Mám vybitou baterii.
When to use
This is useful when your phone has run out of battery and you need to explain the situation, perhaps to ask for a charger, a place to plug in, or assistance.
How it works
The sentence translates to 'I have a dead battery.' 'Mám' - I have 'vybitou' - discharged, dead (accusative feminine singular, agreeing with 'baterii') 'baterii' - battery (accusative singular).
Grammar
The verb 'mít' (to have) is used here. 'Baterie' is a feminine noun. The adjective 'vybitý' (discharged) must agree in gender, number, and case with 'baterie', hence 'vybitou'.
Notes
Explain why you need help or charging because your phone has no power.