How to tell Android that 80% battery is actually 0%

I have a phone whose battery is dying at 80%. How can I calibrate Android so that it's aware of the actual battery capacity? I have root access. To be clear, I'm not asking to increase the capacity or re-calibrate the battery itself, only make a change to Android so it can reflect the actual battery capacity. I'm pretty sure the battery is fake since even when it was brand new it would die at 30%, so I have no expectations of getting more life out of the battery itself.