Biggest bottleneck in an 18650 pack

Announcement - Help us fight the BOTS! Please report all spam including stuff in your inbox!

Aspendell

Member
Joined
Jan 31, 2017
Messages
70
Ahh, I see where you got your #s now.

going kinda off-topic but..
It's worth noting, tinned copper wire has significantly more resistance for DC. For this reason we have to nearly double the cross-section of speaker wire that most want to sell on ebay if we are using it for DC current.
In AC current it travels primarily through the core of the conductor, but in Direct Current the electrons like travel along the surface of the conductor, hence the tinned part of tinned copper.
This is also why the very best DC cables (Welding cable) you can buy will have way more strands in the cable. Typically like 300-600 strands in large gauge cables. But our AC house wiring is solid wire, cuz it don't care.

So, at lower currents, it's no big deal. But we have to take it into account when we are sizing our wires. And I suppose that if we had bussbars much over a couple feet, we would have to oversize those accordingly too as they are mostly solid conductors.

btw I subscribed to your Uchannel :D
 

Korishan

Moderator
Joined
Jan 7, 2017
Messages
6,522
Ahh, that makes a lot of sense. Hadn't thought about the AC vs DC traveling through the wire differences. I thought all current flowed on the outside. But that does make sense since DC devices have stranded vs AC devices have solid. It's all coming together now :p

So, having a bussbar that multi-stranded would be better than having one thick one, then.
 

daromer

Moderator
Joined
Oct 8, 2016
Messages
5,659
Yeah that is a big issue i agree :) I'm trying to find out the actual resistance of the fuse wire i bought. Going to try to do a test later today by measuring resistance and voltage drop over a longer wire but its hard since i cannot measure that low voltage nor resistance with good enough %

Il be back as Arnold would say :)
 

Aspendell

Member
Joined
Jan 31, 2017
Messages
70
It's funny. When I wake in the morning and read my email alerts, it tells me that:This thread is Biggest bottleneck in an 18650 pack.
lol, not sure if that is Mike trying to tell me something :p

My first couple packs won't be strictly parallel like many of you, but rather 7S, 10p. So about 7 rows of 10 cells in parallel. This will give me around 25 volts with 25 amps of max capacity, with every other row swapped pos and neg.
So at this point I think I've resolved to go with the nickel strips for my bars between each S, then fuse at least to the negative of the next S cells. 2x 8mm x 0.15 Ni Plated Nickel Strip Tape For Li 18650 Battery Spot Welding
This will give me additional capacity on the busses for when I series connect packs later for 48v systems.
I've had this type ordered for about a month now. But this will maximize the surface area of my micro-busses to minimize losses. Then connect 7s JST plugs to each pack so that I can connect

http://www.ebay.com/itm/172460021203?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT


image_kiawag.jpg

to monitor each row's voltage, and even balance them at 50ma when needed.
Hopefully I'll have all this Utube stuff figured out to be able to produce a video after I complete my first one.

Unrelated: I got a pack full of what appear to be 15650s, that are testing around 2800 mah, which is more than my average for 18650s. Has anyone tried using these in your packs yet? They fit just fine, just not quite as tight.
 

Janis

New member
Joined
Jan 22, 2017
Messages
14
Korishan said:
Ahh, that makes a lot of sense. Hadn't thought about the AC vs DC traveling through the wire differences. I thought all current flowed on the outside. But that does make sense since DC devices have stranded vs AC devices have solid. It's all coming together now :p

So, having a bussbar that multi-stranded would be better than having one thick one, then.

No, it doesn't.
Before to go on, please, take a look in Wiki and, if still curious, in specialist forums, like this.
The essence: on DC or household AC, it's the cross section of the wire what matters. The skin effect starts on higher frequencies.
On 50 or 60 Hz AC, the solid wire is used for house wiring due to simpler fixation and better conductivity at the given diameter. Cables going from sockets to devices are stranded, of course. In the later link You can see that, for example, the max current for the 14 AWG wire is 24 A for solid and only around 10 A for popular multi core.
So, stranded wire must be oversized and used in places were it's inappropriate to use solid one.
For a DC and especially buss bars, the solid are the best.
 

Korishan

Moderator
Joined
Jan 7, 2017
Messages
6,522
Because the interior of a large conductor carries so little of the current, tubular conductors such as pipe can be used to save weight and cost.
At what point would this even be feasible??


But I do see how just having thicker wire would be beneficial for DC as it creates a stable field that doesn't try to repel itself like in AC. I don't know a whole lot of magnetic fields and such, but enough to understand some of the principles :)The math equations make me go :( :huh: :exclamation: :-/ :s :sleepy:


Thanks for the info, Janis :D
 
Top