Smoke Detectors are a pretty important part of our lives, and a lot of people forget that you need to change their batteries to make sure they are working 24/7, 365.
The most commonly used batteries in smoke detectors are lead and alkaline AA and 9V batteries. These are made by both Duracell and Energizer.
These days, there are lithium smoke detector batteries, which can last for up to a decade, so you don't have to change it yearly.
Rechargeable NiMH batteries are not recommended for smoke alarms as they lose their voltage faster than non-rechargeable alkaline batteries.
Lead and alkaline batteries need to be changed yearly. In Australia, the beginning of daylight savings (that's in October for you Queenslanders) is the time you are supposed to replace the battery. There is no need to change it twice a year as that is a waste of batteries, they're designed to last for the year they're in use.
If you're using lithium batteries you only need to change it every 10 years. However, if this is the case, the battery is probably built into the smoke detector, so you'll have to change the whole thing.
Smoke detectors are also designed to let you know when the battery is dying. They will often beep quietly (But loud enough to annoy you) until the battery is changed.