 280ah cells on hand.
You connect up 7 of them in parallel equating to the equivalent of a nominal 3.2 , 1960ah battery.
Attach the last cell. Do you think that having 1960ah on tap when you connect the last cell is a good ideal? Nothing stands out as you make the attachment going from a loose connection to tight. Busbar might get warm for a little bit, but one thinks nothing of it.
But now, if your cell is only rated at C3 max current, we have a problem. And a reason why cells on the end can be underperformers. It's happened to me when I got in a hurry and didn't charge my replacement individually first.
Just a precautionary tale to always take internet-guru advice with a grain of salt. Or at the very least ask yourself: "Is my setup exactly like his?"
If not, you can fall into the trap of promoting urban-legend. In this case, it would be "don't worry, I've proven that you can connect cells willy-nilly and not worry about it!". All that concern comes from old EV'ers who are just worry warts. Not so - just trying to save some headache$.
The point I was making that I've run into this over-current problem in the past when I was in a hurry and can be overlooked if all you are doing is testing two cells. Things change when you are doing more than 4.
Constructing an 8-cell paralleled battery for initial top balance? Let's say you have eight, 280ah cells on hand.
You connect up 7 of them in parallel equating to the equivalent of a nominal 3.2 , 1960ah battery.
Attach the last cell. Do you think that having 1960ah on tap when you connect the last cell is a good ideal? Nothing stands out as you make the attachment going from a loose connection to tight. Busbar might get warm for a little bit, but one thinks nothing of it.
But now, if your cell is only rated at C3 max current, we have a problem. And a reason why cells on the end can be underperformers. It's happened to me when I got in a hurry and didn't charge my replacement individually first.
Just a precautionary tale to always take internet-guru advice with a grain of salt. Or at the very least ask yourself: "Is my setup exactly like his?"
If not, you can fall into the trap of promoting urban-legend. In this case, it would be "don't worry, I've proven that you can connect cells willy-nilly and not worry about it!". All that concern comes from old EV'ers who are just worry warts. Not so - just trying to save some headache$.
4 Nm is equal to 700 pounds of axial force. I think that's pleanty provided the terminals and busbars are squeaky clean. I noticed during the video once he torqued the nut to a certain point, continued torquing made little if any difference. I was hoping to see a torque wrench.
I would not attempt to connect 2 cells in parallel with drastic SOC's. He was careful but I would not want to try this at home..
Don't miss the forest for the trees here!
What he has shown in regards to CURRENT, is that you can safely connect two cells together in parallel with no worry. At most you'll witness a 1C event, provided you aren't doing this to a massively over-discharged cell. So yes, common sense.
So if you are putting together a large string, (say larger than 4) out of an abundance of caution, parallel them in sets of two, and put those sets together until your string is complete. Then top-balance.
So I'll take some of my harshness back. He inadvertently demonstrated a GREAT way to put large strings together if you do it in sets of two initially. And then put the sets of 4 together. Keep your construction symetrical.
What he has shown in regards to CURRENT, is that you can safely connect two cells together in parallel with no worry. At most you'll witness a 1C event, provided you aren't doing this to a massively over-discharged cell. So yes, common sense.
So if you are putting together a large string, (say larger than 4) out of an abundance of caution, parallel them in sets of two, and put those sets together until your string is complete. Then top-balance.
So I'll take some of my harshness back. He inadvertently demonstrated a GREAT way to put large strings together if you do it in sets of two initially. And then put the sets of 4 together. Keep your construction symetrical.
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Yes I would like to see the experiment continued with 3 full or 7 full cells and adding the low cell last. Probably more amps will move but not 3x or 7x. Of course the test was staged to produce more amps. The initial test was more realistic for newly arrived cells and much less current was flowing. By all means check the voltages are close before you assemble a stack. Maybe connect them in order from lowest to highest.
I haven't. I understand your point. Say one connects 3 cells in parallel with a full SOC to a 4th cell with a low SOC. Nope...not going to do it with even just 2 cells.
I'll take it back - he inadvertently showed a GREAT way to put cells together to minimize the over-current risk.
Just do it symetrically. If you have a large string, say 8 cells you just got out of the box, and they show no damage and aren't well below say 2.7v, just pair them up. Then pair up those pairs and so forth.
Don't just line 'em up and attach to each other one at a time. Pair them, and pair the pairs symetrically!
Now enter your standard top-balance routine. Huh, why didn't I think of that? Ok, thumbs up for the video.
Just do it symetrically. If you have a large string, say 8 cells you just got out of the box, and they show no damage and aren't well below say 2.7v, just pair them up. Then pair up those pairs and so forth.
Don't just line 'em up and attach to each other one at a time. Pair them, and pair the pairs symetrically!
Now enter your standard top-balance routine. Huh, why didn't I think of that? Ok, thumbs up for the video.
That's how I did my top balance. But I did it despite my lack of knowledge (sometimes I get lucky in spite of myself) . I did it that way simply because that's the way the bus bars had to go together. Bus bars on pairs. Then bus bars on top of the existing bus bars to join the pairs.
I did end up with a bad connection during the top balance and the end cell wasn't charged up to what the other seven cells were.
