It’s a long time since laptop Mac models recharged in simple ways: when the battery was lower than 100%, plugging in the mains power adaptor resulted in them recharging to 100%. Yet in our mental model of how they should work, that’s still what we expect. I’ve had many users ask me, when their MacBook/MacBook Air/MacBook Pro is connected to mains power and indicates the battery is less than 100%, why it doesn’t always recharge.

Changing charging policy

These questions have become significantly more common among users whose Macs are running recent versions of macOS, including Catalina and Big Sur. That’s because Apple made changes to battery charging policy with those more recent versions of macOS, and possibly with firmware updates which have been applied to older versions in the last year or two.

Lithium batteries, as used in Apple’s laptop Macs and iOS/iPadOS devices, are very widely used in everything from miniature electronic devices up to electric vehicles, and there’s extensive experience using different designs for various purposes. They’re not all the same, and are carefully engineered for specific applications and uses.

Those used in Apple computer products last longest when they’re not repeatedly recharged whenever possible to stay at 100% of their charge capacity. Long term experience shows that keeping those batteries as close to 100% charge all the time shortens their working life. Yet many laptop Macs spend much of their working time connected to the mains power supply.

To prolong the working life of their batteries, Apple has therefore introduced changes to their charging policy which can allow their battery to be partially discharged before recharging starts. Users may therefore see their Mac not charging its battery even though it’s connected to mains power.

Thermal cutoff

Another great enemy of battery health is operation, particularly recharging, when the battery is too warm. Not only does this shorten the battery’s working life, but it can lead to damage resulting in swelling, and in worst cases can cause fires. Most, perhaps all, Mac batteries therefore monitor their temperature. If the battery controller considers that the battery is too hot for charging, then – although it may still be connected to mains power – charging of that battery will be stopped. This is the battery’s equivalent of thermal throttling of processors and SSDs.

You can see this happening in recent tests of an M1 MacBook Air reported here by Thomas Kaiser. When the gas_gauge_battery thermal sensor reached 44˚C, battery charging was disabled.

Not only that, but charging a battery increases heat production, and many laptops get noticeably warm, to the point where they can be hot enough to redden the the thighs if they’re resting on bare flesh. This is reduced by the M1 models’ low energy consumption and proportionately lower heat production. However, the MacBook Air relies on passive cooling rather than active measures such as internal fans to blow cooling air through internal components. The M1 MacBook Air is therefore more susceptible to thermal limitation of battery recharging.

This is complicated by observed differences between charging laptop models using different ports. Although that affects a thermal sensor outside the battery, high readings there may be taken into account when your Mac makes decisions about charging as well as CPU use.

Checking temperatures

There are several utilities which will display readings from a Mac’s thermal sensors, and provide a good idea when thermal protection is likely to come into effect. You can also use the powermetrics command tool to see what is going on, for example
sudo powermetrics -n 1 -i 1 --samplers thermal
should produce a summary of thermal pressures. You can get further information on the samplers available from
powermetrics -h
These are currently limited on M1 models, which supports battery but not smc.

If you’re concerned that your Mac’s controls aren’t working correctly, try resetting its SMC, although that doesn’t work for M1 models, which don’t have one.

Apple has a whole section of its support website devoted to batteries.