Identify your unit first
Smoke & CO detector batteries
Alarms are not all the same. Identify your exact model below — sealed 10-year packs, 9V or AA replaceable units, hardwired alarms with backup, and accessories each have different answers. If you cannot identify the unit, do not guess.
Important
Smoke and CO alarms are life-safety devices. Match your exact unit below — or use the label on the alarm — before buying anything. We only recommend a battery when the maker’s specified type, quantity, and any replacement limits are known. Wrong chemistry or a disabled alarm can be dangerous. Never leave an alarm without power to stop chirping; test after any authorised change; replace the whole unit at the maker’s end-of-life date. This site is not fire-safety advice.
What kind of unit do you have?
| Generation / model | Battery | Replaceable? | Notes |
|---|---|---|---|
| First Alert SMI100 (battery smoke) | 1 × 9V alkaline | Yes | First Alert product specs: one replaceable 9V alkaline. Not a sealed 10-year unit. |
| First Alert SMCO100 (smoke + CO) | 2 × AA alkaline | Yes | Combination alarm — AA, not 9V. Replaces older SCO5-class units. |
| Kidde i9050 (battery smoke, 9V) | 1 × 9V alkaline | Yes | Classic 9V battery-only Kidde. Successor retail line is 20SDR (2× AA) — confirm the printed model. |
| Kidde 20SDR (AA smoke) | 2 × AA alkaline | Yes | Current DETECT AA smoke alarm — not 9V. |
| Kidde i12060A (hardwired + 9V backup) | 1 × 9V backup | Yes | Mains-powered with replaceable 9V backup — not a battery-only design. |
| Kidde i1040 (10-year sealed) | Sealed — replace whole unit | No | Sealed lithium pack for the life of the alarm. Do not buy a 9V or AA for this model. |
| Nest Protect 2nd gen (battery) | 6 × AA lithium (L91) | Yes | Google specifies Energizer Ultimate Lithium AA (L91) only — not alkaline or rechargeable. Replace all six. |
| Nest Protect 2nd gen (wired) | 3 × AA lithium backup (L91) | Yes | 120V wired with three L91 lithium AA backups — different quantity from the battery-only Nest Protect. |
| Firex i4618 / i4618AC (hardwired + 9V) | 1 × 9V backup | Yes | Kidde-era Firex — discontinued. 120V with 9V backup while still in service. |
| Firex i5000 (hardwired + 9V alkaline) | 1 × 9V alkaline backup | Yes | Kidde specifies alkaline backup. Unit replacement path pointed at 20SAR. |
| Firex PAD / AD / ADC (classic hardwired) | 1 × 9V backup | Yes | Pre-2009 Invensys Firex family — OEM guide lists approved 9V cells. Kidde acquired Firex in 2009. |
| Ring Smoke/CO Listener (accessory) | 1 × CR123A (listener only) | Yes | Listens to alarms — not the detector itself. CR123A does not apply to the alarm. |
Matching devices
Still not working?
If you cannot identify the alarm model and labelled cell, stop — do not buy a generic pack from search ads. Use the manufacturer label or manual.