Where do I find the expiration date for the PLB battery?

The date is located on the back of the PLB, on a label, underneath the holster or bracket. Above the label it says “Replace after use or by the date shown”.

Why does the battery need to be replaced after 5 years?

Things happen to beacons over five or six year. Every drop, every time it is left in the sun, every time it is exposed to extreme temperatures etc. induces some wear and tear. The wear is cumulative. A check-up every five - six years is just a good idea when you are dealing with life saving equipment. It's not something you want to carry until it fails...unless you know when and where your next emergency will occur!

Additionally, the regulatory authorities that approve 406MHz devices require that users replace batteries at ½ the useful life of the battery.