Should I say
Some people use the word "flat" to describe their phone when its battery is dead.
or
Some people use the word "flat" to describe their phones when their batteries are dead. (I wanted to use "their" here to refer to the phone's instead of the people's)
or
Some people use the word "flat" to describe their phones when their battery is dead. (I wanted to use "their" here to refer to the phone's instead of the people's)
Which one is grammatically correct and why?