They're all more or less correct: [...] = optional word My phone has 1% battery [left/remaining/until it dies]. My phone is at/on 1% [battery]. My phone's battery [level] is at/on 1%.

US - "My phone is down to 1%." "My battery is down to 1%" "My phone is at 1%" "My battery is down to 1%" "My phone is nearly dead." "My battery is dying."

I agree with Lincoln. In the UK if you said "my phone is at 1%" you would be understood too. Or as others have said: - My phone has 1% battery (left/remaining)

My phone battery is showing 1%.

You could say “my phone has 1%” or “my phone has 1% battery left”. It’s a bit more common to hear “my phone is at 1%” (in the US anyway).