Power Banks dead after 7mths of storage

[Android Central Question]

I know that I am asking about power banks and not cell phones but I am stuck.
I sold 100 power banks to a client in Oct 2017. He gave some out in Dec as corporate gifts. Many left unused. He now is constantly asking me to replace them in drips and drabs and is a huge cost now as they are branded. Why are they dead? Do the cells die if unused? Please help...

 

[anon(9485947)]

were they power banks from a generic supplier? there are a lot of dodgy ones floating about. some with lower capacity cells than quoted.. others with poor lifespans due to low grade batteries.

As the vendor, I think you would have to take it up with your supplier.

 

[Mooncatt]

If the power banks are pre-charged from the factory, then they very well could be dead. Storing at 100% will kill a battery, but most manufacturers ship them with only a partial charge just for that reason. On the other end of the spectrum, they will self discharge over time. If they dropped below 20%, that also causes internal damage that can kill a lithium based battery.

And that's not even taking into account user abuse that is still common, especially if they are draining the batteries completely.