I do not know the EMB, but assume that the electric system is mostly DC as well and therefore somewhat comparable to that of the DH8. That said, does the high APU load occur together with unusually high battery charging currents? If so, it might be that the batteries have been drained a bit by either having run the aircraft systems on them a while (if the previous crew was too skimpy to fire up the APU after the last leg and no GPU was in use either), or a GPU with a too low voltage output that has discharged the batteries.
When the GPU is connected, we normally disconnect the batteries to avoid this scenario and we are required to reject any GPU that delivers less than 25V. For an APU-driven engine start, we are not limited by APU output, but by battery charging current (it needs to be below 40%), and this seems to keep the APU generator happy.