I would tend to say something like:
My battery is low.
I need to charge my battery.
My battery is about dead.
My phone is about dead.
My phone is almost dead.
My phone needs recharging. (Suggested by J.R. in comment)
My phone has low power.
This is understandable. There could be a bit of ambiguity. "My phone has low power" could be taken to mean that your battery charge is running down, or could it mean that your phone just puts out a weak signal, no matter how high the battery charge state is. What I'm trying to say is "has low power" could imply that it is a permanent characteristic of the phone and not just temporary because of the battery charge. I don't think a native speaker would tend to use this.
My phone is low on power.
This is understandable, but probably would not come as readily to the lips of a native American English speaker as the alternatives I gave you.