The Ex-Pompey boss, 70, was earlier confirmed as Gianfranco Zola’s successor until the end of the season.
Zola’s run of only two wins from 24 fixtures has left Birmingham 20th and only three points above the relegation zone with three games to play.
Redknapp told TalkSport: ‘I got fed up sitting around doing nothing. I will come and live up here until the end of the season.
‘If I can keep them up then we can sit down and talk about going forward.
‘I got a phone call last night from the people at Birmingham.
‘I drove to London, had a meeting for 10 to 15 minutes, sat down and said “Yeah, I will do it, no problem”. I got home and my wife said: “Are you mad?”’
‘Birmingham is a proper club. It is a good club. They are in a precarious position and we have got the worst goal difference as well.
‘I’ve gone in there, said I will come and do it. The money I am not interested in.’
Redknapp’s last job in English football came at Derby last season, as a football advisor alongside then-manager Darren Wassall.
He previously led QPR to promotion from the Championship via the play-offs in 2014, and Pompey as league winners in 2003.
He also won the FA Cup as Pompey manager in 2008.
Redknapp is due to meet the squad tomorrow before being unveiled as Birmingham’s new manager in a press conference.