Smoke & CO alarms
First Alert SMI100 battery
1 × 9V alkaline
How to replace it
| How to open | Open the front-access battery compartment. Replace with one 9V alkaline matching polarity. Close compartment fully and press Test/Silence. |
|---|---|
| Orientation | Match +/− contacts in the drawer |
| Low battery indicator | Low-battery chirp (about once per minute on many First Alert models) |
| Notes | First Alert product specs: Battery Type 9V Alkaline · Number of Batteries 1 · Battery Replaceable Yes. Do not use rechargeable cells. |
| Access | Tool-free |
Important
- Life-safety device — use only 9V alkaline as specified on the product page; test after replacement.
- Rechargeable 9V packs are not approved for this alarm family per First Alert support guidance.
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 1 × 9V should restore First Alert SMI100 if the battery was the problem. Work through the checks below before replacing the device.
- Check orientation
Confirm the cell is seated correctly: Match +/− contacts in the drawer
- Check the battery type
This device needs 1 × 9V (alkaline). Rechargeable cells are not supported for this device. Same size does not always mean the same battery.
- Check known model-specific issues
Life-safety device — use only 9V alkaline as specified on the product page; test after replacement. Rechargeable 9V packs are not approved for this alarm family per First Alert support guidance.
- 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 SMI100 is still sold new. Only buy another if a fresh battery and the checks above did not help.