A CMOS (often called RTC) battery provides a small amount of power so your device can retain basic firmware information when it’s turned off.

• Keeps time and date accurate when the system is unplugged.
• Maintains essential firmware preferences like boot order and time format. Some newer systems store settings in non-volatile memory, but the battery still powers the real‑time clock.
• Helps avoid setup prompts after a full power loss.

If the clock resets or you’re prompted to enter setup after outages, replacing the CMOS/RTC battery can help. You’ll find individual coin cells and pre-wired RTC batteries in various form factors on this page. Results may vary by model, so checking your device’s manual or part number is a good idea.

Most PCs can start without a CMOS battery, but you may see quirks after full power loss (unplugging or removing the main battery).

• Clock resets to a default date/time until the operating system syncs online.
• Firmware settings revert to defaults, which can change boot order, disable RAID profiles, or reset memory profiles.
• Startup prompts like “press a key to continue” may appear until you re-save settings.

If AC power stays connected, you might not notice issues until the system is completely disconnected. Replacing the RTC/CMOS battery can reduce these interruptions, but behavior varies by motherboard and device design. After replacement, you’ll usually need to re-enter date/time and any custom settings before saving and exiting firmware.

Not always. While many desktop motherboards use a 3V CR2032 coin cell, plenty of laptops, mini PCs, and specialty equipment use different formats.

• Coin cells with leads: Pre-wired 2‑pin or 3‑pin assemblies wrapped in heat-shrink.
• Alternate sizes: Some devices use CR2025, CR1220, or similar.
• Rechargeable types: Certain systems use rechargeable coin cells or small 3.6V packs.

Match voltage, connector type, and size to your original part. Avoid swapping a rechargeable cell with a non-rechargeable one (and vice versa). Checking the old battery’s part number or your device’s documentation can help you pick the right RTC/CMOS replacement from the options available on this page.

There are a few common signs that point to a weak RTC/CMOS battery.

• Time and date keep resetting after the device is unplugged.
• Firmware/BIOS settings don’t stick, like boot order or drive mode.
• Startup warnings such as checksum errors or prompts to enter setup.
• Intermittent boot behavior that improves after re-saving settings.

These symptoms can have other causes, but replacing the CMOS/RTC battery is a simple, low-cost step that often helps. After installing a new battery, load defaults, set the date/time, and save changes. If problems persist, consider checking storage devices, cables, or consulting your device support resources for further diagnostics.

Replacing a CMOS/RTC battery is usually straightforward, but take basic precautions.

1. Power down and unplug the device. Remove the main battery if it’s a laptop with a removable pack.
2. Discharge by pressing the power button for a few seconds.
3. Ground yourself to reduce static (touch a metal surface).
4. Open the case carefully and locate the coin cell or wired pack.
5. Note orientation and connector, then gently remove the old battery.
6. Install the replacement with the same voltage, size, and connector.
7. Reassemble and boot, then set the date/time and save settings.

If your device is sealed, uses a soldered battery, or is under warranty, consulting a service manual or a professional may be advisable. Dispose of used coin cells responsibly.