Busbars, wires and terminals

Batteriapan

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Feb 20, 2017
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35
Hi all,

I'd love some general feedback on my project.

I'm working on a 7s20p. Arather small solution I hope to be able toscale up two or three times over as I get my hands on more cells, i.e. upgrade to7s40p and then to7s60p.I wish to do this without having to replace any existing wires etc.

The busbars/wireson my 20p-packsare made from double folded andtwinned 1,5mm diameter copper wire enforced with solder(so about 7mm^2, or 9-8 AWG). That shouldhandle40 amps, or1kW@24v. Planning for a 600W inverter and no more than 1kW PV-panels,I feel thatshould be OK.

Does this seem to add up? Is there perhaps something I'm not considering?


I'm now looking at connector terminals to pair up the packs and hooking themup with aBMS. How does these look to you?


http://www.ebay.com/itm/Crimp-or-So...inals-Cable-Bolt-Hole-All-Sizes-/262287387962

I was considering the 10mm^2 wire/5mm holes variant. Is there som better solution you would recommend me?


Here's a picture of the negative pole onone of my packs, taken prior toenforcing the busbar with solder.

image_ceyobw.jpg

Due to receiving nickel covered steel plate rather than pure nickel I ended up with identical setup on both poles of the pack, i.e. copper busbar andfuses on both sides. The fuse wireis good for 6,6 amps and burns up at about twice that current. I feel confident it should not be very lossy at all given the power drained by a600W inverter.

Thanks for reading all the way here :D
 

Korishan

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Jan 7, 2017
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Looks pretty good. The lugs on ebay are good. Altho a little pricey, imho. Comes to over a $/lug. However, I'm from US and that's high for me. You could look around on banggood or purchase some lugs further away (tho not sure if they will be genuinely copper with plating).

Or, you could always go full out maker mode and get some copper tubing. Cut the tube in lengths like 1" or so. Put a cable or screw driver in about 1/2 way to hold that part open, and vice press the other end flat. Then just drill out the hole to the size you need. And finish it off with a good crimping, fill with solder, and voila! :)


As far as your pack goes, looks good. But why are you soldering the whole busbar? Seems a little excess to me.
 

ozz93666

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Feb 22, 2017
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345
That all looks very good and the busbar is more than adequate for the size of pack ..

When it comes to thickness of busbars , it's not so much can they "handle " the current ... they can more than do that without even getting warm , it's about keeping the resistive losses down , in low voltage systems these can be significant , and they increase by the square of the current , so they are much more if running at the high currents you propose (2A per cell) ....Even at those high currents , you are well below 1% loss with your busbar and that size of pack , so it's fine ..

Many don't like to run these old cells at 2Amps,they think it's too much for them ... not enough data to know if they're correct or not.
 

daromer

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Oct 8, 2016
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The important thing about the current is that not many of us test above 1A... And if you havent done that you dont know much at all about it.

I have done several with 2A and upt to 4A but in general a cell that gets slightly hot at 1A will get damn hot at 2A. Even the cold ones can get hot and you also get quite alot less capacity out if you go CC when testing.


Regarding the pack it looks good.
 

Batteriapan

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Joined
Feb 20, 2017
Messages
35
Thanks for the feedback!

Clarification on the 2A drain per cell that some of you've calculated: I'm probably not going above 150W @ 230 AC output in the beginning, i.e. 400-500mA per cell. As the packs grows from 140 cells to 240 to 380, I'll start pulling more effect out of the inverter.
 

Elmo

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Feb 19, 2017
Messages
189
I've "sat in" on a lot of discussions about lithium cells by people who should know over on endless sphere , my understanding is that laptop cells are limited in current output both deliberately by design and also by necessity.

The by design part is so that dangerous amounts of current aren't available from laptop cells due to internal resistance. I learned about high current at low voltages being potentially hazardous a long time ago when I absent mindedly put a charged AA nicad in my pocket and burned my leg when it shorted out to my change and keys. It wasn't the cell that got hot so much as the metal it shorted through.

The necessity part is due to wanting more amp hour capacity and not needing high current, you devote more of the cell to materials that create electricity and necessarily less to materials to conduct the electricity around through the cell.
 

daromer

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Either you get higher current or higher capacity. Laptop cells are the later.

If they intentionally have limited the actual current by higher IR i have no clue of. sounds weird if so by doing that :)
 
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