QA testing 18650 battery packs - strategies and equipment? - Page 1
Forum section: electronics / testgear

author: ajb
author_rank: Super Contributor
timestamp: March 08, 2021, 11:43:30 pm
content: We're considering a project that would involve dealing with thousands of 2S 18650 battery packs, which is not something we've had to deal with as none of our products to date have used rechargeable batteries. Part of this project planning involves figuring out a QA process for these packs we'd be buying in from an outside supplier to ensure they continue to meet requirements over a number of orders, adding up to a few thousand packs per year.I'm not fully sure what level of testing we'd want to do, but I'm imagining we'd want to put a few samples of each batch through several charge/discharge cycles to verify capacity, and maybe a quick AC-IR test on a larger fraction or even 100% of the incoming packs.So assuming that test strategy makes sense, which I'll be doing some further reading to verify and I'm open to input on, I'm not sure what equipment we'd need to best accomplish that. I see that Hioki and a couple of others make a few models of benchtop quick test devices which seem reasonable for AC-IR testing (like the BT3561A), but I'm having a surprisingly hard time finding equipment for capacity testing. There are tons of little gadgets designed for individual cells or for RC battery packs of various sizes, and a few more industrial-looking options for testing hundreds of cells at a time or giant EV packs, but I haven't turned up much in between. I know a battery charge/discharge test setup can be put together from standard test equipment, but I'd prefer to find something a little simpler to set up and hopefully easier to maintain and easier to train people who aren't EEs to use.Are there equipment options out there that I just haven't found? Or am I going to need to just put something together myself? Any other input from anyone who's done this sort of thing before?

author: tunk
author_rank: Super Contributor
timestamp: March 09, 2021, 12:17:05 am
content: No expert on this, but in addition you could check self-discharge.Will the packs come with a balancing circuit?And maybe ask the supplier what type of cell they will be using.

author: threephase
author_rank: Regular Contributor
timestamp: March 09, 2021, 11:56:41 am
content: For 18650 cell testing I use a setup from BK Precision. An 9206 DC Power supply coupled with an 8601 DC Load and utilising their software that links the two together to carry out a charge and discharge cycle.https://www.bkprecision.com/battery-testI have a Hioki BT3554 impedance meter, but to be honest I do not find impedance testing that useful on Li-Ion cells. By the time you start to see an increase in the reading, it is already obvious from the performance of the cell that there are issues. On VRLA impedance measurements work better as the impedance increases before capacity is overly affected.

author: ajb
author_rank: Super Contributor
timestamp: March 09, 2021, 04:43:39 pm
content: I'll take a look at that BK Precision page, thanks!We'd be getting these packs with wire tails with a connector, so one of the things I like about impedance measurements is that if we take the measurement at the connector we'd be implicitly testing the wire/termination as well as battery performance. It won't catch all possible assembly problems, but it would catch some. Getting a four-wire measurement at the connector would be tricky, but these would be JST XH and it looks like just enough of the contact is exposed through the housing that I can build a fixture that will get two pogo pins onto each contact to make it work.

author: threephase
author_rank: Regular Contributor
timestamp: March 09, 2021, 06:06:45 pm
content: Yes to some extent measuring the impedance at the connector will confirm the whole assembly. I would advise to speak to the actual cell manufacturer or obtain the cell data sheet. Sometimes I get a specific impedance value for the cell with a few milliohms tolerance, other times I get a specification of less then a certain value of milliohms for a new cell. The problem is when measuring multiple cells, I get a range of readings spread across 10 to 15 milliohms. If there is a BMS in the battery pack that will also affect the reading but should be more consistent. Cell temperature also affects impedance readings.I have no idea why I did not say this previously, but a pulsed DC load test at two different values will allow you to calculate the internal DC resistance of the pack and the DC load will also be able to carry that out for you and save you investing in an impedance meter. A pulsed load test will also be less temperature dependent than impedance. With this method you are less dependent on a 4 wire connection methodology, but could still be used if desired.Good luck with whichever you choose.

author: masterx81
author_rank: Frequent Contributor
timestamp: March 09, 2021, 06:10:12 pm
content: Look here:https://www.eevblog.com/forum/testgear/program-that-can-log-from-many-multimeters/It support a lot of devices, and you can program it to do and log anything you like.