Experiments have shown charge of a piezoelectric actuator is proportional to its displacement for an extensive area of operating. As a result, accurate estimation of charge can be an equivalent of displacement/position estimation for piezoelectric actuators, a key advancement towards precise sensorless nanopositioning. As a drawback, existing charge estimation methods take a significant portion of the excitation voltage, i.e. voltage drop. Digital charge estimation have been shown, in the literature, to have the least voltage drop compared to other methods. Digital charge estimators have only one analogue element, a sensing resistor. This paper initially investigates digital charge estimators of piezoelectric actuators to extract an aptness criterion to (i) maximise the accuracy and (ii) minimise the voltage drop. Experiments show that estimators with a constant sensing resistance cannot satisfy the aptness criterion at different operating conditions; while, all existing digital charge estimators use one or, exceptionally, a few intuitive uncalculated sensing resistances. That is, existing estimators witness evitable inaccuracy and/or unnecessarily high voltage drop. This research tackles this defect through development of adaptive charge estimators with varying resistors, which fulfil the aptness criterion in the entire operating area.