Few components in our Macs are victims of more stories, rumours and superstition than the batteries in MacBook series, and those inside Apple’s devices. I’ve heard all sorts of tales, from those who feel they should repeatedly discharge their Mac’s battery until it almost won’t start up, to others who reckon theirs should never be charged past 80%. This article tries to dispel myth from reality.
Rechargeable batteries
When I got my first car, it came with a large lead-acid battery consisting of plates of lead suspended in acid, termed electrolyte. Every week the electrolyte level had to be topped up with distilled water, and the aim was to keep your car’s battery as fully charged as possible at all times. Technology improved, and by the time that Apple launched its Mac Portable in 1989, although it relied on a lead-acid battery, that contained gel instead of liquid electrolyte, and was sealed, although it also ensured the Portable weighed a ton.
Although nickel-cadmium (NiCd or Nicad) batteries had been invented in 1899, they didn’t become popular until the late 20th century. I used them to power miner’s headlamps when I went caving. Because of what was known as the Memory Effect, their use was part of an elaborate ritual: every time they were to be used, we charged the battery fully, then discharged it completely during use. This practice was widespread, and founded on a report that Nicads remembered how fully they were charged and discharged. If you didn’t do both as completely as possible, then the charge they could hold steadily reduced until they had to be replaced. It turned out this was a largely false interpretation of some exceptional observations.
For some years, the nickel–metal hydride battery (NiMH) replaced Nicads, and became popular in notebooks and laptops. Charging and battery maintenance became more complex, and sites where there were multiple computers with NiMH batteries often bought expensive battery ‘conditioning units’ to keep them in peak condition. Although evidence of their benefits seemed very sketchy, for many this conditioning became essential.
In the 21st century, lithium batteries are king, either in their original lithium-ion or newer lithium-polymer form that Apple has made standard since the first MacBook Pro in 2006. Over the years since, they’ve shown only two worrying signs: a rare tendency to catch fire, something Apple has been able to largely avoid, and swelling and failure due to prolonged use without being discharged, something Apple has addressed in its battery management software.
Definitions
Aside from basic electrical specifications such as voltage and charge capacity, there are three important terms I use:
- Battery endurance, sometimes referred to as life, is the period of time over which a battery can sustain steady voltage and current, so as to power a Mac notebook (or device) before it must be recharged.
- Battery lifespan is the number of charge cycles you should expect a battery to deliver before its charge capacity falls below 80% of its original value when new.
- A charge cycle is the notional discharge of 100% of the battery’s full charge capacity. Although this could occur in one continuous period of use, it’s more usually the sum of several shorter periods of discharge. If your MacBook Pro typically discharges from 100% to 75% each day that it’s used, and is recharged after use on most days, then one charge cycle takes four days, over which the total discharged reaches 100%, i.e. four time 25%.
Charging and management
Gone are the days of battery ‘reconditioning’: Apple’s batteries, and their charging in particular, are carefully managed by macOS (and its other OSes) to optimise both endurance and lifespan. This takes into account the specific battery, power demands made by that Mac or device, the battery’s charging and discharge history, and battery temperature.
When a suitable mains/AC adaptor is used, charging is managed so that rapid recharging takes place until the battery reaches 80% of its total charge capacity. This ensures that recharging takes the shortest time, and will normally only be restricted if the battery temperature rises above a specific temperature. Not only does that minimise the risk of fire during charging, but it protects battery lifespan. Once charge reaches 80%, the charging rate is reduced to a slow trickle, again to protect lifespan, until the battery is 100% fully charged and charging stops.
To guard against prolonged use without significant battery discharge, management software determines when the battery needs to be discharged even though it’s connected to a mains/AC power supply. This is typically enforced by preventing recharging until the charge level falls below about 80%, then allowing recharge back to 100%. However, because the user may require the battery to be charged to full capacity (as it may need to run without recharging for as long as possible), there is a manual override.
How should you manage your Mac’s battery?
If you’re not a diabetic, how do you manage your blood sugar? Of course you don’t, because your body manages it for you, making you hungry when you need sugar intake, for instance. So too with Apple’s battery management: don’t try to second-guess what it should do and try to manage your battery, but use it normally and with care.
When your Mac’s battery is starting to fall below 70-80%, if you can connect it to recharge the battery, do so. When it reaches full charge again, although it may be convenient to disconnect the power supply, it will stop charging anyway. Don’t feel that you have to put the battery through cycles in which you discharge it as fully as possible before recharging, as that’s not required for a good lifespan.
When you know your Mac could require additional power for a longer period, such as when making a large backup, or upgrading macOS, either charge it up first or leave it connected to its power supply for that procedure.
Some prefer to run a daily routine, in which their Mac is charged fully early in the day, then runs from battery alone during working hours, before being put to sleep overnight. So long as that delivers the endurance you need, don’t feel that’s going to reduce its lifespan. But don’t think that you’ve got to manage how your Mac manages its battery, let it get on with it.
Storage
There’s one significant exception to much that I’ve written above, and that’s management of batteries in Macs and devices that are being stored, and won’t be used normally for some weeks or months. It’s generally agreed that you shouldn’t store devices with lithium-polymer batteries at high or low charge levels. Instead, you should aim to discharge the battery to around 60% when it’s put into storage, then to recharge it periodically to maintain a level around 50%. How often you need to do this depends on the rate of discharge of the battery when that device is shut down. Apple suggests that might be every six months, although it will vary according to the device.
Sometimes recharging is forgotten, and a Mac falls into a low-battery or deep-discharge state. If that has happened, then it may take 20 minutes or more of charging to wake it up, and restore normal function and battery charging. If that doesn’t work, contact an authorised Apple service provider or Apple store.
Heat
The lithium-polymer battery’s worst enemy is heat, whether in use, during charging, or storage. As much of the world gets significantly warmer, you should aim to keep your Macs and devices at an ambient temperature of between 10˚C and 35˚C (50˚F to 95˚F) at all times, particularly when in use. Be cautious when temperatures exceed 22˚C (72˚F), and avoid all exposure to temperatures above 35˚C (95˚F) even when shut down, as they will cause permanent battery damage.
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