Excel is the wrong tool for dealing with extremely large and/or precise numbers.

Excel can only handle I believe 14/15 digits of significance after which point it truncates beyond those characters. Its also a grey area as to where this happens (final value in a cell definitely, using the results of a calculation mid calc maybe works until the end, etc).

Testing this is simple, type in a blank cell digits (say 20 digits to make a whole number) and hit enter, then look at the number you entered...it will have trailing 0's now.