What’s the signalling voltage on hardwire smoke detectors? Mix and match

No Demand

The demand for interconnected is associated with new homes in many jurisdictions. I don't know of any places (there may be exceptions, probably California if anywhere...) where they are required to be interconnected unless a home is new or, possibly, substantially renovated. When a home is new or is substantially renovated, it is easy to run wires between the detectors and to add wires from the first detector back to a power source.

So the need for "battery" is made obsolete by the need for "interconnected". You don't need battery if they are interconnected, you don't need interconnected if you are in a situation where you don't want to run wires back to a power source (because if you don't want to connect to power, you probably don't really want to run wires for interconnecting them either).

End result:

• Battery power for individual detectors in older homes. (Unrelated, but now typically "10-year batteries" instead of "replace battery when you hear the chirp").
• AC-powered for interconnected detectors in newer homes.

I don't think dual-sensor has much to do with it, though due to power requirements (and possibly a higher price-point) that may be more common with AC-powered, interconnected detectors.

I didn't catch the "wireless" part originally. For a bunch of reasons, I would prefer wired to wireless for smoke detectors. Wireless at many levels (from Bluetooth to WiFi to cell phones...) has enough problems that for life-safety I'd prefer wired, when practical. As long as this is new construction, wired seems quite practical, and new construction is the primary demand source for interconnected.

Smoke detectors are exempt from the sealed-battery requirement in Maryland law

Since smoke detectors are either powered by the central alarm panel's battery during an AC power failure or monitored for power failure and/or battery issues by the alarm panel in a wireless system, there's no reason for them to have a sealed battery. As a result, given the definitions in Maryland PSC 9-101(e) and (f):

(e) “Smoke alarm” means a single or multiple station device that detects visible or invisible products of combustion and includes a built–in internal alarm signal.

(f) “Smoke detector” means a system–connected smoke sensing device tied to a fire alarm control panel or a household fire warning panel.

, and that the sealed-battery requirement in Maryland law expressly exempts both system smoke detectors and centrally monitored wireless systems, anyway:

(a) This section does not apply to:

(1) a fire alarm, a smoke detector, a smoke alarm, or an ancillary component that is:

(i) electronically connected as a part of a listed centrally monitored or supervised alarm system; or

(ii) capable of sending and receiving notifications by:

1. a low–power radio frequency wireless communication signal; or

2. a wireless local area networking capability; or

(2) any other device that the State Fire Marshal designates as exempt through the regulatory process.

(b) On or after October 1, 2018, a person may not sell a battery operated smoke alarm in the State for compliance with this subtitle unless the smoke alarm is a sealed, tamper resistant unit incorporating a silence/hush button and using one or more long–life batteries.

, go ahead and install the wireless detectors you are planning on installing.