How long the computer remembers the current date when power is disconnected

[Pawelk198604]

As kid i i was always surprised why the computer can remember current date and time despite being unplugged from power.

TV or video recorder always need to be reprogrammed after being discontented or blackout.

Now i know that computer BIOS has it's own battery, i just wonder how long such battery can last?

[Nuke]

i have never owned a computer where the rtc battery had died. i have a 11 year old computer and its cell still works.

[ZedNova]

Years and years most of the time. My dad's PC is approaching 13 years old and the Battery hasn't ever needed to be replaced.

Your PC will most likely fail or be horribly outdated before the CMOS battery dies. There are exceptions though.

[magnemoe]

It lasts for years, an motherboard tend to have an battery like you have in watches, think how long they last in the watch, in the computer they don't have to show anything just run the clock ticks and only then powered off.

The only cases I have run into flat batteries is probably because people has shorten out the battery by laying the motherboard on metal like the computer case door while assemble it or similar and this has drained it.

I think the first generations of vcr did not have battery, but later had, the old tv sets used dials to select frequency of channels and did not need batteries, not owned any electronic ones without.

Note that TV sets last longer than computers so if you have an 15 year old tv the battery might have died.

[Nuke]

in addition to keeping the rtc ticking, batteries on older mobos were used to preserve bios settings on a ram chip. these were later replaced with flash and this has made the batteries last much longer. the rtc was also later moved off of its own chip and onto the south bridge. the smaller process had also cut power consumption. frankly im surprised we still use batteries, a supercap could do the same job and could be recharged while the computer is on. i guess its a matter of if it aint broke dont fix it.

[Stargate525]

I've had it happen once, after three years. The battery had to be replaced, but soon after the motherboard went too. I'd be more likely to blame the motherboard than the battery. And considering that the CMOS battery isn't drawn from whenever the computer is on...

[andrewas]

An ATX power supply has a rail, +5VSB, which is active even when the computer is off. This is used to provide the voltage needed to turn the PSU on when you hit the power button, to support wake on LAN, and to keep the BIOS memory charged. You only draw from the battery when the computer is unplugged or turned off at the wall.

[Nuke]

bios memory was replaced by flash long ago.

[magnemoe]

bios memory was replaced by flash long ago.

Still the battery has to keep the real time clock running. Just checked and even small and cheap motherboards has a clock battery. Part of the reason is that its a cheap solution.

[cantab]

Removing the battery cleared the CMOS data on my motherboard. Though that is from like 2009, maybe things have changed in the last few years. Or maybe there's code that says "if the battery is pulled, wipe the CMOS because that's what people expect".

As for why the non-rechargeable batteries are still used, and not say rechargeables or supercaps, bear in mind it's not unreasonable for a PC to be out of use for several months on end, then put back into use. It's nice to not have to worry about needing to set up the BIOS/UEFI again.

[Kruleworld]

My last job i had to replace a few cmos batteries in the 5yo dell desktops. they'd loose their bios settings when unplugged

My dad's PC is approaching 13 years old and the Battery hasn't ever needed to be replaced.

yeah, but was it running on the battery for all that time? or was it plugged in/turned on?

[magnemoe]

My last job i had to replace a few cmos batteries in the 5yo dell desktops. they'd loose their bios settings when unplugged

5 year is unusual short, I suspect it was because of the quality of the batteries, old cheap Dell desktops tend to use the cheapest parts possible and doing stuff like not having pci-e 16 slots as they costed 50 cent with the effect that you could not use graphic cards on them.

[cantab]

Yeah, I've worked with a lot of Dells and they often show signs of penny-pinching. For example the motherboard design supporting four RAM slots, but only two are actually installed, there's just spaces where the others should be.

[Laie]

yeah, but was it running on the battery for all that time? or was it plugged in/turned on?

Anecdata from my job (of which there is plenty) indicates that the average life expectancy of a modern-day MB battery is somewhere around five or six years. As with light bulbs, there is significant spread -- some give up after two years, others seem to last forever, but about six years seems typical. That's considerably longer than their stated shelf life, so whatever demand the board puts on them, it can't be much.