What cells to choose?

[Gifting]

I´m planning to build a 72v 20ah (only around 20ah due to limited space). I have never build a battery pack before so I have been doing quite a lot of research but I can´t seem to figure out the best cells to use.

I´ve been looking at Samsung 21700 40t cells because a lot of people recommend them and because I will be able to draw a lot of amps from a batterypack made by these cells using the copper nickel sandwich method. However if I use these cells (or any other 21700/18650 cells) I will have to invenst in a spotwelder. I have been looking at a kWeld with the Turnigy lipo battery recommended on the kWeld website and the total cost for welder and battery including freight cost will be around 400 euros. Since I don´t know how many battery packs I will be building this seems a little bit expensive and it got me thinking that there might be other ways of building good battery packs without spot weldning? I´ve seen people using eig c020b cells for example with bus bars that screws on to the cells. Eig c020b seems impossible to find and order but maybe there are other similar cells? I have been looking at some Lifepo cells that also allows bus bars to be screwed in place but it seems hard to find Lifepo cells that are as space efficient as the Samsung 40t? Another thing is that I live in Europe so I would prefere a EU (or possibly a Chinese) supplier.

Any help is appreciated!

 

[amberwolf]

To help select cells, we need to know what the battery needs to do for you.

How much capacity is required? (not the 20Ah you note that you're limited to, because with different cells that might be a different number)

How many amps peak will your system draw from the battery?

How many amps continous will your system draw from the battery?

What are the measurements of the space you have to put it into?

Are you willing to use alternate locations / mounting methods than whatever you've presently settled on?



Some other notes:

--Most pouch (flat) style cells (not the plastic box style like CALB / etc, but the ones like the EIG, Leaf, etc) will be less volume for the same amount of cell, because all of the cylindrical cells will leave an unavoidable gap between each other that is just air. So generally flat cells will fit more battery in the same space than cylindrical cells, as long as your space is a box-shaped space that accommodates that type of pack (where cylindricals can be rearranged in a number of shapes that larger flat cells can't do efficiently).

All the EIG C020 cells will be a little on the older side (AFAICR they haven't been made in a while; mine are over a decade old, I think), and though they do last a long time, all cells do degrade with age. If you do want some NOS unused cells, Jimbob01 has a thread in the items-for-sale-new section; he's in the UK so if you're not it may cost quite a lot to get them shipped to you (probably more than the cells cost). They *are* nice cells, easy to DIY with.

There are plenty of similar cells out there, though not many have premade cell holders and interconnects from the manufacturer like EIGs that make them very easy to DIY with. But as long as they have long enough tabs, there are quite a few ways to clamp them together without soldering or welding, many of which are in various threads around this forum. Some are titled with A123, JonesCG has at least one thread with smaller cells in very large packs for motorcycles/racing/etc, etc.

There's plenty of Chinese Li-Ion / LiPo flat pouch cells out there from Chinese vendors; the catch is finding a cell that will do what you want *and* that you can actually be sure to get from whcihever vendor you choose as actual new top-grade cells. There are numerous places selling non-top-grade cells as if they were, some of them literally recycling garbage cells and even remarking them as different brands / models than they really are (a big problem in 18650 stuff), so you have to vet whatever vendor you use to be sure they really will sell you the real thing you're after (or be sure you don't care and will just use whatever you get).


Note that flat pouch cells will generally need some form of physical compression across the flat faces of the end cells of a stack, yo'ull need the specs for how many lbs/sq in from the cell manufacturer to do this "right", to ensure that they don't puff up while in use (charging or discharging). If they do puff up, they're generally damaged and don't perform as they should (and potentially are a fire risk, but mostly just performance issues), and you'll end up replacing them to fix that. There's info on this in the various pouch-cell build threads, among others.

Cylindrical cells don't need that because their casings do it for them.

 

[Gifting]

Thank you for your thoughts. I´m sorry for not being clear in my questions

As I mentioned I have been looking at the Samsung 21700 40t. If I do 20s and 5-6p I will have 72v and 20-24ah in theory. Since these cells are rated 35a I will be able to pull 175-210a (depending on 5 or 6p) in theory and that will be more than enough for me. My needs are 80a continuois. My plan is to stack 2 10s on top of eachother and the cells are 21x70mm so in theory that would give me a 105Wx140Hx210L mm pack for a 5p and that seems very space efficiant to me. I know that these meassurments will not be accurate in real life due to spacers, bms and so on but still.

So I guess what I was trying to ask is if there are other cells out ther as space efficiant as these Samsung cells that doesn´t require spot welding?

 

[amberwolf]

So I guess what I was trying to ask is if there are other cells out ther as space efficiant as these Samsung cells that doesn´t require spot welding?

Well, that part I did answer, in that any good pouch cell is more space efficient because they're flat and you can make a pack that doesn't have a significant volume of air (wasted space for packs like these) inside it between all the cells, whereas all cylindrical cells built into packs that are not a simple single-cell-width tube *will* be full of wasted air space.

And that any of them with long enough tabs can be clamped together in one of a number of methods, not needing soldering or welding.

Now we just have to locate *which* cells would work for you, and determine which clamping method you can do with the parts, tools, and skills available to you.

 

[jimmyhackers]

https://www.ebay.co.uk/usr/jimboblogan01?_trksid=p2047675.m3561.l2559

this is the ebay guy i bought my eig cells from. uk based. not sure if he ships to europe.....worth an ask maybe?

chaps pretty cool. ask him how many you want and he will do you a custom listing if you cant find the right number for sale on his shop.

not sure if youve read my other threads about the EIG cells.... dont get them wet....adding some sort of corrosion inhibiter on the screw terminals and bus bars would be a good idea. make sure you get stainless screws/bolts.

i did end up soldering them....was a bit of a mission...but it yielded slightly better results (lower internal resistance, less weight)

https://www.youtube.com/watch?v=z86Mt6t6Cn8
watch this video and near the end youll see a person smash a hole through one with what looks like an ice pick.
spoilers: no fire

 

[calab]

You choose the cell based on what discharge you want, what capacity (ah) you want and what physical size.
I like 25r's but the 21700's look good to, I am just not familiar with their characteristics (discharge amps and ah). My Grintech battery is 21700, 15ah gets me around town :wink:

 

[999zip999]

Ok what ? So the question is what size ? Do you have a pic. What watt controller or ampage and motor are you going to use at what amps.

 

[calab]

Well if you get holders then its a set spacing between the cells.
If you have a 3d printer maybe you can offset the cells so its a narrower dimension or maybe someone sells offset holders.
Choose pouches or prismatics, maybe lipo.

 

[Dak77]

I´m planning to build a 72v 20ah (only around 20ah due to limited space). I have never build a battery pack before so I have been doing quite a lot of research but I can´t seem to figure out the best cells to use.

I´ve been looking at Samsung 21700 40t cells because a lot of people recommend them and because I will be able to draw a lot of amps from a batterypack made by these cells using the copper nickel sandwich method. However if I use these cells (or any other 21700/18650 cells) I will have to invenst in a spotwelder. I have been looking at a kWeld with the Turnigy lipo battery recommended on the kWeld website and the total cost for welder and battery including freight cost will be around 400 euros. Since I don´t know how many battery packs I will be building this seems a little bit expensive and it got me thinking that there might be other ways of building good battery packs without spot weldning? I´ve seen people using eig c020b cells for example with bus bars that screws on to the cells. Eig c020b seems impossible to find and order but maybe there are other similar cells? I have been looking at some Lifepo cells that also allows bus bars to be screwed in place but it seems hard to find Lifepo cells that are as space efficient as the Samsung 40t? Another thing is that I live in Europe so I would prefere a EU (or possibly a Chinese) supplier.

Any help is appreciated!

If you end up going the cylindrical shape route , and spot wielding isn't an option, I would recommend the NESE modules. I've been using them on a daily commuter for about a year now and they've been trouble free. Individual cell dimensions matter , though, so be sure to let him know if you're using a Samsung 40T or 50E for example. The difference in diameter can cause problems . 50E is slightly bigger.