Yes. You need one... but don't buy the one from Ingress. It is on the expensive end of things.
This Kamashi has been fine for me. $14 on Amazon
I'm got some KmashiK-MP816 that someone gave me after she had trouble with the power button.
It seems to work fine now, but I haven't really used it much, since I also carry a wall wart that uses a removable USB cable, and there's not enough portable density in my area to warrant more than an hour or two of continuous play.
I did get a longer cord for it, though. The little two-foot thing it came with is quaint, and definitely not Ingress friendly.