I have a 2412 also theres 2 yellow control boxes in the cabinet. Each one has its own back up battery .
The larger yellow control box under a plastic cover holds the 6 volt battery it backs up the absolute encoders so the machine knows its position when its off.
It lasts about 2 years.
On the small yellow control box is a small 3 volt battery it supplys power to the memory that hold the machines paramerters when the machine is off. This battery last from 6- 10 years .
When the batteries get low the machine will give you a alarm every time you power up the machine . When you first get this alarm you have about 2 weeks of battery life left . I keep a spare battery in my toolbox.
Most important aways change batterys with the power on.