Is it necessary to remove the CMOS battery if it is going to be stored for long periods? Also, will the BIOS be effected if the battery has been removed for long periods or will it still be okay?
the BIOS will be fine, that information is programmed into the chip. the setting the user sets up however will be cleared and will be needed to be re-entered when the board is again powered up
Is it suggested to leave them in or to remove them when storing for long periods? Does it harm it at all or is it just more convenient to leave them in? Does leaving them in drain the battery after many years?
How long?
Probably a non issue.
The drain is minimal.
If the battery should fail, they are cheap to replace.
The default bios settings will be available.
Probably more convenient to keep it in the motherboard so you don't lose the battery.
personal preference. I prefer to remove it, batteries CAN leak, removing the battery eliminates that possibility. that is the only reason I can think of to remove it.