Extern Battery Pack might be braking my usb cables?

I bought a 12000 mAh battery pack from eBay and it charges my iPhone perfectly fine but I noticed that suddenly it stoped charging one day and my usb cable just didn't work when I plugged it in the wall. I just assumed it broke and I bought another usb cable and everything worked fine and it charged my iphone fine but then it stoped working again and when I plugged it in the wall it didn't work either? Why is this happening? I don't want to be buying new usb cables every week

This seems a little strange, somewhat.
Hmm. What are You doing with the usb leads what no longer work?
Are you snapping them in-two so it renders them useless, by doing so
it means you're not picking up, one of the 'duff ' leads.

I think someone may be playing a sneaky "trick" on you, by substituting
the newly purchased one you've just bought, with a one that doesn't
work, each time, whilst Your back is turned, so to speak.
Or, some person is sabotaging, the new lead once again, while you're
not looking.

It is very unlikely to be the leads, that are at fault if they were going to fail
they would do so probably before the first charge had finished.

I suspect someone may be a tad jealous of the bargain You've purchased
on Ebay, And is fooling you into thinking the charger is at fault.
I may be wrong, of course… But I would consider foul-play is at work here.

The solution is simple… Put the Charger & Lead(s) away in a 'secret' place,
of which, is known only to Yourself, No-one else, and I mean No-one!

Do this immediately after charging, keeping them in a safe place until Its'
needed, And I bet You the 'Problem' does not occur again.

If by some chance it does, then someone is replacing the lead whilst It is
being charged… Very Sneaky… Thing to do.

Get rid of / or destroy any duff leads that you've used/ bought in the past,
And render Them Unusable!
That way " they'll " not have anything to swap over… Hopefully,
Fingers Crossed. (Hth).