# Change number of 18650 cells per pack to make equal amps capacity?



## timk225 (Sep 19, 2014)

I just read the 60 AH / 90 AH paralleling thread and it brought up a question I had.

I'm planning to test used laptop battery pack cells and get enough cells to make my EV pack. It'll be labor intensive but CHEAP. And I'm all about being cheap, or.... let's say "economically efficient". 

I was planning on 4000 2 amp cells, paralleled in groups of 100, to end up with roughly a 150 volt 200 amp pack.

The Question:

Let's say after all my testing, I have a bunch of 18650 laptop cells with 2000 mah capacity, and a bunch with 2500 mah capacity. Capacities will vary, I'm sure.

Now if I were to assemble 100 of the 2000 mah cells into a 3.7 volt 200 amp block of batteries, would I then need to use only 80 of the 2500 mah cells in another block of cells to get the same 3.7 volt 200 amp output? Would that be the correct way to do it?

And if I had a bunch of 2300 mah cells, use 87 cells in that block? And only use 71 of the 2800 mah cells in another block? The goal being that the capacities of the individual cells in each block adds up to 200 amps of capacity?

From what I read in the other forum topic on 60 AH / 90 AH paralleling, having different amp capacities would cause the smaller capacity pack to drain harder, so it seems reasonable to assume that I must match up my blocks of cells to make equal amp capacities per block of cells, lest I get a messed up situation otherwise.


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## rwaudio (May 22, 2008)

timk225 said:


> I just read the 60 AH / 90 AH paralleling thread and it brought up a question I had.
> 
> I'm planning to test used laptop battery pack cells and get enough cells to make my EV pack. It'll be labor intensive but CHEAP. And I'm all about being cheap, or.... let's say "economically efficient".
> 
> ...


I would recommend mixing them within each cell block. Make each cell block the same capacity using the same number of cells. That keeps the surface area the same as well as the number of connections sharing the current. Internal resistance has just as much effect as capacity on currents that may flow within a single cell group. It's not ideal, but it's basically the way all packs are made. Even a single 100Ah prismatic is made of 100's of thin "cells" all packed together in a plastic block. They are likely going to be close to each other in performance but they won't all be the same. Same with a volt/leaf pack. The Volt is 3P, there will be minor differences cell to cell.


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## Karter2 (Nov 17, 2011)

Interesting question. !
i would agree with RWA that mixing them would be best.
And use top ballancing
Whilst the lower capacity cells may reach the discharge "cliff" first, the majority of the paralleled higher capacity cells will support them and hold the voltage up until LVC is reached.
This is the advantage of paralleling groups of cells before linking them into series strings.
However, you do need to ensure each parallel group is closely matched on capacity, and if you use too many low capacity cells it would obviously reduce the useable total pack capacity.


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## timk225 (Sep 19, 2014)

What about when you go to charge these paralleled cells of different capacity? The larger cells will draw off excess voltage from the smaller cells, as the smaller cells would fill up first?


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## rwaudio (May 22, 2008)

timk225 said:


> What about when you go to charge these paralleled cells of different capacity? The larger cells will draw off excess voltage from the smaller cells, as the smaller cells would fill up first?


The voltage across the cell group will be the same (very close) so all of the cells will fill up evenly (be at the same state of charge) even if some of the cells are taking more Ah's than others.

Be careful with capacity variations like Karter2 mentioned, you want each cell group to have the same total capacity.

My method of doing this is to test 16 cell groups and record their capacity. To make the point I'll use easy numbers:
group 1 = 29Ah
group 2 = 30Ah
group 3 = 31Ah
group 4 = 32Ah

My goal is to have 32 cell groups, so if I parallel group 1 and group 4 I get 61Ah, if I parallel group 2 and group 3 I also get 61Ah. It doesn't really matter what the individual capacities of the cells within the groups are as long as they are healthy functioning cells with the same total capacity.


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## ga2500ev (Apr 20, 2008)

timk225 said:


> I'm planning to test used laptop battery pack cells and get enough cells to make my EV pack. It'll be labor intensive but CHEAP. And I'm all about being cheap, or.... let's say "economically efficient".
> 
> I was planning on 4000 2 amp cells, paralleled in groups of 100, to end up with roughly a 150 volt 200 amp pack.


Exactly how cheap is this? How much are the raw materials? How much time is involved?

I'm only asking because I believe that I've seen about 3 or 4 posts from you indicating that it's cheap. I'm just trying to do a cost analysis.

Thanks.

ga2500ev


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## dcb (Dec 5, 2009)

I'm looking at making an 18650 reconditioner, that will take one or more cells, and recharge them from dead (start slow, ramp up current as they build more voltage), 

I'd like to automate:
internal resistance
self discharge rate (roughly)
capacity,
etc., then display it and I will write it on the battery with a sharpie (or store it and give the battery a serial number and figure out the best arrangement in a program).


Does it make sense to have the groups also have the same internal resistance and self discharge rate? 

After a certain number of cells in parallel, can you just recondition them and mix them up and let probability do its thing? Or might it be important to group similiar cells for some reason? (lots of info here fyi)


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## rwaudio (May 22, 2008)

ga2500ev said:


> Exactly how cheap is this? How much are the raw materials? How much time is involved?
> 
> I'm only asking because I believe that I've seen about 3 or 4 posts from you indicating that it's cheap. I'm just trying to do a cost analysis.
> 
> ...


I've gotten about 1000 cells, 700 usable all for free. Holders for them are less than $100 (holders for 4000 would set you back a few hundred bucks) I'm going to be using copper clad PCB for my current collector using the Tesla style thin fuse wires to each cell.

My 14S48P module (51.8v 96Ah) will have a total cost of about $125, but quite a bit of time.


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## timk225 (Sep 19, 2014)

Another part of putting paralleled blocks of series that I need to find out more about:

Let's say you have "blocks" of 100 cells, all in parallel. Each block is roughly 3.7 volts and 200 amps or so.

Now you need to connect 40 blocks in series to get up around 150 volts output.

Describe how you would connect each block to each other in series. I saw a youtube video where some guy with a stripped out VW van put two big metal bus bars down one side of the pack, one on the + side and one of the -. 2 inches apart. Big sparks if some metal ever fell on them and shorted it!

If I am using heavy 4/0 cable to take the power up to the controller and motor, I'd need to use the same 4/0 cable between each block of cells? Because at any given point in the circuit the cable would have to be able to handle the full load?


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## rwaudio (May 22, 2008)

timk225 said:


> Another part of putting paralleled blocks of series that I need to find out more about:
> 
> Let's say you have "blocks" of 100 cells, all in parallel. Each block is roughly 3.7 volts and 200 amps or so.
> 
> ...


I wouldn't make single cells, I would make modules with a manageable size/weight. 500-700 cells might make a workable module that's still not too hard to lift, then add some terminals to connect those modules together with bus bars or cables.

4/0 is pretty heavy cable, what kind of currents are you planning for? I used 4/0 in my motor loop in my old set up, and only 2/0 in the battery loop with a 1000A controller.


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## Karter2 (Nov 17, 2011)

rwaudio said:


> I've gotten about 1000 cells, 700 usable all for free. Holders for them are less than $100 (holders for 4000 would set you back a few hundred bucks) I'm going to be using copper clad PCB for my current collector using the Tesla style thin fuse wires to each cell.
> 
> My 14S48P module (51.8v 96Ah) will have a total cost of about $125, but quite a bit of time.


There are folks developing kits for this type of pack....
http://www.diyelectriccar.com/forums/showthread.php/18650-cell-kit-125257.html


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## merkurios (Oct 28, 2020)

I'm wondering what was the end result of your idea? Did you manage to achieve what you wanted? I think that using laptop battery pack cells to get enough cells to make your EV pack is an ambitious, interesting idea, which I fully support, but that’s only because it’s not my car that you want to test on. Out of all the opinions shared on this topic, it seems like rwaudio is the only one who really knows what he’s talking about, so I would take a closer look at what he’s saying. Since you’ve been asking about numbers and how to mix them, at https://www.sunsigns.org/angel-number-55-meaning/ you can find all the meanings of the number 55 from an astrological point of view, and the impact that this number can have on your life.


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