5 Years of battery life is just an estimate for PowerPro and CS02. 3 years for the CS01 and SYS1 command stations. Your milage may vary. Actual life time of the battery will vary with actual system usage. Battery power is only consumed when the system is off. If you left your command station on forever, the battery will only fade due to self consumption....in other words the shelf life of the battery which is 10 years. So one persons experience in battery life will be different than another persion simply because they used run the layout more or less than the other.
What happens when the battery dies?
What happens will depend on the state of the battery.
A weak battery causes increasing unreliable or flakey operation of the system that gets worse and worse the longer you wait to replace it. The signs are corrupted consist, wrong locomotives showing up on the Procab LCD, System Settings getting set to other values and corrupted macros.
A dead battery will always cause the system to boot slower and nothing is remembered from the last operating session. No engines in the recall list nor shown on the display. No macros or consist remembered. All system setting changes you made go back to the default values.