Smoke & CO alarms
Firex i4618 / i4618AC battery
1 × 9V alkaline
This device uses more than one power source. Only the user-replaceable cell(s) listed below can be changed.
How to replace it
| How to open | Turn off power at the breaker. Remove the alarm from the plate, open the battery compartment, replace the 9V backup matching polarity, remount, restore power, and test. |
|---|---|
| Orientation | Match +/− |
| Low battery indicator | Backup low-battery chirp |
| Notes | Kidde Firex product page: 120V with 9V battery backup. Discontinued — Kidde recommends moving to a current Kidde hardwired model when replacing the unit. |
| Access | Tool-free |
Important
- Firex brand is part of Kidde (acquired 2009). This page is for the battery backup while the unit is still in service.
- Hardwired + 9V backup — not a battery-only alarm.
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 Firex i4618 / i4618AC if the battery was the problem. Work through the checks below before replacing the device.
- Check orientation
Confirm the cell is seated correctly: Match +/−
- 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
Firex brand is part of Kidde (acquired 2009). This page is for the battery backup while the unit is still in service. Hardwired + 9V backup — not a battery-only alarm.
- 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.