It was explained to me as "It won't know that a new battery has been fitted and that is what creates the problem. As the old battery deteriorates the smart system will adjust charge to compensate along with possible changes to all the other things Jon mentioned, if you don't tell it that a new battery or a different battery has been fitted it will continue just as before and possibly damage the new battery or create other problems.
I suppose very simply put, it could be compared to a failing Maf sensor being compensated for, if you clear adaptions/trims problems will occur or if you fit a new Maf sensor without clearing adaptions/trims problems will occur.
Graham
Copyright © uk autotalk