Will my calculator run out of battery?

I have a casio fx-991EX which have been in use for 5, 6 years, and never had battery replaced during this time. In the manual it recommends changing battery 2 to 3 years.
The display is fine and the speed is running fast enough, but would it be in danger of suddenly running out of battery and just shut down? I really wouldn't want this to happen in the middle of an A Level exam!

I know there's a solar panel -- in the manual it says it works when the sun icon at top right appears, but mine never appear even in direct lamplight. Does it mean my calculator has a useless solar panel?