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.