Question Acer aspire CMOS battery mister.
Question Acer aspire CMOS battery mister.
Hi. I recently acquired a second-hand Acer Aspire 5920, which had its battery completely drained and its CMOS cell battery also worn out. I figured it out because the BIOS data showed the year 2007, which is far from the current year. So, I replaced the batteries. The twist came when I charged the new battery, changed the date and time to Windows 7, turned off the laptop, and then turned it back on to test if it retained the charge. Everything seemed fine. But...
When I checked the date and time again, they still displayed the correct values!
I must admit, I don’t have an internet connection, and the Windows time is disabled in services.
But the most surprising part was that the date and time in the BIOS also stayed accurate, even when the battery was removed.
What does this mean? Could the cell battery have taken over from the main battery somehow? Was it rechargeable? I’ve built many PCs, swapped CMOS batteries, and know that once they’re gone, data and time don’t come back. This situation is really confusing me.
Any help would be greatly appreciated.
It's clear, because nearly every laptop—including yours—relies on a rechargeable Li-Ion CMOS battery. This battery only gets used when the main power source disconnects due to over-discharge or removal, and its storage capacity is significantly less than that of a standard coin cell. Because all consumer lithium-ion batteries include protection circuits, your original CMOS battery automatically shuts off once it reaches 2.5v to prevent overcharging. Trying to charge it further could lead to a fire risk. Connecting a fully charged main battery reactivated it and allowed it to accept a new charge.
But what caught my attention most was that the date and time in the BIOS stayed accurate, even after the battery was removed.
If you had replaced the CMOS battery quickly, it's likely the leftover power kept the system running.
Perhaps you meant to refer to the BIOS creation date instead of a timestamp? Did you notice the clock was set to 12:00 when you first turned on the laptop after buying it? If not, then the CMOS battery was probably in good condition from the start.
Hi. The laptop's main battery is dead, and the CMOS cell battery is very hard to reach so I couldn't access it.
😉
Also, the data and time shown are for the laptop itself—it lists 00:00 hrs on January 1st, 2007 in the task bar. The build details say 2007, which matches what I checked in the BIOS when I first turned it on and verified the CMOS battery status.
It's clear, because nearly every laptop—including yours—relies on a rechargeable Li-Ion CMOS battery. This battery only gets used when the main power source disconnects due to over-discharge or removal, and its capacity is significantly less than that of a standard coin cell. Because all li-ion batteries in consumer products have built-in protection circuits, your OEM CMOS battery simply stops working once it reaches 2.5v to prevent overdischarge. Trying to charge it further could lead to a fire. Putting a charged main battery on it reactivated the process and allowed it to accept a charge.