# Battery Pack Design



## Csecrist12 (Nov 22, 2019)

I am currently ramping up to start my motorcycle conversion. I am using a '96 Kawasaki ZX600 roller chassis along with some bits from an electric car I picked up. 

I am going to start with the battery as all of my other components are tailored to this and the controller. I've been discussed with a fellow member on here about these items and what my charger can hold and produce. 

I am using A123 26650 LiFePo4 batteries I picked up on a trade. A deal I couldn't pass up. I configured the system to be a 32S 15P set up and to fit it in the area, I am doing the serious connection as a folded set. The photo below shows along with the battery diagram. 

My question is, what parallel connections am I needing as far as the length of the battery pack? I have 2 photos. Both the same battery packs. One is blank for anyone to drawer on it and help with some feed back and the other is what I am thinking for the parallel set up.

Also, for the BMS, I am using the ZEVA EVS modules. I have 3 units (Each able to have 12 series connections) so the last unit will only have 8 connections and room to grow the pack if needed. Where and how do I connect those wires for the BMS?

Blank Pack:








Parallel Connections: I would also have them on the back sides of each battery.









Any thoughts would help greatly.


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## MattsAwesomeStuff (Aug 10, 2017)

Your second drawing is correct. You want all 15 in parallel to be in parallel at the smallest scale possible. Not just the end of the string.

As to BMS wiring, it depends on your BMS. Follow its instructions.


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## Csecrist12 (Nov 22, 2019)

I really appreciate the feedback as you have a lot of knowledge in this so thanks for giving your advice.

Would it be something like this? Or would the parallel connection have to be in between the first and second and 3rd and 4th row as well? Basically like a ladder for all connections? Just want to make sure it's right the first time around and not fry anything. I am also getting the single connector clips to have some structure on it. I'll lay out the 32S strand x 15P block out then fold the section in 4's to get the dimension i am needing. I am going to do some test folds first to make sure its possible along with some insulator material where the fold connection is and in between each row so it doesn't short circuit.


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## _GonZo_ (Mar 23, 2009)

Why are you using this cells (I suppose 18650) for this pack?
I know that Teslamotors does that. But now it is not the best, cheapest or easiest way to create a battery pack this size.
You have plenty of different prismatic cells now to choose from with very reasonable prices.

Tesla did that way because several reason at its time, and keep on doing it because they can not throw away thousands of millions of dollars on its manufacturing development. (But they already have changed to a much larger cell)
But knowing the actual big availability of prismatic cells it is an error to follow that path.


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## Csecrist12 (Nov 22, 2019)

_GonZo_ said:


> Why are you using this cells (I suppose 18650) for this pack?
> I know that Teslamotors does that. But now it is not the best, cheapest or easiest way to create a battery pack this size.
> You have plenty of different prismatic cells now to choose from with very reasonable prices.
> 
> ...


I am using A123 26650 LiFePo4 because I received them on a trade. I didn't have any money on the items I had and he had some batteries he wanted to trade so I made the swap! Cheap and easy honestly. 

Is the photo with the red strips something correct or do I need more parallel connections? I am about to purchase a spot welder along with getting ahold of my major wires for everything.


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## _GonZo_ (Mar 23, 2009)

Csecrist12 said:


> I am using A123 26650 LiFePo4 because I received them on a trade. I didn't have any money on the items I had and he had some batteries he wanted to trade so I made the swap! Cheap and easy honestly.


OK then sorry for my comment. If you have them already in stock and they were free, it si the way to go.
I am actually doing the same in one of my projects with the battery pack: I know that to use small cells is not the best but I already have several hundreds of them in stock, so I decided to go same way...



Csecrist12 said:


> Is the photo with the red strips something correct or do I need more parallel connections? I am about to purchase a spot welder along with getting ahold of my major wires for everything.


It is need those parallel bus bars as well in the other side of the cells, maybe they are there but can not seen in the last picture.
The current flowing on that parallel connection bus bars is very small (in a perfect pack it will be 0) so they do not need to be very big.


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## MattsAwesomeStuff (Aug 10, 2017)

Csecrist12 said:


> Would it be something like this? Or would the parallel connection have to be in between the first and second and 3rd and 4th row as well? Basically like a ladder for all connections?


I'm not precisely enough sure of what I'm looking at, or what you're describing, to be confident either way. But, yes, like a ladder makes sense.

Connect cells at the lowest points possible. If you have 10s20P, 200 cells total, then you should have 20 cells in parallel over and over and over, and then all those chunks of 20 cells wired in series.

Do not connect them in 20 chains and then just tie the ends points together. That's not good enough.


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## TeZla (Feb 18, 2021)

My unprofessional opinion, and basic understanding of 18650 batteries,

I would connect every parallel connection at each series connection. To my understanding of how it all works, if you have 15 strings of cells all connected in parallel between the positive and negative terminal, it would take the failure of only 1 cell to 'take out' a whole string, reducing the total of the battery in output amperage and capacity. Whereas with every series level of cell the whole 15 are connected in parallel of one cell dies, the rest of the cells in that bank will somewhat cover for the dying cell, the output of the battery will still diminish, but not as severe as the string setup.
The downside to that, There is a case where a cell failing can then end up 'always empty' in comparison to the other cells in that bank, with a constant discharge acting as a big resistance for the other cells to 'balance' into, this continues until the battery catches fire or runs out of charge. I'm fairly certain Tesla mitigates that scenario by implementing a fuse on each cell. If a cell ever fails, its mini fuse wire burns out and its no longer electrically part of the pack.
I'm not sure if its possible to buy that nickel sheet fuse wire pre cut, maybe laser cut? ready to be spot welded to the top end of each cell. I'd include that if i was making this battery

Should note, when you put your parallel straps on, Put it on the negative terminals of each series, don't worry about the positive terminal, they are directly connected to the negative terminal in its series string(except at the positive pack terminal of course)

At the end of each parallel, leave a bit of a tag that you pre solder before installing it, That's where you'll connect your BMS tap wires. Read the instructions for your particular bms, But typically they begin cell wring at the negative terminal of the battery pack and number upwards from there towards the positive terminal, one connection for each cell level in the string. I think with the Zeva units that you have, the last bms you'd just leave the rest of the wires unconnected once you get to the last connection. it should either automatically detect the cells not connected or you can probably configure it somehow, But im sure it'd work.


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## Csecrist12 (Nov 22, 2019)

TeZla said:


> My unprofessional opinion, and basic understanding of 18650 batteries,
> 
> I would connect every parallel connection at each series connection. To my understanding of how it all works, if you have 15 strings of cells all connected in parallel between the positive and negative terminal, it would take the failure of only 1 cell to 'take out' a whole string, reducing the total of the battery in output amperage and capacity. Whereas with every series level of cell the whole 15 are connected in parallel of one cell dies, the rest of the cells in that bank will somewhat cover for the dying cell, the output of the battery will still diminish, but not as severe as the string setup.
> The downside to that, There is a case where a cell failing can then end up 'always empty' in comparison to the other cells in that bank, with a constant discharge acting as a big resistance for the other cells to 'balance' into, this continues until the battery catches fire or runs out of charge. I'm fairly certain Tesla mitigates that scenario by implementing a fuse on each cell. If a cell ever fails, its mini fuse wire burns out and its no longer electrically part of the pack.
> ...


That does make sense as far as fusing and the series and parallel connections. I'm not sure if it's any different for the 26650, which is what I have. I have also read about these needing vented as well? Not sure if that is true or not.

I will create a 3D model of the pack unfolded and show the parallel connections to better understand the connections properly. I'll also try and label the positive and negative sides of the batteries.


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## Csecrist12 (Nov 22, 2019)

I have attached the unfolded version where the red strips are the parallel connections on the negative sides of the battery cells. Is this more accurate of what is needed? Then I can add the BMS wires to the parallel red strips for balancing. So the very top of the photo is the battery pack positive and the very bottom is the battery pack negative. Obviously the connections between cells are + to -, + to -, so on and so on.


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## Csecrist12 (Nov 22, 2019)

Another thread I am on mentioned about nickel strips only being able to handle 10A per cell. I have a 32S15P pack which would be 150 Amps based on the P-count, would I need thick copper bus bars on the ends to handle the amperage or? If so, where would sources be to buy the thick copper bus bars and how thick and dimensions would I need? So the ends would see the 105V by 150 A for power consumption of 15.75 KW? I'm not sure that is adding up correctly?


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## TeZla (Feb 18, 2021)

Yep, That unfolded picture is pretty much perfect. Connect your bms tap wires to the cell at each red strip, Starting negative terminal first.
26650's are basically just the big version of the 18650. Functionally they are the same, just bigger. More capacity, more power output, bigger...
They shouldn't need venting, they are a fully sealed cell. if they are venting there is a big problem occuring.

There will likely be a metal shop of some sort near you, They 'should' be able to get copper bars of a suitable thickness or width, Its also available from online electronics retailers or if your really feeling cheap you could hammer some copper pipe flat and solder to that.






Busbar size and calculation


Busbar Bus bar A bus bar (also spelled busbar, buss bar or busbar), is a str...




bralpowerassociate.blogspot.com




According to the chart in there, a solid copper bus bar 15mm wide x 3mm thick can conduct 170 Amps. That'd work for your battery.



Amazon.com


I've found some copper bus bars on Amazon that are the right size, Shop around for something local


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## Csecrist12 (Nov 22, 2019)

TeZla said:


> Yep, That unfolded picture is pretty much perfect. Connect your bms tap wires to the cell at each red strip, Starting negative terminal first.
> 26650's are basically just the big version of the 18650. Functionally they are the same, just bigger. More capacity, more power output, bigger...
> They shouldn't need venting, they are a fully sealed cell. if they are venting there is a big problem occuring.
> 
> ...


TeZla,

I appreciate the feedback and will definitely get the copper bus bars and do a good search on them as well. As far as the series connections, I was discussing with another individual about the current being carried on the nickel strips and make sure they're capable of the current running through them as where the parallel connections see a small amount of current. I bought some 8mm W x 0.2mm T pure nickel strips from a reputable buyer. Would I be okay using that for the series connections? 

I also thought about making packs of 8S15P resulting in 4 of those packs for a resulting pack of 32S15P, being connected by bus bars so they can better be managed. Would this be a decent idea? Also, could I have a bunch of holes in the bas bars between each back to connect them with some bolts and hardware? What hardware material would be recommended for this?


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## TeZla (Feb 18, 2021)

Not sure on the nickel strip rating, Id have to look it up. but yea, it should be fine to use them for the parallel connection. With every series connection your total amperage is distributed over each strip. It may be 150A peak, but its only 10A peak on each strip.

Yea, a bus bar at each of the folds in the pack is probably a good idea, would definitely make pack assembly and disassembly easier, allows you to reconfigure the battery if you really wanted. Might be expensive and redundant though. If you did have bus bars at each fold, you'd need to make sure the wire connection between them is capable of carrying as much or more power than the bus bar. Or Connect the bus bars on the pack fold with other bus bars that stretch between each fold. 
Yo dawg, I heard you like bus bars...
Hardware to connect bus bars? In a perfect word, Copper bolts and nuts. Stainless is the next best choice.mild steel will do in a pinch but its not the best.
Having electricity flowing through 2 different metals wile they touch each other will cause galvanic corrosion. some metal combinations react more than others.


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