How does laptop 'limit battery charge' work?

Question

Charging while using the device reduces battery lifespan*. The charger gets confused in the Constant Current Phase because of the additional current drawn by the device.

Windows OS and some laptop firmwares offer the option to "limit charge to 80%". How does that work to extend battery lifespan?

_{Not tagging laptop as that's only my main, but far from single, concern.}

_{Had I been asking "how to charge my laptop" I would have gone to https://superuser.com/. Not the case. I seek the physics of it.}

_{* - this interpretation of mine is wrong, for more information see the comments and answers}

Answer 1

Charging a LiIon battery to the highest possible voltage gives it maximum capacity, so that is the preferred setting if you intend to use your laptop as a portable computer, because it will run for a longer time.

However, higher voltage will cause the battery to age faster. So, when connected to mains, keeping the battery at 100% charge all the time is not the best. A lower state of charge puts less stress on the battery and will allow it to keep its capacity for a longer time.

The value is a compromise: 50% would be even better for battery preservation, but it is unpractical: in case you actually need to take the laptop away from a socket, runtime will be low and the battery will end up in a deeper discharge, which also ages it faster. So, 80% seems like a good choice.

Basically the end of charge voltage on a LiIon battery is always a compromise between how many years you want the battery to last, and how much runtime you want out of it.

There is another factor: at 4.2V the charger needs to proceed with caution and reduce current because this is close to the overcharge limit. So when the laptop draws pulsed current according to CPU load, the battery may go through many tiny charge-discharge cycles depending on how the motherboard is designed.

If the battery is at a lower state of charge like 80%, the charger does not have to be "careful" to avoid overcharging at all, so it can provide a lot more current without risk, which means, if it is well designed, the charger will provide for all the current the laptop uses, and the battery will not go through these constant microcycles. That's another reason it will stay in good shape for a much longer time.

Answer 2

Charging while using the device reduces battery lifespan

No, it doesn't. The link you provide is from a context very different than a laptop battery. Usage does not interfere with the battery management system.

... the option of "limit charge to 80%". How does that work to extend battery lifespan?

Lithium-ion batteries age primarily with every charge-discharge cycle. There are however secondary factors such as losing 10% to 20% of their rated capacity per year, even if just stored, as metals builds up on the anode.

This and other secondary aging mechanisms are proportional to the cell voltage and hence the lifespan is extended by generally keeping the cells with a lower charge.

Temperature also plays an important role in aging and laptops with "optimized for lifespan" BIOS options might also reduce the charge speed to keep the battery cooler.

Answer 3

The things that wear out a lithium ion battery the most are charging it to 100% and running it flat. They are best stored at around 50% charge, and used between about 20% and 80% of full charge.

But laptops spend a lot of their time plugged in and charging. So the battery can sit for hours constantly being topped up to 100%. Capping the charge at 80% gives you less charge when you need to run the device on battery, but means that the battery lasts a few years more.

Answer 4

1. Your understanding of what that question is saying is not complete. A simple 1 IC lithium-ion charger is different than the very complicated power management ICs in a laptop or modern phone. Cut off and topping are handled differently, there are multiple independent paths, advanced monitoring etc. So believing that using while charging will result in a lower lifespan is short sighted.

2. Using while charging is probably not the main reason a partial charge is used. Even an 85% charge feature will still allow using while charging. And because of 1, this is even an option. Without independent charge control vs the load, you could not stop at 85% without disconnecting the load. And stopping at 85% has the same issues as stopping at 100% then discharging then charging up again. See 3.

3. The nature of lithium-ion cells is that they have a limited voltage range corresponding with their charge capacity. The higher you charge the more strain you produce on its chemical components. By limiting how much of a charge you put in, it's less stressful, resulting in lower capacity loss issues. Like any battery chemistries which has their own pros and cons and aging issues (remember NiCD and "memory effects") lithium-ion cells have issues with high charges, over discharging, storage etc. You literally have to baby them instead of the normal human usage patterns we have become accused to using (charge and discharge multiple times a day, leave charging for hours after 100%, use in extreme temps both external or internal, etc).

Answer 5

Charging and discharging the battery to extremes will result in dendrite growth. This is what causes batteries to swell and distend, but it also affects utility in that capacity is reduced and by extension effective life.

Keeping between 30% and 70% will go a long way to extending the battery life. 80% also significantly, but I do not have metrics to support this.

Most electric vehicles will recommend 80% or 90% as a limit to extend battery life, but also because of the declining rate of kwh being transferred as the cells approach peak voltage (~4.2V).

As for Chargers, I find that if the battery is heavily discharged, and the load from the device is high, the charger cannot supply the device load and charge the battery. Just a poor match.

In fact, if I have a fully discharged Samsung Galaxy (let's say ~ 3.7V), and I tether my laptop via USB as well as running bluetooth and a WiFi hotspot, I cannot charge the phone from the USB port (500mA presumed). But at 30% it can charge.