How to charge laptop battery

Lithium batteries is a complex topic which I will try to explain.

It is said that you should never fully drain a lithium battery because when it is fully drained it enters a low power state where it may not be able to be "re-awakened" even when attempting to recharge the battery for days. Not sure what causes that to happen. I have read it enough times to believe it is possible that might happen, but I would say it is rare. I have fully drained the lithium batteries for my laptops several time without any issues. I would say you should simply re-charge the battery once it drops to 5%.

Regarding "over-charging" or keeping the laptop plugged into the AC outlet all the time when the battery is at 100%... This is generally a bad thing to do. It is also counter-intuitive as well. The battery provides power to the laptop when not plugged in to the AC outlet, that part is simply enough. However, it gets complicated by chemistry within the battery. Generally speaking, you get the maximum amount of available power when the battery is fully charged, but when the batter is close to / near the maximum capacity something occurs called "Lithium Plating".

Lithium plating refers to the chemical process where the lithium ions combines together to form "plates". When that happens they can no longer hold a charge. With fewer lithium ions available the maximum battery capacity starts to fall which means shorter battery life. The more often the battery is near it's maximum capacity the more likely lithium plating can occur. At least that is what observations indicates; empirical evidence.

To help prevent this from happening to batteries some manufacturers include power management software to automatically limit the battery to a 60% - 80% charge when plugged in. It would only charge the battery to 100% if you manually override the software. This allows you to keep the battery attached to the laptop while always keeping the laptop plugged into the AC outlet. This in theory should help extend the battery's useful life because this in theory helps delay the formation of lithium plates. You would override the software when you know you will be traveling around with the laptop so you can get the full use out of your laptop.

The counter-intuitive part is that the more often you want the maximum battery life because you are traveling around, the more often lithium plating can occur which causes the maximum battery life to degrade more quickly.

Sooner or later the battery will begin to fail no matter how careful you are at maintaining maximum capacity. The battery in My old IBM ThinkPad T40 lasted about 3 or 4 years before there was any noticeable decrease in battery life. After that point battery life was cut in half (2.5 hours vs 5 hours). After some conditioning which involved fully charging and fully draining the battery for around 6 - 8 months the battery life did somewhat improve. I was able to get about 3.5 hours of use.

However, the battery capacity of my Lenovo IdeaPad Y470 was cut in half after only about 20 months. My attempts to re-condition the did not improve the battery life at all. Strangely enough, I did not even run it battery very often. Most of the time the Y470 was simply plugged into the AC with the battery removed (80% charge). When I need to bring the laptop around with me (about 3 or 4 times at most per month) I allowed it to be fully charged. Battery life was never the strong point of the Y470 which generally maxed out at just under 4 hours.

At least a new battery for Asus X550C does not seem to be very expensive. A quick search indicates $42 - $70, but not sure how much an official replacement battery from Asus costs.