ELK 6021 Questions

I currently have my windows hard wired with contacts in the frame. I am replacing my windows, and the company says drilling into the frames voids the warranty. Im thinking of going with the wireless sensors such as the Elk 6020 or 6021. Does anyone have any experirnce with these sensors?
 
In particular, do they work well?
Average battery life. One says 2 years and the other says 7 years?
 
What if the sensor looses its battery power when the system is armed, does it trip the system, or just alert you that its dead?
 
Thanks
Brian

Personally, I wouldn't go with Elk's wireless, but that's me as a pro (and being negative according to some).
 
The system reports a low battery condition long before the battery is stone dead to be rectified. If it is not, the device will cause supervisory errors and should also appear as faulted to the system before it is even armed.
 
If the system is armed and the device "goes missing" or faults, it will set off the alarm as it's viewed as a tampering condition.

Generally a battery like a CR123 will last 4-7 years easily. Coin cells are typically less, but if it's a point that is not opened/closed often, battery life is going to be longer.

I've installed approximately 60 of the ELK-6020 sensors. Out of those, I had an issue with (2) of them; one had an invalid TXID (serial number) the other arrived with a dead battery. Elk took care of both issues. No other issues to report.

Thanks
SO my next issue is, does anyone have examples of wiring up a double hung window? The manufacturer says I cant drill the frame, or it will void warranty. How do I span both sashes? My current windows are single hung. 
 
Thanks
Brian

Can you provide a picture of the window?
 
If double hung window, wireless sensor should be placed on one sash and magnet on other.