I have a BRK carbon monoxide detector. It is flashing red and chirping to indicate the need for a new battery. I have no manual but am experienced in installing new batteries in many other smoke and carbon detectors. What is the trick to getting the 9V battery installed right? It just won't snap in and the device is designed so you're working blind - you can't really see what you are trying to do. Detector came with house, which is 3 years old.