This is the original Acer battery for your Alienware Area-51 M5500 Laptop

There is a $14 Cdn ($13.86US) shipping and handling charge.
100% OEM Compatible - one year warranty

The above batteries will have different model numbers on the product pages since they are after market replacement batteries which are as good or better than the originals.  The current or mAh (milliamp hour) rating of a battery is an indication of the battery capacity or how long it will last on a charge, the higher the better.