In my opinion, a PLB (Personal Locator Beacon) is a device that does one thing very well; it sends a signal to a satellite constellation that tells an established SAR system where you are. Your PLB being active at all is your 9-1-1. Because it cannot do anything else, its battery should always be at or near full charge.

Simply because of normal usage, a satellite phone's battery may be at a low state just when you need it most.

my $.02