Ferroelectrets, with their internal bipolar charges on the surfaces of the voids can be viewed as a collection of a large number of individual tiny electret ionization chambers. Electrets in ionization chambers are discharged upon irradiation with ionizing radiation. The discharge of cellular propylene is studied upon irradiation with alpha-radiation from a ²²⁶Ra source, while in-situ measuring the piezoelectric effect. Piezoelectric coefficients are derived from piezoelectric thickness extension resonances and from the second harmonic current upon application of a sinusoidal voltage signal. The piezoelectric coefficient gives a direct measure of the effective internal surface charge density within the voids of the cellular material. Compared to electret ionizing chambers, a much better temporal stability of charges in ferroelectrets is observed. The small air volume in the voids of cellular polypropylene is believed to account for the remarkable charge stability.