The electric smoke detector with an on board battery back up is excellent, but immediately the low battery began to chirp and I tested it and yes, it was low. I replaced it with a 9V Lithium from Energizer. 10.99 for the detector and 7.99 for the battery. At the end of the day, a small price for a life saving device.
https://www.amazon.com/Kidde-i12040-Wire-Battery-Backup/dp/B0000CBHOR