PS-131

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.