A piezoelectric battery
I guess you wonder if the generated electric field stays if a piezo crystal was kept under a constant tension without letting it take back its original form.
The crystal would keep its voltage, but there are some amount of charged particles in the air and even in glass and on the crystal that gradually they would get organized by the field of the crystal to a formation which neutralizes the generated field and make it unobservable.
You can help it happen much sooner by connecting the surfaces of the crystal together for example with a voltmeter. You should see the crystal with metallized opposite surfaces as a capacitor which can be charged by letting a force make tension to the crystal but a slightest leak would discharge that capacitor sooner or later. New charge needs new deformation.
If the tension is released the collected charged particle formation disintegrates.
Unfortunately I do not know enough of material physics to be able to say does the crystal gradually get reorganized under the tension so that the tension and generated field disappears. I guess that a million years of hammering by thermal motion causes something.