Best Portable Pump

I'm at school between 4-6 hours a day. During that time I have to pump at least once. Right now I'm working with a manual pump. Don't get me wrong it works in a pinch, but it takes to long to pump. My electric pump only works with a wall adapter, and the only place I can pump privately is the women's restroom. I was thinking about purchasing a battery pack that works with my medela pump in style, but would rather keep that pump at home and have another for on the go. Any recommendations? I was thinking about purchasing the medela pump in style advanced that I can take anywhere.