until recently I was not aware that such a thing exist. So I want to share this link with you:
The camera battery lasts 4 hours only and I cannot use a dual battery grip because the OAG and guide camera is in the way. As I use a power bank for the dew heater anyway this is a very simple solution for me.
I was afraid it could add digital noise to the image and made some test shots (darks) with the battery and with this supply. I cannot see any difference in the images.
Hope this helps someone.