Why would a car start stalling after you put in a new battery?

Don't assume the battery had anything to do with the stalling. If a stalling proiblem was associated with the battery, it would have done it right away. And if it did, it would usually need to relearn the idle. The computer needs reset. It could be that the battery wasn't the problem to begin with, it was the alternator, or something in the charging system. So you put in a new battery, and it's got some juice, but not too much.. It works fine for a few starts, but then the engine dies, and you can't start it again. Likely the battery isn't getting a charge. Another theory is that your trunk light isn't turning off when you close the trunk, so it drains the battery, and on short runs the battery doesn't get charged enough. Had the same thing happen to me in a 2001 Dodge Stratus. Fools replaced the battery but forgot to attach the thing that made the computer remember everything (sorry dont know all the tech terms). Had to make it learn how to idle in park, then in drive, then with the air conditioner on, etc.