What does it usually mean when your carbon monoxide detector chirps twice every 30 seconds or so, even after you have changed the batteries?

It usually means your alarm was manufactured since they enacted standards that require it to permanently "fail" when the sensor has reached its expected useful lifetime (e.g., 6 to 8 years). You throw it away and get a new one, making sure it has a date on it.

Probably needs new batteries.

It usually means the CO detector has exceeded its lifespan and should be replaced.

You bought Energizer batteries. I've had a lot of trouble with lithium types going bad after a short period of time.

They wear out over time. You need to replace it. Check with your local fire department as to best placement for the new one. This old one may not be in the ideal location and the fire department people are very helpful. The instructions that come with it are not always right as different areas require different placement. We found this out when ours needed to be replaced.

Replace the detector.

An alarm isn't a little chirp every few seconds. The alarm is an earsplitting screech and you will NOT be here asking this question in the case of an actual alarm. Your detector is due for replacement.

NOTE: if your CO detector needs replacing, strongly consider replacing your smoke detectors as well.

NOTE also: if you don't have any gas burning equipment in your home: stove, furnace, dryer or water heater - you don't actually need a CO detector. Any of those things being present means having one is a good idea but if your whole home is all-electric, skip it. Remove it and toss it.

take it down and blow into the detector ... might be some airborne grit stuck in there

Maybe you fitted the battery incorrectly? The battery is faulty. Or the detector is shot.

Either you fitted defective batteries or the detector is on its way out.

you need a new one the batteries are not working properly