| Breiti |
There is no offical rule that says how many damage a powerful charge does, but all monsters with powerful charge seam to use the following formula: "2 x primary attack dice + 2x str mod".
For example Rhino (Gore 2d6 and str 22 (str mod 6)):
2x 2d6 + 2x 6 = 4d6 + 12
So as a GM i would go with this formula for animal companions.
Breiti