1. A device for ascertaining an amount of charge that is able to be drawn from an energy storage unit, up to at least one cutoff threshold, comprising: a charge predictor for calculating the amount of charge that is able to be drawn from the energy storage unit, up to the at least one cutoff threshold, wherein the calculated amount of charge is based on the product of a time value and a measured or a predetermined discharge current, and wherein the time value is based on an amount of time before the at least one cutoff threshold is met; and an estimator for ascertaining at least one state variable, based on at least one operating performance quantity of the energy storage unit, wherein the charge predictor includes a mathematical energy storage model that receives the at least one state variable as input and provides an estimate of a variable value as a function of time, and wherein the time value is an estimated point in time when the variable value meets or exceeds the at least one cutoff threshold.