Low battery warning

That is correct, sleep mode does not utilize / see the CMOS battery upon waking up. However when it's booting from off mode it does check the CMOS battery. I would recommend getting it changed, however it you don't the worst thing that will probably happen would be that you will have to reset the clock any time you completely remove power from the computer.

That is because while a computer is in sleep mode, it is still using power to keep the memory. The battery is only used when the computer is turned off. You should replace the battery as soon as you can in order to keep your computer running properly.

Please replace the battery when you have a chance. This will not only help you to remove that warning... but also to keep your system date current.