What is the best battery backup solution for a Sony Vaio?

I have a Sony Vaio that gets about 10 minutes of life when I am not plugged in. I need to use my computer a lot when I am not near any power source, and so I am wondering what the best solution is? Do I just need to buy another Sony battery as a backup? Is there such a thing as an external battery for a laptop?|||just buy another battery. Look for 10hrs battery life (as minimum)