Re-use 18650 Li-Ion laptop cells to build a battery pack

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moveon70's picture
Last seen: 6 years 3 months ago
Joined: Saturday, September 26, 2009 - 13:00
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Re-use 18650 Li-Ion laptop cells to build a battery pack

At work, we throw away lots of laptob batteries into the re-cycle bin.
The goal of this project is to salvage them and use them from my 24V scooter.
This was my second attemp. I had posted on this befor, but the BMS was defective, and also the pack was too weak.
This time, I was much more thurough.

First step is to get lots of batteries and open them up. Check th voltage of each cell.
If the voltage of the cell is below 2.5V then discard the cell. Holding a low voltage will permanently reduce the capacity of the cell.
Next clean up each cell. Pull off the Nickel tabs. I used a dremel tool to grind away the spot weld bumps from the nickel.
I ordered a charger for the 18650 cell off ebay and charged each cell, and then confirmed that the voltage was 4.2V
At this point I had about 150 cells. My goal is to use only the best 80 cells for my pack, so I needed to weed out the weak ones.
I let the cells sit for 20 days and re-measured the voltage. I removed all cells where the voltage had self disscharged to below 4.15V.
Next was the load test. I bilt a little load tester which used a 20W 2.5Ohm resistive load. I put each cell under load and monitored the voltage. After the cell was under load for 1min I recorded the voltage while still under load and wrote the value on each cell. I wanted to use the cells that could hold their voltage under load. In this case, I selected the cells that had >3.72V while on the 2.5 ohm load.

Now that I had all the cells picked out, it was time to spot weld them.
I build a capacitive disscharge spot welder using 0.04F capacitor at 48V. Others have used 12V and a 1-4F capacitor.
and bought my Nickel strips from Sunstone
Or from All-Battery
You can see that it works pretty well.

I decided to make 16 banks of 5 cells each.
Here you can see the cells ligned up in my fixture with a nickel strip waiting to be spot welded.
Next I bought some shrink tubing and shrink wrapped each set of 5.
You can buy shrink wrap at Battery Space and All Battery
Here you see the result of a lot of work.
16 banks of 5 all spot welded and shrink tubed.

A pair of 5 are connected in parallel, so there are a totall of 10cells in parallel for each "cell" on the BMS.
And bought my BMS from Battery Space
This is an 8cell, 30A BMS. I have 10x 18650 in parallel for each "cell"
And finally, the whole pack is shrink wrapped with just the two wires comming out the side.
I mounted it in my little stand up scooter and it works really, really well.
The Schwinn scooter was designed to hold 2x 7ah SLA batteries. My pack is designed to fit in the same space.
If all these cells were new, this would be a 20aH battery, but these cells are used, so it will be a bit less.

This whole project was a lot of work. It took a lot of research and also labor.
I got the cells for free, but after building the spot welder ($50), buying multiple sizes of shrink tubing ($45), and I am on my third BMS at $40 each because the first two of a different brand were poorly designed. If you add it all up it cost more to build then to buy a LiFePO4 cell, but the learning experience was priceless.

Sunstone circuits, Nickel strips for battery tabs

Last seen: 1 year 9 months ago
Joined: Friday, December 1, 2006 - 09:01
Points: 712
Re: Re-use 18650 Li-Ion laptop cells to build a battery pack

Thanks for the write-up, and the referrals to supply sources!


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