Hello bolegna,
You can use the following filename template: frame_{frame:04}_.obj, for example.
In this case the naming would be frame_0001_1.obj, frame_0002_2.obj and etc.
In the next update we will fix the minor bug that adds the frame number to the end of the filename when formatted numbering is used.
If your OS doesn't allow to use "colon" sign when selecting the export path, you can modify the path string manually in the batch process dialog.