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I do have a bunch of 18650 LiPo cells, salvaged from various sources. Now, I happened to come across a variety of power bank cases on Amazon which mostly look like this:

power bank case, with 20 parallel contacts

Even a cursory look tells me that these are designed for 20 cells connected in parallel, without any individual cell balancing. As far as I understand it, this is a complete no-go, especially if you're considering salvaged cells with mixed capabilities?

Or is this configuration actually safe to use, because the individual cells can't be charged above 4.2 V (or whatever the cutoff voltage of the charger is)?

Addendum: I just found Can I use multiple 18650 batteries in parallel with a standard MicroUSB 3.7V charger board?, and based on that answer, it would seem my original hunch was correct? If one cell shorts out, then this thing would go up like a napalm charge?

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    \$\begingroup\$ Balancing is not an issue with 1S20P, but cells with low capacity but also low ESR would suffer from any high load situation. For regular 5 V USB out, this isn't an issue. For 20 V USB C at mat load, perhaps. When I build mine, I match voltage and preferably all fully charged, since there will be a short between the cells. \$\endgroup\$
    – winny
    Nov 16, 2023 at 7:59
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    \$\begingroup\$ The product page says "Each battery slot is charged and tested separately" but indeed, I wonder how they could possibly do that given that all the contacts seem to be connected together on each side... \$\endgroup\$
    – jcaron
    Nov 16, 2023 at 17:13
  • \$\begingroup\$ Yeah, I don't buy that either. 😑 \$\endgroup\$ Nov 17, 2023 at 7:04

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Those are a fire hazard. Even if someone can sell something, it does not mean it should be sold.

I recall seeing a picture of similar device that had melted itself. What likely happened was bad contact to one battery, and it didn't charge but others charged. When the power bank was moved, the uncharged battery made contact and all the other, 19 (or less) fully charged batteries, dumped current to the empty cell that became shorted to the terminals, pushing through a lot of current, which the battery and metal contacts could not handle.

Batteries should not be paralleled with contacts that can get intermittent connections, and that also allows user to put in batteries with unmatched state of charge without balancing the cells.

I think that device would not pass electrical safety and compliance tests if you tried to import and sell it by the book.

If you buy it online, in some cases, you might even be held responsible as the importer if you bought it from an overseas online shop, and it burns down your house. The insurance company can get very interested about these things. So far they have just warned not to buy cheap dangerous products of unknown quality from online shops, as the safety and compliance for standards of those products might be unknown.

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This does not look safe. Maybe alright for personal use but would you want to sell this to anybody else?? NO. 20 parallel cells mean enormous prospective fault current which could burn big time. You could get 20 small fuses like say 1 amp and protect each cell this would help. You could get more expensive and very slightly longer protected cells if they would fit.

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