Adrian Ramos should have scored but shot narrowly wide, before Ronny finally produced a deserved equalizer.
"There was probably a bit of frustration in that," Hertha coach Jos Luhukay said. "He started three games and wasn't so fortunate. Then he comes on and scores. Thank God."
Hertha then had goalkeeper Thomas Kraft to thank for saving Artur Sobiech's point-blank effort in the last minute.
Hannover striker Didier Ya Konan was taken to a hospital with a severe-looking left ankle injury sustained in a tussle before halftime.
"It was a bitter blow for us. For the players to see that at halftime didn't do our prospects any good in the second half," said Hannover coach Mirko Slomka. "Still, you have to say we were bad in the second half. The Berliners had really good chances. We're lucky to get the point."
Hannover had been bidding to win five games in a row at home for the first time.