I recently purchased an ASUS X54C-ES91 or (K54C-ES91) Notebook. It came with a 4 cell 14.4Vdc, 2600mAh, 37Wh battery.
I would like to replace it with one that has longer run time. I found one that is an 8 cell with 14.4Vdc, 4400mAh, 63Wh. The only difference being the 37Wh vs. 63Wh.
What about a 6 cell with 10.8V, 4400mAH? My sons ASUS notebook has one of these, I tried it in mine and it seemed to work fine even with the lower volts.
By the way, ASUS's Tech support has been baisically usless.