It's not hard to remove and replace it. My wife's ES1 has an internal battery the length of which is the width of the laptop by 3" wide and barely ¼ thick. Look up your model on e-Bay to get an idea from the photographs if it's simlar.
You'll need a .5 Posidrive srewdriver and an old bank or hotel room entry card then it's only a matter of removing the sixteen visible screws under the base of the machine. They're all the same size so don't bother to keep them separate.
Use the plastic card gently to ease the palmrest/keyboard assembly from the base. It's best to start with the card at 45° to one of the front corners. Once you're in, carefully ease it all round the front and sides and lift until you can see the inside. The...