The former Tottenham manager has agreed to take over until the end of the season — which comes in just 17 days — after Gianfranco Zola quit on Monday.
Blues are three points above the relegation zone after Zola won just two of his 24 games in charge and stepped down after a 2-0 home defeat to Burton.
They go to city rivals Aston Villa on Sunday knowing they could be in the drop zone before kick-off but Redknapp had no hesitation taking the job with three games left.
“Birmingham are a team I’ve not see much of, seen nothing of really, they’ve not been on TV much this year,” he said, after returning to club management for the first time in over two years.
“I’m a bit like that, I tend to dive in. When I got home at 2.30am my wife was asleep and she said: ‘What happened?’ I said: ‘I’m going to Birmingham until the end of the season’. She thought I was mad. Everybody I know thinks I’m mad.
“I got a call at 7pm asking if I could come and having a meeting in London. I went over, met them at 9.30pm and by 9.45pm I had no hesitation. It wasn’t a case of signing a contract for three weeks, I shook hands and said I would do it, no problem.
“It’s a big club and it’s a challenge, a big challenge. After I looked at the fixtures I thought it was an even bigger challenge.
“It’s not easy. All I can do is try and install some confidence.”
The Blues are winless in their last nine games ahead of Sunday’s trip to Villa Park and Redknapp heads into the game after his most recent job was coaching a local U10s team.
He said: “I went over the park two weeks ago and coached some U10s. A guy came and did a bit of work at my house and started telling me how he coaches his little boys’ team and would I come over? I did two-and-a-half hours, Saturday and Sunday morning. I love it. While I’m still fit enough to do it and wanted I’ll always pop up somewhere.
“Villa’s a tough game to start with, a local derby and it will be great atmosphere. I’m sure Steve Bruce will do a good job in the long run but Birmingham’s a big club too and should be pushing for Premier League.”
Former Cheltenham manager Steve Cotterill and ex-Bournemouth boss Paul Groves will work as assistants at St Andrew’s with Redknapp, whose last management job was as Jordan boss last year, saying they must win one against Villa, Huddersfield and Bristol City to survive.
“The other teams are going to win some games, I’ve got no doubts about that. We’re going to have to win one of the three games, at least,” he said.
“It’s a risk but what can happen? No one has a magic wand in football.
“You could put (Alex) Ferguson in the same position and he’s not going to walk in the door and make them play like Manchester United.”