I read this:
The battery's flat on the mobile.
I think we should say in the mobile not on the mobile.
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
If I must choose, I'd say on, but a case could be made for in as well, as the battery is inside the mobile phone.
This is going to be a non-answer, but I would honestly just say this instead:
The mobile's battery's flat.
Avoids the problem and sounds far more natural.