Exporting to XLS

Hi, I have some grids where I need to export data to a xls file, the problem is that some fields are being formatted and change its value.

I have 4 fields, id - brand - model - serial, the last one (serial) consists in numeric digits like 2350099430329024390, but when I export the grid to xls, the serial numbers are formatted in 235009943033000000 (last 5 or 6 digits change to 0000).

How can I fix this? it is a very important feature in my app.

If I export to PDF, XML, CSV or any other the “serial” number is not affected. Also I tried exporting from PhpMyAdmin and it exports it ok.