I have a new smoke alarm, base is installed in ceiling, until will not lock into the base.

If you don't mind changing batteries every 6-12 months during the year then you can pick up these smoke alarms from any hardware or even discount store. The also come in blister packs (2 to 6 or more for value packs) Most will screw mount to attach to wall. You can also use self stick foam tape to 'stick it' anywhere. Try to put them in a high spot on wall or ceiling depending on the surface (crumbly textured surface won't stick well. It will have to be mounted with a screw) The higher locations will give you quicker response time but It's harder to get at without a step ladder. Lower units have slower response time as the smoke has to collect and lower to the point where the alarm is.
*** Note; you will need to check your fire code requirements for placement of alarms. (though once this minimal requirement is met there is no reason you can't add more in other locations for added coverage; like a large walk in closet, baby's room, laundry room, etc.)
* Getting back to your question, I'm guessing you have the updated required powered alarms or need to have a "Powered" fire alarm. The newer homes (well last 20 years is new to me) have low voltage power lines running from room to room where the alarms are placed. If you required to install this type of system then depending on the ceiling access (crawl space, lowered ceiling, etc.) and the need to run the wires "hidden" behind wall and ceiling will increase the installation time and labor costs. Electrician or authorized fire alarm technician will be needed. They should check your fire alarm requirements for you local, indicate locations for alarms, run wires, install and test. Check with several companies in your area and get several quotes. Check with Better Business Bureau for rating or complaints.
* With Smart ware being all the rage. It may be possible to use these "smart devices" to alert you to other situations besides just fire hazards. Break in, baby monitor, door ringer monitor, water (flooding), Carbon Monoxide, etc. But the alarm guy should be able to give you more options. Though I's still up to you how much you want to spend.
* Final; You check what the fire codes require. If they called you first then suspect a scam or a sales pitch to get you to buy something you don't require. You can always go with the 'stick on' alarms if that is allowed for your area. Call the fire Dept in your area. Those are the 'good guys' that want to keep you safe.
Aloha, ukeboy57

usually for wall mounting (if allowed by building codes), the top edge of the smoke alarm should be placed between 4 inches (102 mm) and 12 inches (305 mm) from the wall/ceiling line, or corners... below typical "dead air" spaces.

How old is it? It may be telling you that it's no longer reliable and must be replaced.

This looks like a detector for a monitored alarm system. If it is, make sure you notify the alarm company before removing. No one wants to see the fire dept when replacing a battery. Turn the unit about 1/8 of a turn counterclockwise to remove it from the base (it will be trickier to reinstall than it is to remove, just so you know). Underneath you will probably find a door covering the battery compartment, which is easily opened. Remove the batteries, then place a temporary short (about 3 seconds) across the empty sockets to reset the low battery detection circuitry before installing the new batteries. There is probably an arrow on the underside of the base with which you align a debossed scribe line on the outer edge of the detector when reassembling. If it sounds too complicated, you might want to call a professional service tech. Hope this helps.

Human hearing is not really all that great, especially directionally. Look for the sound to be coming from some other place and bouncing. Check for plug-in CO detectors as they are often the culprit. Try using tape to muffle the sound ports until you find the correct device. Good luck!

The manual here, https://www.detectorsandalarms.com/images/pdf/kidde_0916k_manual.pdf describes replacing the battery.
Basically you, 1) Remove the locking pin if it's installed, with some long nosed pliers. 2) Twist the alarm free in the direction of the off arrow. 3) Install a new battery. 4) Twist the alarm back in place, in the direction of the on arrow. 5) Insert a locking pin 6) Test the alarm.

If this is a new site that has to be OK'd by the building inspector then he is a small minor deity and you must do everything you can to not tick him off or he'll run you thru the wringer, so put it where he says.
If it's a friend who is a building inspector then put 'em where you want them.
Mine are mounted on the ceiling.
Their site says 12".


http://homeappliance.manualsonline.com/manuals/mfg/kidde/smoke_alarm_model_i12060.html?p=3

to program any new sensors first you must access the installer menu by opening the front cover and using the red buttons typing in the installers code,(4321) is the default code. once that is done press the add button, then the sensor/remote button, then trip sensor(keyfob by pressing both the lock and unlock buttons together until the system beeps) or (removing the smoke detector from the base until it beeps). Once the sensor has been learned it will say keychain remote, press the sensor/remote button repeatedly until it says the correct location, then press done. then enter group code 01 for the keyfob and 26 for the smoke detector. then done.

You will need to remove the unit from the ceiling to reveal the battery compartment on the back. It should just need to be rotated counter clockwise a small amount and will then come loose from the bracket. When installing a new battey be sure to press the battery reminder finger down into the battery compartment or the battery door will not close. Call the Kidde (pronounced kid-a) hotline if you have problems 1.800.880.6788