Battery calibration

Without calibration, the charge percentage reading will be incorrect and your device might behave strangely: such as shutting down suddenly even if the new battery "reports" that it is still half-charged, or continuing to run for hours even though the battery appears almost empty.

For phones and tablets:

1. Charge to 100% and leave it charging for at least two more hours.
2. Use the device until it shuts down due to a depleted battery.
3. Charge uninterrupted until 100%.

For laptops:

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

It is best to perform this procedure periodically (about once a month) to ensure the battery remains properly calibrated throughout its battery lifespan. As conditions (and the age) of the battery change, it is only possible to reduce the inaccuracy of the reported data.'''

A good read regarding battery calibration is on this page. This article on the charge indicator is also instructive. What follows is our summary.

The fundamental problem is that there is no reliable way to know exactly at any given instant the amount of energy stored in a battery (it is an electrochemical storage system that changes continuously and degrades; it never behaves exactly the same way from one charge to the next). In practice, the only reliable way to evaluate it is to charge the battery completely, then drain it completely, and measure the difference (the so-called coulomb counting). Obviously, we cannot do this every time we want to check the battery level, so we have to use indirect methods: store all data related to usage and use them to calculate a percentage charge state at any moment. Over time, the calculation tends to drift and become less accurate. And, with a brand-new battery, there is no data to work with, so this procedure leads astray. Calibration helps keep these estimates accurate by inserting new fixed points—"full charge" and "full discharge"—into the battery management system, so it does not have to guess. We are still at the level of the game of pinning the tail on the donkey blindfolded, but calibration tells the battery management system: "Hey, look, the donkey is over there".

This is precisely the crux of the problem. How do you update these "full charge" and "full discharge" indices? The page from the link above by Battery University puts it this way:

To maintain accuracy, a smart battery must be periodically calibrated by discharging it within the device until "Low battery" appears and then recharging it. The full discharge sets the "discharge" reference, and the full charge sets the "charge" reference. A straight line between these two references allows for an estimate of the state of charge. Over time, this line becomes increasingly indistinct, and the battery requires recalibration. Figure 2 illustrates the full discharge and full charge references.

Figure 2: Full charge and full discharge indices. Calibration occurs when you apply a full charge, a discharge, and a recharge. This is done, as part of battery maintenance, directly with the device or with a battery analyzer.

Here we must note two things: (1) according to this page, it is not enough to discharge and then charge: you must start from a full charge. Furthermore (2) the term "full discharge" is ambiguous: the figure seems to indicate that the full discharge indication must be set at 10%, but the problem we are trying to solve is that the % reading is not accurate. How do you know you have discharged the battery "below 10%" if the battery reading is not accurate? You can't! For example, we have installed several batteries that gave a "low battery" warning and then continued to work at full speed for hours with a battery charge indication of 1%. In short, "calibrating" a battery by discharging it "below 10%" is of little use. It is like entrusting someone with a car that has a broken fuel gauge and telling them to drive until the gauge indicates 1/4 of a tank.

It seems understood that the graph above must indicate the actual chemical state of the battery and not the % indicated to the user, which is a very different thing. The software that displays data to the user might claim that the battery charge is near zero when the actual chemical state of the battery is near a 10% charge value. This is done deliberately to prevent the battery from ever dropping below a safety level that would cause damage to the battery and prevent the system from restarting. In short, for safety reasons, the system shuts itself down when there is still some battery charge, but it does not show the user the value of the reserve amount. A commenter on the article linked above pointed this out:

• The low battery warning is implemented in the device software purely as a means to prevent potential data loss during use and is completely independent of the battery management system.
• Even if you leave the device on until it shuts down automatically due to insufficient charge, the battery management system will maintain the charge level high enough to prevent damage to the battery pack.
• The battery level indicator you see displayed on the screen is in practice the amount of USABLE charge and NOT the absolute battery charge level. This is why you can set the "low battery" warning to whatever value you want: it is not intended to protect the battery (which is done automatically by the battery management system), but rather to give you enough time to save current work or connect the charger.
• Therefore, if you intend to calibrate your device's battery, you must let it run after the warnings have appeared until it shuts down automatically, and this BEFORE performing the recharge: otherwise, the battery may not be discharged enough to be able to record the battery-discharged index of the battery management system, which would make your attempt to calibrate the battery incomplete.

Remember that there are two different (but connected) systems at play: the battery management system, which evaluates and monitors the battery's health, and the software user interface (and associated power management software), which reads data from the first system to show an indication of the battery charge level and condition and responds to various set references (such as shutting down based on the battery-discharged index setting).

It would be nice to be able to confirm all this by checking Apple's official battery calibration procedure, but it seems they have removed it from their support site on the basis that their new batteries are factory-calibrated and are not user-replaceable. In any case, the following can be found cited in various forums:

To calibrate the battery of a portable computer:

1. Connect the MagSafe charger and fully charge the battery.
2. When the battery is fully charged, the light on the MagSafe charger connector turns green and the battery icon in the menu bar indicates that the battery is charged.
3. Leave the battery in a full charge state for at least two hours.
4. In the meantime, you can use the computer, provided the power adapter is connected.
5. Unplug the power adapter with the computer still on and continue to use your computer.
6. When you see the low battery warning, save your current work and close all applications. Leave the computer on until it automatically goes into sleep mode.
7. After your computer has gone to sleep, turn it off or leave it in sleep for at least five hours.
8. Connect the power adapter and leave it connected until the battery is fully charged again.

Note that Apple does not believe your system knows when the battery is fully charged or discharged and wants to leave it charging for two extra hours to be sure before proceeding with the calibration.

This seems in line with what we have learned and is in agreement with other advice found around the web for DIY battery calibration.