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DIY Pouch Cells 24V Powerwall


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HI All,

 

So, Before is start testing and arranging cells, I have a question.

I have the possibility of getting a lot of these cells: https://www.aliexpress.com/i/32951551702.html

The one in the link is the 10a. The ones i can get is 20a, 2x10A spotweld with a positive and negative (black and red) cable. (no BMS)

The specs are as followed:

20a, 0.2c charge/discharge, 3.7v

Some of them is very new, others not so new, hence the testing and sorting.

I can get about 300 of these cells. more or less...

So the plan is, if all goes good, and the cells are working, to have a 7s40p (40P for if some cells is bad) setup.

Each cell will be fused with a 5A fuse.

BMS to be used: https://lithiumbatteriessa.co.za/collections/bms/products/100a-ant-smart-bms-with-bluetooth-and-lcd?variant=35449073434791 as it can do 3.7V Cells.

I Currently have an Axpert RCT 3000kva 2.4KW, 24V with a Victron 702 BMV

Ive done the (excell) math's and this is what i get: Based on 2400w, 100A draw, 24v Battery:

Load per cell: 0.35A

Series string the load will be 2.5A (40 Series in Parralell) 

Based on this, the A draw will be way with in Specs. (20a 0.2C = 4a)

 

SO...

Before I start the journey, does anybody have any experience with there type of cells, and is it possible to build a powerwall with it? 

  

 

 

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I would be very weary of using LiPo cells at all, never mind potentially mismatched cells in a large pack.

The thermal characteristics of LiPo cells can result in a single cell 'stealing' all the charge and overheating.  If you can't individually monitor cell temperatures (or at least clusters of adjacent cells), you will never know until it goes boom...

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thanks @JustinSchoeman 

In not 2 familiar with these sells aswell. Im under the impression that they are the same as 18650 cells, only the shape and capacity differs. - But i might be completely wrong. 18650 is like "hoender tande"

Ive read somewhere, not sure where, that if you stick within the C rating, it is a very stable. Hence fuses on all the cells.

But, im here to learn. a Man dont know what he dont know. 

 

 

 

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thanks @JustinSchoeman,

Of all the research ive done, almost all point to the fact that these LIPO sells are safer than 18650. Due to the fact that they have "protection Circuit" and dont use a liquid electrolyte. Lipo use a gel base electrolyte.  Not sure how it measure in real life, as i never really came across 18650 that caught fire/explode. But then again, no one really talk about the bad experience they had with home made projects! 

I know that a lot of DIY power walls are build with 18650 cells. 

Hence I'm coming here to find out if this build is possible, the pro and cons. and ideas to make it safer. or did anybody attempt this before.  

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You are confusing issues.

18650 is a packaging. Cells are generally packaged in one of 3 formats:

1) cylindrical (of which 18650 is a specific type of cylindrical packaging)

2) pouch

3) prism

Inside that packaging is a cell with a specific chemistry.  Popular chemistries are:

a) Lithium Ion (Li Ion)

b) Lithium Polymer (LiPo)

c) Lithium Nanophosphate (LiFe)

d) Lithium Titanate (LTO)

Generally, you can get any cell chemistry in any package type.

Individual cells seldom have any built in protection circuitry, except when they are specifically packaged for standalone use (i have only seen this in some cylindrical cells and more rarely in pouch cells).

When you say '18650 cell', you could be talking about just about anything - although most DIY powerwalls seem to use recycled laptop batteries, which are typically Li-Ion with no built in protection.

LiPo pouch cells are marginally safer.  If your cells have individual built in protection boards, then they are safer still - but still be careful, they are still subject to runaway in some conditions.

 

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