Smoke & CO alarms
First Alert SMCO100 battery
2 × 1.5V alkaline cell
How to replace it
| How to open | Open the battery compartment. Replace both AA alkaline cells matching polarity. Close fully and press Test/Silence. |
|---|---|
| Orientation | Match +/− in the tray |
| Low battery indicator | Low-battery chirp / indicator |
| Notes | First Alert SMCO100 specs: Battery Type AA Alkaline · Number of Batteries 2 · Battery Replaceable Yes. Replaces model SCO5. |
| Access | Tool-free |
Important
- Combination smoke + CO alarm — not a smoke-only unit. Battery answer is 2× AA alkaline per First Alert specs.
- Do not assume 9V; this model uses AA.
Sources
Helpful for this device
Optional tools for opening, testing or charging — separate from the battery answer.
- Ansmann Energy Check LCD
Quick household cell check (good / low / replace)
- ZTS Mini-MBT Multi-Battery Tester
More informative state-of-charge style checks across common formats
- Battery tester
Quick good / low / dead check for household cells
New battery but still not working?
A fresh 2 × AA should restore First Alert SMCO100 if the battery was the problem. Work through the checks below before replacing the device.
- Check orientation
Confirm the cell is seated correctly: Match +/− in the tray
- Check the battery type
This device needs 2 × AA (alkaline). Rechargeable cells are not supported for this device. Same size does not always mean the same battery.
- Check known model-specific issues
Combination smoke + CO alarm — not a smoke-only unit. Battery answer is 2× AA alkaline per First Alert specs. Do not assume 9V; this model uses AA.
- Confirm the battery itself
If you have a battery tester or multimeter, check the cell. A low open-circuit voltage often means replace; for rechargeable AA/AAA, a capacity test is more informative than voltage alone.
- Still not responding?
If a correct, fresh battery installed properly does not restore the device, the fault may be elsewhere — the device may need service or replacement.
Need to replace the device?
First Alert SMCO100 is still sold new. Only buy another if a fresh battery and the checks above did not help.