This letter presents a new method for the characterization of electret transducers, which are typically used in electrostatic vibration energy harvesters (electret e-VEHs). This is the first method allowing to accurately measure the value of the equivalent voltage source representing the electret in lumped parameter models of a wide range of electret e-VEHs. An accurate value for this parameter is critical for design, analysis and optimization, given the increasing complexity of e-VEHs electrical interfaces. Until now, there was no universal method allowing the measurement of this parameter, because of practical difficulties with some geometries, and because of charging non-uniformities. In this letter, the new method is presented, with insights on how to maximize the measurement accuracy. It is then applied to a state-of-the art MEMS electret e-VEH.