What the cell does
The small coin on a motherboard (often called the CMOS, BIOS, or RTC battery) keeps the real-time clock and some firmware settings when the machine is unplugged.
When it is flat you may see wrong dates, lost BIOS settings, or rare boot quirks after power loss — not a blank screen by itself in most cases.
Usually CR2032 — not always
Many desktop boards and some laptops use a CR2032 (about 20 × 3.2 mm, 3 V lithium).
Thinner CR2025 / CR2016 cells are the same diameter family but not drop-in swaps. A thicker CR2050 needs a matching tray or spacer — see CR2032 vs CR2025 vs CR2016 and CR2032 vs CR2050.
Read the marking on the empty cell or the board silk-screen. Laptop service manuals sometimes name a different coin or a wired pouch — follow that over a forum guess.
Safe swap habits
Power down, unplug, and discharge residual power if the maker says to. Note BIOS settings you care about before removal — some boards forget them when the cell is out.
Match polarity (+ usually faces up on a holder). Do not use a rechargeable LIR2032 unless the board documentation allows it — voltage and charging differ. See rechargeable vs disposable.
Keep coin cells away from children. Recycle the spent lithium coin — see dispose and recycle.
Quick takeaway
Confirm CR2032 (or the exact code on the tray) before you buy. Same diameter is not the same thickness. Replace with a matching disposable lithium coin, then check the clock and BIOS once.