DuPont Self-Charging Smoke Alarm (PS-131) for $5.99 shipped. Here is a screenshot for your reference. It contains a 10-year self-charging power pack which automatically re-charges from the household current when the light is on, and remains fully charged if the light is on for about 2 hours per week, and will operate for 30 days from a full power pack. It uses an ionization type sensor to detect the presence of smoke. It usually goes for about $10 or more.