No generic recommendation
Search phrases such as “smoke detector battery” or “smoke alarm battery” do not mean every unit takes a 9V. Sealed 10-year packs, replaceable 9V or AA trays, hardwired alarms with backup cells, and listening accessories are different systems.
If you cannot identify your exact model and the manufacturer-specified battery, do not buy from a generic ad — read the label or manual.
Architecture classes (identify yours first)
Replaceable 9V battery-only — examples on this site include First Alert SMI100 and Kidde i9050 (1 × 9V alkaline where the maker says so).
Replaceable AA — examples include First Alert SMCO100 (2 × AA) and Kidde 20SDR (2 × AA). Not 9V.
Hardwired with backup — examples include Kidde i12060A and several Firex units (mains plus a labelled 9V backup).
Sealed long-life — example Kidde i1040: sealed lithium for the life of the alarm; replace the whole unit, not a tray cell.
Lithium AA only — Nest Protect 2nd gen battery and wired models specify Energizer Ultimate Lithium AA (L91) counts; alkaline or NiMH is not a substitute.
What must be known before any cell answer
Model or generation identity; whether power is sealed, replaceable, or hardwired+backup; the exact type/chemistry the maker names; quantity; and any replacement restrictions (including end-of-life whole-unit replace).
Life-safety rules
Replace batteries only as the manufacturer instructs. Test after every authorised change.
Do not leave an alarm without power to stop chirping. Replace the whole unit when it reaches end-of-life.
Accessories are not the alarm
Devices such as a Ring Smoke/CO Listener use their own cell (CR123A on the listener). That does not tell you the alarm’s battery — identify the detector separately.