How to test massive amounts of Lithium batteries at the same time?

I have a few thousand Lithium-Ion cells harvested from old laptops that I would like to test (measure capacity). They are mostly in 18650 form factor, but the are a couple of different ones too. I think they all charge to 4.2V though.

I have a cheap battery tester that I bought for 10 bucks which can test up to 4 cells at the same time. But as you can imagine this takes forever. I mean testing 4 cells can take like 6 hours sometimes...

So I am interested in building a custom testing device that can take 32 or more batteries at the same time. Ideally the design would allow me to easily add more, in case I decide that 32 is not enough.

From what I understand, the testing simply charges the batteries up completely and then discharges them at a constant current measuring how long it takes until they are empty to calculate the mAh (capacity).

And in the end I need a display or something to read the capacity (and maybe also the time it took to charge and the internal resistance) from. For each individual cell.

Ideally I would like to use a 12V/16A PSU that I have lying around.

Has anyone done something like this before? Or do you have any tips on how to do this?

I think the main issue that I have is that I need a PCB or multiple PCBs that on the one hand can do the testing for one or more cells, but on the other hand allow reading the data from a micro controller easily.

(Obviously I would like to keep things as cheap as possible)

I'm fairly sure that a few months ago someone else on here was discussing a similar project. Unfortunately I can't remember who or the title of their of their topic, but I suggest you try and find it. At a guess I'd say you are looking for something between 3 and 6 months ago.

And in the end I need a display or something to read the capacity (and maybe also the time it took to charge and the internal resistance) from. For each individual cell.

In which case I suspect you're going to have to spend several 10s of thousands of hours checking cells on an individual basis.

If you want to charge and discharge cells in series you need a method of monitoring individual cell performance and SOC such as the Revolectrix PL8. There will be alternative units that can accommodate more than 8 cells but I'd surmise these are even more expensive.

If you charge/discharge cells in parallel there's no way you can isolate the performance of individual cells unless you feed each with individual power lines to permit monitoring (and control) of current and voltage.

I wait with bated breath to be advised otherwise :slight_smile:

Being your sourcing them from discarded laptops, are you just looking for the one that still has some life left in it?

Wait till you see how much testing is required to see if that battery is going to last more than hundred more recharges.

Frankly your best bet likely would be to buy more of the same battery tester you have.
Buy 10 more and get 44 batteries testing at once. You could “test” almost 1,500 batteries a week.

I think if you check the internal resistance and the heat produced during charging/discharging you can make an educated guess about the battery life. Obviously capacity will degrade over time.

Yes, I could buy the same battery tester 10 times, but that would be really expensive and to be honest I'm not too happy with the one I have. I can't interface it with an Arduino or the like and it doesn't monitor heat. And it's not capable of reviving deep discharged cells, I always have to do that manually which is a pita imo.

And since I don't trust the tester I only have it running when I'm at home. Effectively I am charging 4 cells a day.

@jackrae I'm fairly confident that I checking the results won't take 10 hours per cell. 10 seconds would be more realistic. But it is indeed correct that current control needs to happen for each cell individually. That's what my cheap battery tester does already.

@PerryBebbington I couldn't find it unfortunately. Maybe I'm not using the right keywords..

A 32-way battery capacity tester is nearly 32 battery capacitor testers worth of hardware, except the microcontroller can be shared.

You'll need 32 logic-level MOSFETs to switch the cells during discharge at the undervoltage point, you'll need 32 way
analog switch array to be able to monitor every cell's voltage, you'll need 32 dummy loads, 32 LEDs to show
the status of each cell might be good, and some way to be able to show the capacity of a particular cell when
you come to remove it.

So the first step is to make a capacity tester for 2 cells to prove everything, then scale up. For driving multiple
MOSFETs 595 shift register chain can be used, for multiplexing lots of analog readings several 16-to-1 analog
multiplexer chips can be used.

In scaling up try to be modular, say scale to 8 cell modules, and then build 4 of them.

Well, ideally I wouldn't solder everything together on a component level. I was thinking maybe there are PCBs on aliexpress that have all the circuitry required for the testing of a single cell. Then I would just need to buy 32 of those and connect them with an Arduino. I also don't see the need for 32 LEDs. One display and a button to cycle through the cells should be enough.

You might want to check out this guy How to capacity test an 18650 cell out of a laptop battery pack - YouTube