Lithium-ion Battery

What is a Charge Cycle?

Excerpt from Barclay Sloan’s “Should You Keep Your MacBook Plugged In?” (2021)¹:

A cycle is complete discharge, followed by a full charge. You complete one charge cycle when you’ve used (discharged) an amount that equals 100% of your battery’s capacity — but not necessarily all from one charge.

For example, you might use 75% of your battery’s capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle. It could take several days to complete a cycle.

Do note that the capacity of any type of battery will diminish after a certain amount of recharging. With lithium-ion batteries, the capacity diminishes slightly with each complete charge cycle.

Barclay Sloan

The battery, over time, will ’naturally’ degrade even when plugged in. Nothing can stop a battery from degrading over time.¹

How to charge a Lithium-ion battery?

Casey Johnston

One of the worst things you can do to a Li-ion battery is to run it out completely all the time. Full discharges put a lot of strain on the battery, and it’s much better practice to do shallow discharges to no lower than 20 percent.²

Casey Johnston

Keeping a Li-ion battery fully charged is not good for it either. This isn’t because Li-ion batteries can get “overcharged” (something that people used to worry about in The Olden Days of portable computers), but a Li-ion battery that doesn’t get used will suffer from capacity loss, meaning that it won’t be able to hold as much charge and power your gadgets for as long. Extremely shallow discharges of only a couple percent are also not enough to keep a Li-ion battery in practice, so if you’re going to pull the plug, let the battery run down for a little bit.²

Casey Johnston

Running the battery out very quickly by drawing a lot of power at once is another way to cause it a lot of strain. For example, running a graphics-intensive game on a smartphone or a notebook for a couple of hours while unplugged is worse for the battery than depleting it over several hours while e-mailing or Internet-browsing (heat is a factor here, too).²

Keep it between 20% to 80% for optimal longevity

While it’s not strictly forbidden to charge lithium-ion batteries to 100%, as most devices with lithium-ion batteries do have internal systems that stop charging once they reach 100%, which minimizing the risk of overcharging, it’s generally not recommended for optimal battery health and longevity.

When batteries approach their full capacity, they experience internal stress due to higher voltage. This stress accelerates natural chemical degradation, leading to reduced overall capacity and shorter lifespan over time.

At high voltages, tiny needle-link lithium deposits called dendrites can also form inside the battery. These dendrites can eventually pierce the separator between the electrodes, causing a short circuit or even fire in rare cases.

Many experts recommend keeping the battery level between 20% and 80% for optimal longevity. This reduces stress, minimizes dendrite growth, and extends the number of usable cycles. Though, the choice of whether to charge to 100% depends on your needs and priorities. If you prioritize convenience and rarely worry about battery life, charging to 100% might be fine. However, if you want to maximize your battery’s lifespan and enjoy optimal performance for longer, keeping the charge level between 20-80% is the better practice.

Lithium-ion Batteries in Cold Temperatures

While they are less susceptible to cold weather damage than lead-acid batteries, their capacity can still be reduced in cold temperatures.

Even though you might see a full charge indicator, the usable energy can drops by about 20-30% at freezing temperatures (0°C or 32°F). The chemical reactions inside the battery slow down in cold temperatures, hindering its ability to deliver optimal power. This could impact tasks like fast charging. The resistance within the battery also rises in cold temperatures, further limiting its efficiency and leading to faster discharge. This means the device might run out of juice sooner than expected.

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