18650 parallel power bank cooks the furthest batteries

Question

Against my better judgement I bought one of these 18650 power banks (see image). The reviews didn't disclose any issues and this sort of tech is so common now I never really thought much of it.

It takes 8 cells and I had 4 of these Samsung INR18650-30Q 3000mAh and 4 of these Sanyo UR18650ZTA 3000mAh. I thought as they're same mAh I wouldn't have any issues mixing them (I appreciate this is not the case now). I add the batteries in no particular order and check they are seated correctly then close the case.

The power bank says 5V 2A max so I plugged it into my phone charger with output of 5V 1500mA via the USB-C.

I have an ammo box I use for charging LiPos so I sat it in there while it charged. An hour or so later I pick it up and the end furthest from the control board is red hot. I disconnect and let it cool. 5 min later I open it and the batteries closest to the control board are cold but they gradually get warmer the further from the board they are. The last battery is still relatively hot. I also notice corrosion (see image) on the metal strip furthest from the control board that was not there prior to charging.

Can anyone enlighten me as to what is happening here. Is the product at fault, am I solely at fault because I've mixed the cells or is it something else? And why is it that only the cells furthest away got so hot?

Answer 1

If the batteries were not balanced before blindly shorting them in parallel, a huge unlimited charging or discharging current would flow.

There likely is nothing wrong in the powerbank itself, or the batteries itself, the product is just dangerous because it allows users to mix and match wildly different cells.

This includes old and new cells, charged and discharged cells, protected and unprotected cells, damaged cells.

I don't know if there are some safety regulations about users being allowed to access the cells dangerously like this, but this does not look very safe to let consumers apply the cells themselves due to how this device just shorts them in parallel.

If you bought this from outside your country, depending on your local laws you may be liable as an importer for the product if it had set your house on fire. So far insurance companies are still ignoring it, but it may be that at some point people can be held responsible for importing unsafe products. As you can literally buy anything that does not fill any sane safety standards, people should be more aware that when buying awesome looking products with unbelievable prices, it may come with serious safety risks.

Answer 2

Something is wacky. 5 volts at 1.5A is 7.5 watts. That for an hour is 7.5 watt-hours and your cells have a capacity of 10 watt-hours each. Assuming your USB block is current limited at 1.5A. Right? Right? We're not talking listed products, not cheap Cheese junk here, right?

That means in 1 hour it would have added less than 10% charge to the group of 8 cells. There is no way it could have cooked any of the batteries.

Did you put the cell in reversed, maybe?

Now, that power bank seems a bit sketch, like it's missing stuff that oughta be there. Just because a certain country sells millions of them does not mean they are quality or non-dangerous. Do the batteries fit a little loose, like the holders are designed for a battery 3mm taller than yours? Check the instructions, but perhaps it is designed only for 18650's with onboard BMS, which is a coin-sized PCB that mounts on one terminal and adds about 3mm to the length of the battery and is underneath the colored shrink wrap. It can be observed as a slight indentation about 2mm from one end of the cell. Speccing 18650s with onboard BMS would allow them to be more slack about the design of the charger, since the BMS will protect the battery.

Answer 3

So I have tested these batteries today and the 7th battery from the control board is showing as 0.0V and the others were all around 3.1V when I test it with a multi-meter.

They were previously working (either brand new and tested or tested and working) batteries I had stored for about 6 months, though I did not test them after taking them out of storage and using them in the power bank as my multi-meter was at another address.

I'm guessing the battery failed in the power bank but it's impossible to say now.

Thanks for the input everyone. (Welcome to delete question if there's no value to it.)