I had the pleasure of being woken up at 0130 this morning to the sound of one of my smoke alarms chirping away every 30 seconds to let me know the 9v backup battery was low. As you can imagine I was impressed having to fetch a ladder to remove the damn thing at that time of the morning.

The alarm (and supplied battery) were only installed about 6 months ago but the expiry date on the battery was March 2016 so it had probably been sitting in storage for quite some time before being installed.
Getting to the point, I'm about to buy some more. There are several types/brands available with different chemistries including Lithium Manganese Dioxide, Zinc Carbon, Zinc Chloride and Alkaline. Which should I be looking at for this application?
Each alarm is normally powered by mains power and the battery is only used during a power failure. Ideally it should be very low self-draining so that it lasts as long as possible. Also, I'm not sure what the "low battery" alarm trigger voltage is on these smoke alarms?
Any suggestions? Spare any Batteriser jokes please ;-)