Batteries die before low battery level entity triggered

[tetsuo13]

Low battery notification added in #20 doesn't appear to be reliable -- in fact I've never received a notification even though several batteries have since been replaced. It relies on using the "_low_battery_level" entity provided by the Z-Wave devices however it appears that the battery dies before the device identifies it as low. For example, the front door sensor died at 62% battery level.

Use a different approach to identifying low batteries. Something that would allow setting a different threshold per device would be nice.

A list of devices that have experienced low battery levels that never triggered their respective "low battery level" entities:

Device	Min Battery Level	Notes
Ecolink Door & Window Sensor	62%	--
First Alert Smoke & CO Alarm	78%	Will stay at 78% for several days. The device will start chirping of low battery as soon as it drops to 77% (typically early morning hours when everyone's asleep), even though the low battery level sensor isn't triggered.

[tetsuo13]

Merged the change for the smoke alarms since I was able to monitor their levels when they depleted enough to start chirping. Haven't been able to notice it with the Ecolink sensors yet. Will create additional PRs as I come across other devices.