Mitsubishi Galant Forum : “when u change the battery how do u reset the engine to idle at the right rpm”

This does not sound like a problem which would have been caused by disconnecting the battery.  Have this checked out by a mechanic or dealership.  You have a larger issue here.

there is a code you are suppose to type in from your stereo to reset the computer.  i am having the same problem.  after a few 100 miles the computer will eventually reset itself.  i bought my car used and i dont have the code but my girlfriend bought hers new and it is written in her owners manual.  i am not sure if it is a universal code because i have an aftermarked stereo so i dont have a way to check it.

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Did you ever figure this out?  This happened to me  a few years back when I changed my battery.  I brought it to a repair shop and all they did was reset the computer.  Now I have to do the same thing and don't want to bring back to a repair shop just to reset the comp.  thx

Leave the battery unplugged for 20 minutes. The car's ECU will be cleared. Then when you start the car, DO NOT drive it or touch anything. Just let it idle for 15 minutes while the ECU reprograms itself.

dont listen to anybody else what happened is when u disconnected the battery the pcm lost its memory and the throttle body is dirty all you have to do is get some intake system cleaner and clean the throttle body. if you dont know where it is follow the air intake hose to the intake manifold and right where the hose meets the manifold is where the throttle body is located. be careful though dont spray cleaner directly in the throttle body spray it on a rag and wipe.

Did you ever figure this out?  Everytime I change the battery on my montero sport, it does the exact same thing.  After a day or so it stops, but I would love to be able to fix it from the beginning than going dead at every light or stop sign

It's a normal condition where the ECU relearns and readjusts itself.