A method of determining the integrity of an electric or hydraulic energy source of a braking control system. The energy source is charged or discharged, and two or more measurements of voltage or hydraulic pressure are taken from the braking control system. A difference between at least two of the measurements is determined in order to determine a change or rate of change in voltage or hydraulic pressure caused by the charging or discharging of the energy source. The integrity of the energy source is determined by checking whether the change or rate of change in voltage or hydraulic pressure meets predetermined criteria. The check may comprise comparing the change or rate of change of the voltage or hydraulic pressure to a threshold—for instance checking whether the change exceeds a threshold, or falls between an upper threshold and a lower threshold.