Wednesday, 15 July 2020

selling shit to the unwary

eBay is a great place for bargains, but equally you can get shit there too. Happily the buyer protection system works. Case in point, these 10,000 mAh 18650 cells:


there were originally 4, but one has been sacrificed on the altar of science.

I was instantly suspicious about these cells as soon as I opened the pack, they were too light. I popped them on the kitchen scales and they weighed 24g each. A typical 18650 should weigh 45 ~ 50g

A typical 18650 should be expected to deliver somewhere around 2000mAh (even a cheap one) or more. However I'd sort of expected these to be "below par" and perhaps be ok to 1500 or so ... sadly for the seller I have a cell discharge tester and a few tests found that these cells were about 350mAh.

That's right

  • good = 3000 (or so)
  • average = 2000 (or so)
  • these = 320 or about a tenth of good and four times less than I'd expected
That's of course simply shit.

So, why is it so (and indeed how is it so)?

Naturally I decided (after getting a refund) to cut the head off the cell and have a look. I found this:



and almost immediately noted the abundance of empty space in there with very little winding of "electrolyte",  becoming obvious when pulling it out of that cell


I count 9 turns ... which is low (commonly its over 20)

Here's a quick video


so, measuring the area of that electrolyte (as shown in the video) I get about 137cm2 which is also very below par with this publication suggesting 846cm2 for the area.

As 137 is about 0.16 of the area then that pretty much perfectly reflects in the reduction of capacity because 0.16 x 2000mAh = 323mAh ... or what I measured. Good thing I didn't use these cells for attempting to build a scooter battery.

So this is of course a working practical example of why you went to school and why they taught maths.
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3 comments:

  1. BLJace27 July 2020 at 15:18

    I didn't learn this in school... but this why i come to your blog lol!

    ReplyDelete
  2. fred7 January 2022 at 23:45

    Hi
    Just a quick update 4 u since you were so helpful in accessing my WW pro 2019 with snapped back suspension and bent hubs
    Ive ridden for a few weeks now and really loving it although there are a few issues which still need addressing
    I rebuilt my machine from scratch and de rusted or replaced the necessary components as i went along
    I find the movement in the handle bar assembly needs addressing - im going to retap the handle bars and replace all the screws to b sure
    The stem constantly needs tightening after pretty much each ride so ill tap and replace where necessary as well
    i rewired the front motor and repaired the blown controller to save expenses - all good
    Love riding it but needs further TLC then i will get to modifying things
    I thought id tell you about a custom lithium battery box from easy battery box seeking funds
    here is the link
    https://easybatterybox.com
    Im looking for a little bit more speed in eco/power and more distance so wanting to add a more powerful battery setup
    pls can i get your expertise and thoughts (not sure where the BMS is mounted) but looks like a good option to me

    ReplyDelete
  3. obakesan8 January 2022 at 08:09

    fred, firstly I don't normally approve any post with a link in it. However after looked at that link in an incognito window I have done so in this case.

    my view is that it will not be suitable for your needs on the widewheel because of space issues.

    In standard form the BMS is tucked inside the blue sealed battery box (as you'll find on my blog here https://cjeastwd.blogspot.com/2020/02/mercane-pack-repair.html

    Naturally the bigger battery has it in a slightly different spot.

    :-)

    ReplyDelete