  Silicon-Germanium heterostructures are a promising quantum circuit platform,
but crucial aspects as the long-term charge dynamics and cooldown-to-cooldown
variations are still widely unexplored quantitatively. In this letter we
present the results of an extensive bias cooling study performed on gated
silicon-germanium quantum dots with an Al2O3-dielectric. Over 80 cooldowns were
performed in the course of our investigations. The performance of the devices
is assessed by low-frequency charge noise measurements in the band of 200 micro
Hertz to 10 milli Hertz. We measure the total noise power as a function of the
applied voltage during cooldown in four different devices and find a minimum in
noise at 0.7V bias cooling voltage for all observed samples. We manage to
decrease the total noise power median by a factor of 6 and compute a reduced
tunneling current density using Schr\"odinger-Poisson simulations. Furthermore,
we show the variation in noise from the same device in the course of eleven
different cooldowns performed under the nominally same conditions.
