"My phone is dead." OR "My phone's battery is dead." OR "My phone has no charge."

It might be more effective to tell them the percentage of charge you have left so that they know the urgency of why you must go and charge it or they know you’re a limitation of how long you can chat or text. “ my phone only has 8% “ [it’s implied that you’re talking about your phone battery]

I like the expression ‘my phone is battery dead’ - it kind of mimics referring to someone, including yourself in some circumstances, as ‘brain-dead’. However, normally we would probably just say ‘my phone’s not working, it needs charging’.

"I need to charge my phone."