Smoke & CO alarms
Google Nest Nest Protect (Wired) 2nd gen battery
3 × 1.5V lithium primary cell
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 power off at the breaker. Remove Protect from the backplate. Open the battery door / cover as Google describes for wired models. Replace all three backup AA cells with Energizer Ultimate Lithium AA (L91). Remount, restore power, and test. |
|---|---|
| Orientation | Match +/− markings |
| Low battery indicator | App / device low-battery warnings |
| Notes | Google specs: wired Nest Protect has three long-life lithium AA (L91) backup batteries — not six. |
| Access | Tool-free |
Important
- Hardwired 120V with three lithium AA backups — different quantity from the battery-only Nest Protect (six).
- Use Energizer Ultimate Lithium AA (L91) as Google specifies.
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 3 × AA should restore Google Nest Nest Protect (Wired) 2nd gen if the battery was the problem. Work through the checks below before replacing the device.
- Check orientation
Confirm the cell is seated correctly: Match +/− markings
- Check the battery type
This device needs 3 × AA (lithium_primary). Rechargeable cells are not supported for this device. Same size does not always mean the same battery.
- Check known model-specific issues
Hardwired 120V with three lithium AA backups — different quantity from the battery-only Nest Protect (six). Use Energizer Ultimate Lithium AA (L91) as Google specifies.
- 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?
Google Nest Nest Protect (Wired) 2nd gen is still sold new. Only buy another if a fresh battery and the checks above did not help.