Why does my GE Model ESL 511C smoke detector (two wire) red light blink about every 30 seconds?

copied from the manual, available at the kidde website

RED AND GREEN LED INDICATORS This smoke alarm features separate red and green LED indicators. The LEDs indicate the following: GREEN LED CONSTANT ON - AC power is present. OFF - AC power is not present. RED LED - Can be seen through the Push-to-Test button. Blinks once a minute - Indicating presence of a working battery. Blinks once a second - Smoke alarm senses smoke and simultaneously sounds an audible alarm. Blinks once every 10 seconds - Smoke alarm is quieting an unwanted alarm (Model ADC only). (Interconnected system only): OFF - Another smoke alarm in the network has sensed smoke and is signalling this alarm.

Yes in normal operation the red light should flash intermittently every 30-40 seconds.this lets you know the battery has sufficient power for detecting smoke.when the battery needs replacing the unit should beep/ chirp in singular sound every 60-90.if smoke I'd detected the difference is very obvious as the shrill beep is continuous and much louder.

Before I can really try to answer, I would like to know what make/model control panel you have (ex: Honeywell Vista20P), the zone number of the affected smokes, and whether or not you get an indication of a problem on your keypad.

http://www.smarthome.com/manuals/7309.pdf

Not sure what you mean; can you be more specific about the problem?

Change the 9V backup battery,with a new one.Push and hold the reset button for a minute or two for the alarm to to go off like about 30-40 sec. for the smoke detector to dectected the 9V backup battery power sourced.The green power LED light indicator will stop flashing also the beeping sounds too.

Grab the detector by the top and twist counter clockwise, then puu away from base. There will be a 9 volt battery you can pull out and change. If it is wired for electric, make sure wires are connected before installation. Line up slots with holes where it came off from and twist clockwise to re-attach.

Yes it is. See link. (pdf)

http://www.sdilink.com/pdf/ESL/CompatIndex_Oct04.pdf

you may need to clean the chamber.

If detector is more then 5 years old it will beep every 30 seconde to wirn you the sensor in the detector needs to be replaced. bying a new detector would be better.