The rules I found for Shock Trooper state (emphasis mine):
Heedless Charge: To use this maneuver, you must charge and make the attack at the end of the charge using your Power Attack feat. The penalty you take on your attack roll must be -5 or worse. In addition to normal charge modifiers (which give you a -2 penalty to AC and a +2 bonus on the attack roll), you can assign any portion of the attack roll penalty from Power Attack to your Armor Class instead, up to a maximum equal to your base attack bonus.
The way I understand this, you need to get at least -5 to hit from your Power Attack (which means you have a BAB of at least +16). However It seems people are not really considering this point when discussing the usefulness of Shock Trooper, so I am wondering: do I understand correctly this limitation?