Common pack design mistakes, how to avoid?

fechter said:
The picture didn’t come out.
Yes you want the current in all the cells to be nearly equal. With only 3p this won’t be hard.

Thoughts on this? Terminal.jpg
 
i went on reddit and someone told me i need to not have all my output for the negative and positive terminals of my battery pack on just two tabs.( im using the vruzend 2.1 kit) i understand why and the diagram in the beginning of this thread but everyones batteries i see on here seem to do the opposite of what is suggested or am i missing something? can someone help me understand this? how to have good current flow and even draw. everyone seems to just have a slab of wire across their battery pack. i dont understand how thats different then just having it at the end if every cell is already connected.

here is my diagram vs what he suggested. which i know is the right way but how could i do this? pls help thank you. its the last thing i need to understand
 

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Trying to wire 2 of these exact gotrax scooter batterys packs in parallel to double capacity of scooter. Dunno how to go about connecting them together. Need help, here are the pics from the bottom of battery pack to top & BMS.
 

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I'm back with my 3rd battery build in sight. I'm aiming for a 20S6P configuration but am constrained by the shape of the bike. Here is what I have to work with:

Ignore the Nickle sheet
IMG_20200513_195746-v2.1.jpg

Does anyone have an suggested layouts to make this 20S6P?
 
pwd said:
I'm back with my 3rd battery build in sight. I'm aiming for a 20S6P configuration but am constrained by the shape of the bike. Here is what I have to work with:

Ignore the Nickle sheet
IMG_20200513_195746-v2.1.jpg

Does anyone have an suggested layouts to make this 20S6P?

You don't have to do a 20*6, you can do a 20*2x3 and get the same result. I did this with my 14*2x3 pack and got a perfit 1:1 power share, using nickel STRIPS .... your using sheet so it matters not.
 
RootedSuperuser said:
pwd said:
I'm back with my 3rd battery build in sight. I'm aiming for a 20S6P configuration but am constrained by the shape of the bike. Here is what I have to work with:

Ignore the Nickle sheet
IMG_20200513_195746-v2.1.jpg

Does anyone have an suggested layouts to make this 20S6P?

You don't have to do a 20*6, you can do a 20*2x3 and get the same result. I did this with my 14*2x3 pack and got a perfit 1:1 power share, using nickel STRIPS .... your using sheet so it matters not.

Thank you for the suggestion. The numbers make sense to me but I'm having a hard time visualizing how to connect that. So I would make 3 X 20S2P packs and parallel the discharge leads together?
 
If I did spilt up the 20S6P pack into 20S2P sub packs and put them in parallel; would I also have to paralell all the balance leads for the BMS?
 
pwd said:
If I did spilt up the 20S6P pack into 20S2P sub packs and put them in parallel; would I also have to paralell all the balance leads for the BMS?
No "have to" your call.

But yes that approach lets all the sub-packs to be protected and balanced by just one BMS

simpler is usually better.

Plus the cell/groups will naturally rebalance in use, counteracting the ill effects of unequal draws via the power leads at pack level voltage
 
pwd said:
If I did spilt up the ...; would I also have to ... for the BMS?

I do. yes. I just built a 50Ah 2p Chevy cell pack that I did this so I could run 1 BMS... One 25Ah half, 21 sense lines, and one 25Ah half, 21 sense lines... into +/- discharge leads and a 25 pin DB connector.... Every cell has an output lead and the pairs of 2p come together in a connector to go to the BMS as one signal line.
 
Again, not have to

but certainly is the better way to go.

If only paralleled at the power pair, imbalanced flows creates extra monitoring / balancing headaches

and lower overall lifespan.
 
Thanks guys :bigthumb: No free lunch as they say.
 
I've made a few ampacity tables for different metal strips in order to try to understand what is "reasonable" for weld metal strips.
Here's my original tables in an excel file : View attachment 205307

The thing is, "Ampacity" is NOT a "Cast in Stone " value that is only proportionnal to the gauge (aka cross-sectional area) of the metal used as the conductor. Ampacity not only depends on the gauge (ex : AWG) and the type of metal, but it also depends on the lenght of conductor, the increase in temperature and the magnitude of the voltage drop that you're willing to tolerate. I mean, Ampacity can be really high-rated if you don't mind your conductor rising at 105°C (221°F) ( :x loosing precious watts) in an open-air environnement, and if you don't care about loosing 10 volts in voltage drop just in one meter of conductor. Also, ampacity depends on the length of the conductor used... The shorter, the better.

Standard ampacity tables seldom specify these parameters when they give ampacity values. See for example : American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength.

From these charts, and looking at what Ohms/km (mOhms/m) values correspond to what ampacity values for different gauges of copper, you can extrapolate ampacity values for different metals of known resistivities : Hence I made this table extrapolating data from PowerStream.com (American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength) and from wikipedia (resistivity of different metals : Electrical resistivity and conductivity - Wikipedia ).
View attachment 205284
Just as an indicator of what ampacity values PowerStream.com expects from copper wire, they rate 10AWG at 55 amps (I personally think that would get hot). So if you trust PowerStream.com's ratings, use this first table.

So for you answer
... Is 0.20mm thick Nickel (let's assume 10mm width) enough for each parallel cell ? Well, it depends on the cell you choose to use and what amperage you intend to pull out of it. 0.20 x 10 mm should be okay for 7.9 amps. So if you intend to pull max 7.9 amps out of each cell (as in 31.9 amps in a 4P pack), you should be okay, but the strips might get a little warm or even a little bit hot at these sustained currents...

On the other hand, if you plan to use 20A or 30A high-current rated 18650 cells (ex : Sony IMR 18650 VTC4 2100 mAh - 30A; Samsung 18650 30Q 3000mAh - 20A, LG 18650 HG2 3000mAh - 20A, Sony IMR 18650 VTC5 2600 mAh - 20A) then those Nickel Strips will be a serious bottleneck to what these cell could be capable of delivering with proper wiring. At 20 amps per cell or 30 amps per cell, these 0.20 mm thick nickel strips will heat up and fry (remember this : New alleged Panasonic battery failed to deliver in road test). Even if there'is a copper bussbar collecting current from individual parrallel nickel strips. Even 10A cells (ex : Panasonic NCR18650PF 2900 mAh - 10A or SANYO NCR18650GA 3300mAh - 10A) in my opinion would suffer a bit from bottlenecking if welded with these nickel strips rated for 7.9A. It's a bit like uprgading a car with a new big turbocompressor, but without upgrading the exhaust and intake pipe system ... The restriction from the way too small pipes will make the car underperform, counteracting the potential added benefit of the new turbocompressor ...

Personnally, I don't think the PowerStream.com Ampacity chart is suited to my own battery build. I mean, I wouldn't want my conductors to go over 50°Celcius (to prevent lithium cells degradation from heat). Bare in mind that these conductors would also be enclosed (not in open-air) and I would then be using about 10 meters total of conductor strips... So with theses parameters in mind, the ampacity values has to be seriously derated (see this website for a automated calculator : I moved!!! and this website for Neher-McGrath theory on calculation of ampacity : Understanding the Neher-McGrath Calculation and the Ampacity of Conductors)
So extrapolating again (ampacity for max 50°C), I made another ampacity chart that is more stringent and more suited to my needs. Only 2 volts of drop when max amps approach ampacity. Here it is :
View attachment 1
Just as an indicator of what ampacity values I personally expect from copper wire (based on calculation for max V drop of 2 volts for 10 meter, and max temp 0f 50°C), I rate 10AWG at 30 amps. So if you trust that rating more than PowerStream's ratings, use this second table .

Just to give you an idea, in your house, your mains are required to be at least 14AWG for 15 amps rated circuits.
Well according to the PowerStream charts, 18AWG would be enough for 16 amps. Hence, I think they overrate.
With my own rating standards (from calculations made for 50°C, 2 V drop per 10 meters) ,14AWG would be rated for 17 amps... I think that is more comparable to household wiring expectations.
Hey Guys, this is a Very Old Post. But I think it has overcome the Test of Time! Becoming a Real Treasure. Should be added to a "Resources" Section somewhere.
 
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