A car can often run better after an ECU reset

it is possible to change the battery without resetting the ECU, There are devices that plug into the cigar lighter or just keep a constant power source (for example, a second battery connected via jump leads)

if you simply drive the car harder - like redline it through the gears etc(safely, legally once its warmed up, of course!) it can help the ECU relearn - there is a procedure you are supposed to follow after a reset