Discharge circuit to stress test 18650 cells

[veer]

Hello
I am new to the forum, but I have been a lurker for about an year.
Me and my brother are looking to start my first EV project and I bought 18650 cells from a nearby seller. These are 2900mah 3c cells. I want to stress test these cells and see what sort of temps do these cells work at.

We bought 2 motors in the past for different things, both are BLDC and 72v, one of them has a controller too, but nothing has been wired up as of yet as I want to design the battery enclosure. One is a 3kw and other one is 4.2kw.

So to test these, I was thinking about connecting 20s1p battery from my cells to one of the motors and record the temps from there. But I am not sure about the controller connections as my brother was supposed to help in that, and he is right now indisposed elsewhere. Is there any other way I can stress test these cells at the rated 3c power rating that they list? I think a high resistance circuit would not be able to push the 3c rating of the cells. But can it? What are my other options?

If there is any other thread that I can follow and read to go ahead with this project, please link me there as I don't even know what exactly to search for!

 

[BlueSeas]

There isn't a simple answer to your question. Load testing on the built bike requires a way to induce a load on the wheel. Sort of like a dynometer.

Testing the full 72V pack without the bike requires an electronic load in the kW category.

The easiest way to test cells is at lower voltages. Maybe single cell at 3.x volts, or in small series connections at like 12V. Where you can get the dissipated currents for testing at more reasonable levels.

This isn't rocket science, but it does require a basic understanding of the variables at play and some basic testing equipment.

 

[john61ct]

The first question is, are these good cells?

The next is, are they well matched?

You test each individually of course.

There are many battery testing threads here that discuss reasonably priced electronic "dummy" loads that help automate the process.

$80-120 for a good one. . .

 

[john61ct]

Also RCgroups.com and candlepower forums

 

[amberwolf]

If you want to test the fully built battery, then it sounds like you want a "load tester". There's a few posts discussing various versions of those
https://endless-sphere.com/forums/search.php?keywords=load+tester*&terms=all&author=&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
Not all those results will be relevant, but many of them are.

If you want to test the cells before you build the battery (a much better idea), then you want a "cell tester"; there's lots of threads that discuss testing and matching cells, hard to find in a single search.
https://endless-sphere.com/forums/search.php?keywords=cell+tester*&terms=all&author=&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
https://endless-sphere.com/forums/search.php?keywords=%22match+cells%22&terms=all&author=&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
https://endless-sphere.com/forums/viewtopic.php?f=14&t=26383
https://endless-sphere.com/forums/search.php?keywords=cell+test*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search

Sorry I don't have exactly the answer you're after.

 

[goatman]

you can go to lygte to do cell comparisons, it doesnt give temp but it will give you an idea of capacity at different amps

https://lygte-info.dk/review/batteries2012/Common18650comparator.php

another way is to go to a battery seller site like nkon and look up your battery
nkon puts a link to the batteries data sheet and some of those data sheets put out cell temperatures

panasonic 18650pf/2900mah at nkon, you have to go down and click additional information

https://eu.nkon.nl/rechargeable/li-ion/18650-size/panasonic-ncr18650pf-3-7v-2900mah.html

then click, data sheet. and you get this

https://www.nkon.nl/sk/k/ncr18650pf.pdf

what cells do you have?

 

[john61ct]

Data sheet specs are often a fantasy compared to what actually gets delivered.

The point of being able to load test at the cell level, is to identify the honest sellers

like looking for a needle in a haystack.

And then the Holy Grail, you might have "great quality" instances of your genuine desired model

but are they well matched to each other?

 

[izeman]

I'm also in the process to test my two 15s5p packs made of Samsung 30Q. With a small load of 3A they still have a capacity of 14.300mAh after 3 year. That's pretty good - though it's only about a hundred charges ran through).

To test the whole pack under real life conditions I now ordered G4 50W light bulbs from China for $10 for 20pcs.
I now have 40 bulbs, 12V, 50W each. That's a total of 2000W or 30-40A depending on pack's voltage.
I also had to order bulb sockets. That way I'm free to arrange as many bulbs in serial and parallel as I like.

The standard load woud be 5 bulbs in series = 60V 50W, and then as many strings in parallel to up the power/current.

Current and capacity management is done by the smart BMS.

It will be VERY bright in my garage, but this was the most versatile and cheap solution I could find.