It's obvious Kenpachi knows, that's why she said she could used her sword because Ken-chan wasn't around.He probably doesn't want her fighting.And the fact that she has been the lieutenant for so long an never been unseated in a squad that had two shinigami who became lieutenant of other squads, Renji and Tetsuzaemon, and two shinigamis who have bankai Renji and Ikkaku shows Kenpachi must have knew of her strength.