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Without calibration, the battery percentage reading will be incorrect, and your device may behave oddly—shutting down suddenly even though the new battery “reads” half charged, or working for hours when the battery reads nearly dead.

How to calibrate your battery

For phones and tablets:

  1. Charge it to 100% and keep charging it for at least 2 more hours.
  2. Use your device until it shuts off due to low battery.
  3. Charge it uninterrupted to 100%.

For laptops:

  1. Charge it to 100%, and keep charging it for at least two more hours.
  2. Unplug your laptop and use it normally to drain the battery.
  3. Save your work when you see the low battery warning.
  4. Keep your laptop on until it goes to sleep due to low battery.
  5. Wait at least five hours, then charge your laptop uninterrupted to 100%.

It’s best to perform this process periodically (about once a month) to ensure that the battery remains properly calibrated throughout its lifespan.

Background: What’s calibration anyway?

For a good read on battery calibration, see this pageThis article on fuel gauges is also instructive. What follows is our summation.

The fundamental problem is that there’s no reliable way to know exactly how much energy a battery holds at any given moment. (It’s an electrochemical storage system that is always changing and decaying, and never behaves exactly the same way from one charge to the next.) About the only reliable way to gauge it is to fully charge the battery, then fully discharge it and measure the difference (a.k.a. coulomb counting). Obviously, we can't do that every time we want to check the battery level, so we have to use indirect methods—storing all kinds of usage data and using that to calculate an estimated % state of charge from moment to moment. Over time, that calculation tends to drift and become less accurate. And on a brand-new battery, there’s not really any good data to work with, so the model will be way off. Calibration helps keep estimates accurate by setting new “full charge” and “full discharge” anchors in the battery management system so it doesn’t have to guess. We're still playing Pin the Tail on the Donkey, but calibration tells the battery management system, "Hey—the donkey is over that way."

What do “full charge” and “full discharge” actually mean?

Here’s the nub of the problem. How do you update those “full charge” and “full discharge” flags? The above-linked page at Battery University puts it this way:

To maintain accuracy, a smart battery should periodically be calibrated by running the pack down in the device until “Low Battery” appears and then apply a recharge. The full discharge sets the discharge flag and the full charge establishes the charge flag. A linear line forms between these two anchor points that allow state-of-charge estimation. In time, this line gets blurred again and the battery requires recalibration. Figure 2 illustrates the full-discharge and full-charge flags.

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Figure 2: Full-discharge and full-charge flags. Calibration occurs by applying a full charge, discharge and charge. This is done in the equipment or with a battery analyzer as part of battery maintenance.

Two things to notice here: (1) According to this page, it’s not enough to drain and then charge—you have to start by charging it fully. And, (2) “full discharge” is ambiguous—the figure seems to indicate that the full-discharge flag will be set at 10%, but the whole problem we’re trying to solve is that the % reading is inaccurate. How do you know when you’ve drained your battery “below 10%” if the battery reading is inaccurate? You don’t! For example, we've installed numerous batteries that eventually gave a “low battery” warning and then continued to work full steam for hours on an indicated battery charge of 1%. In short, “calibrating" a battery by draining it “below 10%” is futile. It’s like giving someone a car with a broken fuel gauge and telling them to drive until the tank is ¼ full.

What seems to be going on here is, the graph above is meant to show the actual chemical state of the battery and not the % indicated to the user, which can be quite different. User-facing software may claim a battery charge of near zero when the actual chemical state of the battery is closer to 10% charge. This is done deliberately to prevent the battery from ever discharging below a safe level where battery damage may occur and the system may not be able to reboot. In short, the system always shuts itself down with a little bit of charge left in the battery as a safety measure, but it doesn’t show that reserve amount to the user. As one commenter on the above-linked article pointed out:

  • The low battery warning is purely implemented in the device software as a means to prevent possible data loss whilst using it and is completely independent of the battery management system.
  • Even if you let your device run until it shuts down automatically due to lack of battery charge, the battery management system will still keep the battery charge at a high enough level to prevent damage to the battery pack.
  • The battery gauge that you see displayed on the screen is basically the amount of USEABLE charge the battery has and NOT the absolute total charge of the battery. This is why you can change the “battery low” warning to any percentage you choose—it’s not there to protect the battery (that’s done automatically by the battery management system), it’s there to give you enough time to save your work or connect the charger.
  • Therefore, if you intend to calibrate your device battery, you need to let it run down past the warnings until it shuts down automatically BEFORE recharging—otherwise you may not discharge the battery sufficiently to register the battery management system’s discharged flag, thus rendering your attempt to calibrate the battery incomplete.

Remember there are two different (but connected) systems at play, the battery management system, which monitors and controls the health of the battery, and the software user interface (and associated power control software), which reads data from the former to display an indication of battery charge status and level and respond to various flags (like shut down when the discharge flag is set).

Now what?

It would be great to confirm all this by checking Apple’s official battery calibration procedure, but they seem to have purged it from their support site under the premise that their newer batteries are calibrated from the factory and are not user-replaceable. However, you can find it quoted in a number of forums as follows:

To calibrate a portable computer battery:

  1. Plug in the MagSafe Power Adapter and fully charge the battery.
  2. When the battery is fully charged, the light on the MagSafe Power Adapter connector changes to green and the Battery icon in the menu bar indicates that the battery is charged.
  3. Allow the battery to rest in the fully charged state for two hours or longer.
  4. You can use your computer during this time as long as the power adapter is plugged in.
  5. With the computer still on, disconnect the power adapter and continue to use your computer.
  6. When you see the low battery warning, save your work and close all applications. Keep your computer turned on until it goes to sleep.
  7. After your computer goes to sleep, turn it off or allow it to sleep for five hours or longer.
  8. Connect the power adapter and leave it connected until the battery is fully charged.

Note that Apple seemingly doesn’t trust your system to know when the battery is fully charged  or discharged, and wants you to keep it on the charger an extra two hours just to make sure, before you proceed with the calibration.

This seems to align with what we’ve learned, and matches other battery calibration DIYs around the web.

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