[knotimpressed]

Braving potentially getting spit roasted to ask:

If I'm making my own 18650 USB C power banks, are there any easy to miss risks? I've got the cells in holders, not welded, but the holders are specc'd above the current I need. The cells are unprotected, but the Aliexpress listing for the power management board says specifically to use unprotected cells, as at 6A draw most protection boards don't do well (dubious). The cells are tested and mechanically protected by a thick enclosure. The only EE work I'm doing is soldering 2 high gauge wires from the holder to the board that's doing everything else. I know Aliexpress isn't a bastion of quality, but the seller has good feedback and I checked over the board to make sure there's at the very least a good counterfeit battery protection IC included.

Currently, the concerns I have are: - the holder relies on good contact to deliver 6A without developing hotspots on the terminals - the board from Aliexpress perhaps should not be trusted

If there's anything else anyone can think of, I'm happy to hear it.

[Cyan488]

As an electronics designer myself, I would find it hard to trust the AliExpress board.

Without the design files, running a failure modes and effects analysis on the board is difficult.

There's also no guarantee that each board you get is built identically. Some parts or the whole design could be changed between orders.

If I was designing a power bank board professionally, I'd be putting it through the ringer - environmental, mechanical, component level short circuit, load short circuit, load power injection, input over-voltage, input transient, RFI/EMI susceptibility, etc. Do you trust that all that has been done on a board that is representative of what you've received?

[Panzer04]

There's a difference between acceptable standards for a truly professional product that you'd have to certify and hobbyist stuff.

Beyond short circuit/overcurrent, overtemp, over voltage, under voltage protections, what else would be necessary to ensure safe usage of the cells from an electrical perspective? Ie. What additional protections would a batter management circuit need to be safe in normal circumstances?

[exmadscientist]

JEITA profile compliant chargers require temperature monitoring (usually a 10k NTC thermistor). This is the third (yellow) wire coming out of a typical protected cell.

Many people also like to have fuel gauging but that isn't required.

[knotimpressed]

Definitely agreed on the lack of consistency between orders and even between boards in the same order. I’m doing very low volume for myself and not giving anyone else the banks, so I’m more than happy to check over all the boards for visually obvious issues with a loupe.

As another commenter said, is there anything beyond short circuit/overcurrent for the load side, and undervoltage/overcurrent protection on the cell side that’s crucial for a non professional bank? I’m happy to pop a few boards testing them myself.

[ranger207]

There is a subreddit dedicated to 18650s where people make power banks and the like, https://old.reddit.com/r/18650masterrace/ . Off the top of my head I remember reading about making sure you use the same set of cells together so you don't get unwanted current flow from highly-charged cell to low-charge cell, but I'm sure there's other details too

[knotimpressed]

Absolutely, I’ve browsed that sub a bunch but frankly I trust the opinions here more. I’m using matched reclaimed cells that I’ve tested to make sure they’re still healthy.

[Eduard]

it helps if you give us links and model numbers

[knotimpressed]

Well, there’s only one real part (the cells are interchangeable, and reclaimed) https://a.aliexpress.com/_mKNNnGZ I’ve gone with the purple C and U model.