If the termination is within a covered box, you can just abandon them in place. While this is not an electrical code issue, what you intend to do is ILLEGAL. Smoke detectors must be wired to the power (though they can be battery backed up) and interconnected.

For anyone curious, here is California building code regarding smoke detectors in existing buildings.

LOCATIONS:
A. Centrally located in the hallway (if no hallway, the area adjacent to the bedroom door) giving access to the bedrooms.
B. In any room that opens into the hallway that has a ceiling level, at any location, 24 inches higher than the lowest ceiling level of the hallway.
C. On the ceiling in close proximity to the stairway.
D. On the lower level ceiling in close proximity to the stairway.
E. In the sleeping rooms
One story house . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A, B, E
Second floor with bedrooms of a two story house . . . . . . . . . . . . . . A, B, C, E
Second floor without bedroom of a two story house . . . . . . . . . . . . C
First floor with bedrooms of a two story house . . . . . . . . . . . . . . . . . A, B, E
First floor without bedrooms of a two story house . . . . . . . . . . . . . . D
Basement with no bedrooms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C
NOTE: Hallway (corridor) - A passageway giving access to rooms, usually enclosed by walls between 3 feet and 5 feet in width.
AUDIBILITY: All detectors must be audible in the bedrooms. Connecting the detectors in series is not required.
NOTE: If any area of a bedroom is more than 30 feet from a detector, an additional detector shall be installed in the bedroom.
POWER SOURCE: Smoke detectors may be solely battery operated when installed in the existing building.
All smoke detectors are to be installed in accordance with the approved manufacturerâÂÂs instructions. The above information is not intended to violate any manufactureâÂÂs specifications or instructions, but is to be used for interpretation of the Building Code.

Before you get all snooty you might understand your emboldened code section is talking about iINSTALLING detectors in a existing building, not RIPPING OUT the existing one.

By removing a hard wired SD and replacing it with a battery unit you ARE creating a violation.

My question is WHY in the world do this??? There are tons of very good smokes and CO/smokes out there to choose from. Way more in fact than battery alone.

No where does it say that old wired units must be replaced by wired units. This unit has been nonfunctional for years and contributes noting to the safely of the home. The reason I want battery power is because a. I do not trust the old wiring and b. the other alarms in the house are battery power wireless link and I want to use the same type here so they can be on the same network.

I already own the replacement unit it is mounted about a foot away from the defunct one I simply want to remove the NONFUNCTIONAL old unit and move the new one over the spot.

This post was edited by tinan on Tue, Jul 23, 13 at 11:53

If your house was built after August 1992, it MUST have hardwired smoke detectors. If it was built before then but happened to have them, you can argue that the code allows you to remove the wired smokes, but I'd think you might have a hard time getting that past the inspector.

Our house was built in 1980. I mentioned in the first post that the alarm appears to be that old so obvously my house was not built after 1992. The wiring is sketchy and we have one of those old Zepto electric panels.

Anything I do here will be adding extra safety to the home.

The home was inspected at our time of our purchase with the single hard wired dectector obviously not working, and replaced functionally by the correct number of battery operated units.

No one seems interested in answering my actual question.

This post was edited by tinan on Tue, Jul 23, 13 at 16:10

I'm no electrical expert but it seems you have a couple of choices.

If the alarm had its own breaker, you could either leave it shut off, or actually disconnect its feed wire at the breaker box.

If it's on the same circuit as lights or outlets, obviously that's not going to work. In that case, you could find where it's fed from (a nearby light fixture for example?) and disconnect the wire there.

Or, you could install a ceiling box and cap off the wires inside it. This might be a good idea in the event that a future inspection picks up the fact that there is no hard wired alarm, or if a future owner wants to install one.

I can't vouch for any of those ideas as far as meeting code. You can always call your local building permitting office and ask them what's OK to do in your situation.

Even if I were to replace it with a new wired-in alarm, it doesn't seem safe the way it is with the wires just running loose. There are no studs nearby to fix a box to, the hole in the ceiling is near the middle of an open area. Are there boxes for alarms (as opposed to heavier light fixtures) that can just be attached to drywall?

Unfortunately it is not on its own breaker - I can't even tell which breaker it is on because it's long dead so I'll have to pull the wires out and test them to determine if it is as I guess on the breaker along with the lights and outlets in the hall and upstairs bedrooms.

This post was edited by tinan on Tue, Jul 23, 13 at 17:50

Hard-wired smoke alarms, if code-compliant, are on circuits that are expected to see regular and frequent use. This helps avoid the problem of occupants turning off the breaker and leaving it off.
Get a competent electrician on site who knows how to properly mount a box at that location. Replace the detector with another powered by the house power. Redundancy in detectors is not a sin.

For anyone curious, here is California building code regarding smoke detectors in existing buildings.

I believe you are quoting the old CA Code. Smoke detectors fall under section R314 of the 2010 code and it reads differently.

Jeeze...what a commotion over nothing... Go to Home Depot, get an 'remodel' box that has the little ears that turn out to secure it, cut the hole in the ceiling to match, thread the wires in, cap them and put on a cover. Safety wise, nothing is compromised with the battery detectors in place, code wise, if for some bizarre reason an inspector gets inside your house and gigs you, you just stick another wired detector in place. Myself, I'd just pick up a new 110 detector and put it up. Probably less work than jumping though the hoops dealing with the wiring.

Not to mention hard wired smoke detectors keep electricians employed. Follow the money.

If you have an attached garage then it should be replaced with a combination CO2 smoke detector. It’s required to be brought up to code in FL if a remodel permit has been pulled.