I've a 15-inch MacBook Pro running Leopard with the latest software. Everything except the battery works fine.

If i unplug the power cable and let the system use the battery power it shuts down itself when it comes to 85 percent.

What might be the reason of this problem? Is it just because of the battery or is there a problem with the power unit of the MacBook Pro?

Also. Can these unexpected shut downs give any harm to the machine?