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Old 07-07-2017, 01:27 AM   #1
JaxPilot
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Default Lithium battery back configuration - Technical questions

OK, this is where I need to call on the really smart electrical guys, and the guys with lots of Lithium experience to help me out here. I have 42 Lithium cells, 40AH each, that I need to string together as 14S3P. The cells came already packaged in very nice sturdy enclosures, which I would like to keep, as they protect the cells well. They originally were topped with a circuit board that managed all the cell functions and acted as a BMS with CANBUS, but I am removing the circuit board as it will not serve the needs of the golf cart. I'll have to buy a separate BMS, which at this point appears to be an Orion Jr. Unfortunately, Nick will not sell me one of his BMS, at less than half the cost of the Orion Jr., as he will only part with them for use with one of his battery packs.

The cells are packaged in the orientation as shown in the Modules picture, with the blue cells. That configuration will allow me to leave the modules and cases intact, and serialize the cells in that module with only 36" of copper bus bars (black lines in pic) which will give me a nominal 50.4V / 40AH module. It will then require two connections to parallel (purple external lines) the 3 modules, which will probably have to be cable or braided and insulated flexible copper straps.

The second picture shows making the 3P connections as much as possible inside the modules, but requires either rotating the middle row of cells and modifying the case (positive and negative terminals are different sizes) by milling out the terminal holes in the top cap of the module and/or doubling up on bussbars in a "V" shape. The red and blue positive and negative lines are for illustration, not "as built". I would need 55" of copper to create 4 each of the 3P cells, and then use one cell from each end of the module to jump between all three modules to create two of the cells. I color coded the cells that are 3P to try and show how that would look. In addition to having to jump between cells to create two of the 3P batteries (pink and yellow cells), I would have to make connections to serialize the cells between modules (purple thick lines).

So now the questions I have are these:
1. Is it better to create each of the 3P batteries internal to the module, so that I can assign each of the 14 BMS channels to a specific group of cells acting as one battery?

2. If I did the three modules that were serialized internally, will the BMS then treat all 14 cells in a single module as one cell?

3. Will doubling up on most of the bussbars cause a problem? I do not think I can create on solid bussbar to span 3 cells and align the holes properly, so it will take two bars per battery per side. I'm thinking I can use .0625 thick x 0.5" wide copper, and that should carry the amperage and handle the heat that may be generated.

3. Are the two options drawn correctly? I keep confusing myself trying to make sure I get the 14S3P drawn correctly so I can try to find bussbars, or buy the copper to try and manufacture my own. That's gonna be a pain.

4. These are cylindrical cells, and use 5mm screws to secure the connections. Should I be thinking about crimped cables between cells, or some other option besides solid copper?

4. Am I making any glaring mistakes or omissions in my reasoning or assumptions?

Thanks in advance, I know that several of you have designed and installed packs, so I'm really looking forward to a great education.
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File Type: jpg Parallel 3 Cells x 14.jpg (97.0 KB, 0 views)
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Old 07-07-2017, 03:51 PM   #2
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Default Re: Lithium battery back configuration - Technical questions

After thinking it through, I think this is the only way to reasonably configure the cells. I believe I can use solid copper busbars inside each module, and will have to use cables or insulated flex braid between modules. I'm still not sure on sizing or sourcing the busbars, however.

The red and black lines represent positive and negative busbars, respectively, to create the 3P configuration. The magenta lines represent the Serial connections required to create the 14S configuration.
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Old 07-07-2017, 04:32 PM   #3
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Default Re: Lithium battery back configuration - Technical questions

Not sure I'm following the logic in the new diagram. The first one looked more reasonable except that you have the 3 serial packs in serial again. I hesitate to say correct as I am no expert here. Treating them as three separate packs is a cleaner solution.

As far as interconnects, if your dimensions between posts are equal across all or at least several batteries, you can have the bars cut out of copper stock. Look for someone with a water jet cutter. The longer interconnects between the three packs can easily be accomplished with insulated welding cable of sufficient size for your application. I'd add one or maybe two layers of split loom to resist abrasion and secure them to prevent movement.
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Old 07-07-2017, 05:23 PM   #4
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Default Re: Lithium battery back configuration - Technical questions

Hi Gregorio. It could be my terminology. I intended to reference individual cells, then three paralleled cells as a battery, and then a physical enclosure as a module, and all of that combined as a pack. It's probably confusing because I have 3 enclosures with 14 cells in each, and I have to span physical enclosures to create two of the paralleled batteries. The revised diagram is supposed to reflect 4 parallel batteries in each enclosure, alongside a partial battery.

Cells 1,9,10 are wired - to - and + to + to create one 120AH, 3.6V battery. That pattern is repeated with 2,8,11 | 3,7,12 | 4,6,13. That gives me 4 paralleled batteries, comprising 3 cells each. The two left over have to be parallel connected to a cell in the other case to be completed.

Then I tried to show the 4 batteries connected + to -, to give me a 14.4V, 120 AH module. That is repeated in the other two cases, with connections between physical enclosures to maintain the serial chain. Then I am working my way back across the three modules to pick up the remaining two batteries that have cells spanning the 3 cases, ending up with 50.4V 120AH battery pack.

Clear as mud, eh?
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Old 07-07-2017, 06:05 PM   #5
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Default Re: Lithium battery back configuration - Technical questions

I hope I'm not going to confuse this more but if you have an enclosure consisting of 14 3.6v cells, putting them all in serial gives you 50.4v, right? Why not create 3 50.4v modules and then parallel them? Logically, this seems to be the most straightforward but am I missing something that is physically inherent to the batteries that prevents this?

Yeah, mud!
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Old 07-07-2017, 07:00 PM   #6
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Default Re: Lithium battery back configuration - Technical questions

Quote:
Originally Posted by Gregorio View Post
Why not create 3 50.4v modules and then parallel them? Logically, this seems to be the most straightforward but am I missing something that is physically inherent to the batteries that prevents this?
Could the BMS properly balance the cells in this config?
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Old 07-07-2017, 09:54 PM   #7
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Default Re: Lithium battery back configuration - Technical questions

Quote:
Originally Posted by Rickochet View Post
Could the BMS properly balance the cells in this config?
No, the BMS can't, and that's why I have to jump through these hoops. The BMS as I understand it works much better and within a much tighter tolerance with the 3P configuration, than it could with a 14P. Trying to balance 14 cells with the micro currents would, I believe, result in fairly (relatively) large and probably unhealthy voltage variances between cells. Everything I have read leads me to believe that keeping voltages tightly coupled is best for safety and longevity.
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Old 07-08-2017, 12:49 AM   #8
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Default Re: Lithium battery back configuration - Technical questions

That explains the complex arrangement. Never thought about BMS limitations
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Old 07-08-2017, 01:07 AM   #9
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Default Re: Lithium battery back configuration - Technical questions

Could we perhaps see an actual pic of the cells and/or blocks? To know what size & shape the actual terminals are, and their mobility/ "twistability". These imaginary pics are not giving me a good mental picture of what could, or could not be done.
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Old 07-08-2017, 07:32 AM   #10
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Default Re: Lithium battery back configuration - Technical questions

This might be totally useless in this thread but somewhat interesting. This is a config tool I used when I was tinkering with RC model Lipo packs. Click on the down arrows to change configs. With the RC Lipos the balancing is done by the external charger.
https://commonsenserc.com/page.php?p...po_wiring.html
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