Laptop screen dark when booted up on battery

Hi , My M1530 lap top boots up ok when mains adapter is plugged in , however if i boot it up on battery power the screen is dark , the image is there but it appears that the back light is not coming on.

I'm guessing it has some thing to do with the power saving options , but i can't see how to change, any help gratefully received,

Regards Rob.