Bluetooth Headphones Battery Question

[FallenAngelDraco]

I have an old pair of headphones that I would like to turn into Bluetooth Headphones. To do this I am planning to follow this instructable(http://www.instructables.com/id/DIY-Bluetooth-Headphones-Sony-MDR-V6-wireless-mod/). However, I would like to make one change which is the addition of a USB power bank in order to prolong the time of usage. The power bank will be plugged into the receiver for as long as I am using it which ranges form 3 to 8+ hours a day.

My question is will the power bank being constantly plugged into the receiver cause the power bank to deteriorate and not be able to hold as much charge as it would have when new. (I had a laptop that was constantly charging and now it runs out of charge in half an hour rather than the advertised 8 hour life.)

[Audioguru]

A good battery should last a long time. But a cheap battery probably fails soon because it has too much rice in it.
I bought a new bicycle made in China to replace my old one made in Canada, the same manufacturer. The brake cable broke after 2 days because its steel had too much rice in it.