But a heads up for everyone doing this. Please, please remove the laptop battery before running it 24/7. Otherwise this is a serious fire hazard. The hardware and especially the battery are not designed to run under these conditions.
1) Source? Well, as it has been also written here, I hear that from every IT-Department I worked with at work. Also learned it once the hard way and got a swollen battery. It was an old old laptop (around 2003 IIRC) and old laptop batteries are an own story, but since people want to repurpose old hardware, I think this is a reasonable hint. Also Linus Tech Tips brought this up in one video.
Also, the newer your laptop the better it might handle it, but I wouldn't bet on it.
2) Why? From my understanding the constant on/off charging and heat development (also due to faster dust buildup. If the fan runs 24/7 it sucks a lot more dust into the case) is the problem. Especially if you run heavy tasks over a longer period, the hardware will heat up to a level that it is not designed to do.
I will grant that the lighter the load you put on that hardware, the less all this precautions matter. But my principle is, that if something is not designed (and likely not tested) for something, proceed with caution if you understand what you're doing, or leave it be. So if you want to host your static website on it, you might never ever be near any problems. But I still wouldn't recommend it.
If your laptop is already 6-8 years old then if you want to keep the battery in as a UPS system it may be worth considering purchasing a new replacement battery and installing that (if it has not already been replaced)
Most computers will only need a small screwdriver and 10 minutes of work to swap the battery, although you will want to check youtube/ifixit for your specific model first just to make sure the battery isn't glued in or otherwise difficult to remove and replace.
While we're here, turn off TurboBoost in BIOS to keep them running cooler with the lid closed, I've found it really helps when tasks get a little bursty.
Every thread about always on laptops leads me to this question. People leave their laptops plugged in all the time.
I've run about 5 laptops 24x7 over the years for at least a year a piece. No issues.
I put an HP Envy onto a shelf in the garage, and the battery swelled enough to ruin the touchpad and case.
In all the laptops I have owned, the only problem battery was idle, unplugged and uncharged.
https://www.uschemicalstorage.com/how-to-store-lithium-batte...
The charging method has been standardized a long time ago - they are charged to ~96%, then cut off, if they fall below ~94% they are topped up again.
There is a significant loss of battery life if you do that for years. I used to unplug some of them and do 2-3 full discharge/charge cycles, that usually helped increase the battery life.
Some laptops use the battery as additional power when running at wattage beyond what the AC can provide.
If you want to be safe with your model, you can try removing the battery while the laptop is plugged in - it should not shut off, the circuit for charging the battery and powering the machine are separate.
I lost more batteries in storage - they still discharge, and even though the cells could be revived, the BMS won't let it recharge if it falls below a certain voltage, rendering them dead (unless you find it fun to restore them).
What if the battery is soldered in place like many machines produced post 2014?
What does that mean?
My laptops run 24x7, not as server but as docked workstations. I have never considered that as a safety risk, and definitely have not considered it outside of norm. At work for a public sector client, we have two floors (few hundred?) laptops plugged in and running 24x7 (ask from IT department is to leave them on overnight for maintenance). Sanity or eco-friendliness aside, this is a first I heard that laptop cannot / should not be run around the clock plugged in...