Different Li-Ion 18650 types.

[Nigel Goodwin]

We use quite a lot of 18650's, both for repairs, and for products we build, and usually buy them in boxes of 100. We normally buy proper Samsung ones from a UK battery supplier, and these are rated at 2600mAH - however, recently (while I was off work following an operation) the supplier notified us that they are no longer able to supply Samsung ones.

They offered an alternative (which I haven't seen) but in the meantime my boss ordered four 18650's from Temu (just to test), at a much lower price, and these claim to be 3000mAH. They have the same pink heatshrink covering, and instead of saying 'SAMSUNG' the label says 'SA SUNG'

Strangely, considering the limitations of Li-Ion, as supplied the Temu cells all read 3.0xV (too low) where the Samsung cells all read 3.5xV (correct) - which doesn't bode well.

However, I've connected them to one of my own designs, as it squirts serial debugging data out of a serial port, and it fully charged the batteries (four in parallel) in 7:55:59, so just short of 8 hours - I like to charge them gently and fully. This is pretty well in line with the Samsung cells, although I've never bothered to write down the charging time for new cells.

Currently I'm running them in the equipment, again taking advantage of the debugging data, and they have been running for about four and a half hours now, and are down to 3.96V - it'll continue after I've left work, and will shut down once it gets to where my software considers the batteries are flat, and I'll check the final time on Monday.

We've still got some brand new boxed Samsung cells left, so I'll repeat the same test on Monday/Tuesday for a comparison.

My concern is the 3V terminal voltage of the cells as provided, as it's below the recommended discharge voltage - what do people think?.

 

[schmitt trigger]

My understanding is (and please correct if wrong) that although a Li Ion battery will not self-destruct like a Pb-acid battery if stored at low voltages for extended periods of time, nevertheless the operating life (meaning the number of charge/discharge cycles) is compromised.

Temu is the poster child for “Let the buyer beware” company. They do have some amazing value stuff, but other things are pure Scheiße.
But at least you’re doing due diligence on these batteries, to have a documented understanding of the product’s quality.

HINT; I know that what I am going to suggest is a lot of work, but if you could duplicate this test after a few dozen of charge/discharge cycles, it would provide additional and valuable insights.