Do modern laptops come with rechargeable CMOS batteries?

Not strictly Linux-related but I hope you can help. I have bough a Packard Bell (Acer) Easynote that I mainly use as a desktop replacement. I have removed the battery and I unplug the mains at night.

Would this drain the CMOS battery? Should I leave the battery or the mains plugged in at all times? Or would the mains recharge the CMOS?

There's no mention of this on the Packard Bell website and their tech support is notoriously useless.

Any input appreciated.

Hi there,

Originally Posted by ottavio I have removed the battery and I unplug the mains at night. Would this drain the CMOS battery? Should I leave the battery or the mains plugged in at all times? Or would the mains recharge the CMOS?

the short answers are: No, maybe, and no.

To elaborate this a bit: The CMOS battery is usually a small Lithium cell. In desktop mainboards, the most common type is a CR2032, which is the size of an average coin. Lithium cells are not rechargeable (as opposed to Lithium-Ion cells, don't get them mixed up).
The current requirement of CMOS memory and real time clock is extremely low, it's in the low microamps. The Lithium cell has a nominal capacity of about 100mAh or more, so that yields a lifespan of ten-thousands of hours, that is, several years. Experience is that a Lithium cell lasts about 5..10 years, so it's rare that they have to be replaced.

Whether you should leave the notebook's power supply plugged in is a difficult question. Ten years ago I would've recommended not to do that. Permanent charging shortens the life of most rechargeable batteries.
However, the charging circuits in modern devices are so clever nowadays that I think this is not an issue any more. They monitor the battery's charging state permanently and decide based on the battery's specs and health whether it is being charged or temporarily disconnected from power.

I would leave the power supply connected permanently.

[X] Doc CPU

I tend to try not to leave my netbook plugged in when I'm not using it since if there's a problem with the charging circuit it could be a fire risk. I think the risk is very low, and I don't lose sleep over it if I leave the machine plugged in, but it seems silly to me to not unplug it when even a tiny risk exists. If you're very energy concious then the mains adaptor for most laptops is going to use a trickle of current also -- again not significant but it you're being careful with energy use out of principal then, again, it makes sense not to leave it plugged in.

Hi,

You can hardware hack to use a rechargeable CMOS battery. Modern designs use a blocking diode to control the charge presented to CMOS via battery or power sub system.

No real gain to change the CMOS battery to a chargeable battery since the load on the current designed battery is low for CMOS. Of course battery life will depend on the systems off time and usage.

Thanks all of you for your replies. One more thing:

Originally Posted by Doc CPU Lithium cells are not rechargeable (as opposed to Lithium-Ion cells, don't get them mixed up).

and:

Originally Posted by Doc CPU Whether you should leave the notebook's power supply plugged in is a difficult question. Ten years ago I would've recommended not to do that. Permanent charging shortens the life of most rechargeable batteries.

If the CMOS battery is not rechargeable, there should be no issues with leaving the power cord plugged in?

What I am also interested if, when the laptpop is powered up, via either mains or battery, would the CMOS still be powered by the Lithium cell?

Thanks

Originally Posted by ottavio Thanks all of you for your replies. One more thing: and: If the CMOS battery is not rechargeable, there should be no issues with leaving the power cord plugged in? What I am also interested if, when the laptpop is powered up, via either mains or battery, would the CMOS still be powered by the Lithium cell? Thanks

The CMOS battery (Lithium cell) does the clock and settings in the BIOS and nothing else, it does NOT recharge (other than keeping the settings). So whether you are using the battery of the device or mains power it does not matter to the CMOS battery since all it does is to keep your settings and the clock ticking over in the BIOS. To your answer, yes it keeps on working no matter what power supply (battery or mains) you use.

Hi,

I thought the OP was wondering about a recharge of the CMOS battery. CMOS battery just provides the means to keep the things going whenever the AC power is removed. You can hack the power design but why.

The current flow to the CMOS is controlled by a blocking diode circuit to either switch flow from battery or AC. Simple! To provide a rechargeable battery for a small load is not necessary. I have systems that are well over 10 years old and still have a valid level from the battery.

I had settled with the PECONET009's comment, now onebuck's post has got me proper confused. Assuming this is a laptop we're talking about

Originally Posted by onebuck CMOS battery just provides the means to keep the things going whenever the AC power is removed.

So are you saying that when the AC is plugged in, the CMOS is powered by the AC itself, not the CMOS battery? And:

Originally Posted by onebuck The current flow to the CMOS is controlled by a blocking diode circuit to either switch flow from battery or AC.

When you say battery do you mean CMOS battery or laptop battery?

Thanks

The CMOS battery is a simple cell that is used to power the RTC and some other things of the CMOS when the computer is not powered. The CMOS battery doesn't know whether it is in a laptop or a desktop -- it only works when the motherboard has no power across its terminals.

Hi,

Originally Posted by ottavio I had settled with the PECONET009's comment, now onebuck's post has got me proper confused. Assuming this is a laptop we're talking about So are you saying that when the AC is plugged in, the CMOS is powered by the AC itself, not the CMOS battery? And: When you say battery do you mean CMOS battery or laptop battery? Thanks

As to the CMOS battery, it does not matter whether Desktop or Laptop. When there is source power (AC or Laptop battery) the CMOS battery is not supplying power to CMOS. The blocking diode circuitry prevents the CMOS battery from providing power. Once you loose AC or Laptop battery then the flow will be allowed through the circuit from the CMOS battery.

Look at: https://en.wikipedia.org/wiki/CMOS

& here: https://en.wikipedia.org/wiki/Diode
Hope this help clear things up for you.

I have never seen a motherboard designed that provides any power to the cmos battery so that it can be recharged.

I can't remember how few batteries I have changed over the decades. They used to be small real time clocks on some. They have been odd soldered on ones and some fairly big batteries.

At home I always remove the power to the entire system by a power strip. I don't need to supply 45 watts of standby power or AC to the room cause I am cheap. You won't gain anything from leaving it powered on except maybe some standby/hibernate issues or wake on lan. Leaving power to even a powered off system may allow some other damages from weather or power issues.