While some of Apple's new laptops are built with internal batteries to increase power capabilities, other systems still have easily removable batteries. In similar systems, the batteries are interchangeable, and while it may be obvious that you can swap batteries among these systems, some people may also have concerns about doing this. Some of these might include whether or not the system would read battery status properly and if charge values would be similar across multiple machines.
Recently MacFixIt reader David wrote in with such concerns:
"Since I own two batteries for my Intel Core Duo MacBook Pro I was wondering... where is the battery condition information stored, in the battery or the computer? The reason I ask has to do with conditioning/calibrating the batteries. If I successfully calibrate one battery and then do the same for the second, can I swap batteries and be confident the calibration will be accurate for whichever battery I am using at the time?"
Battery calibration adjusts the battery itself, and not the computer's reading of the battery. A full calibration pushes the battery to its limits and resets the output levels that the battery tells the computer is 100 percent of its capabilities. As batteries age, they lose their ability to hold a full charge, so what is considered "full" for a given battery needs to be reset periodically.
These levels along with other statistics such as charge status, number of cycles, and other information are stored in the battery and are just reported to the computer. This information can be found in the System Profiler utility (available in the Utilities folder, or by selecting "About this Mac" in the Apple menu and then clicking "More Info..."). In System Profiler there is a Hardware section, and if you click the Power subsection, then you will see battery statistics (number of full charges, charge status, capacities, remaining capacity, etc.)
While it is most common to use the computer to calibrate the battery, this technically can be done by any system that will charge it up fully, and then drain it fully. The battery will hold these charge values and report them to any system that can read them. Therefore you should be able to calibrate a battery in one system and then use it in another.
The one caveat to swapping batteries is the power level at which the system will assume you are running low on battery power will be managed by the system management controller, and if there is a faulty setting in the system management controller, then the computer may go into precautionary sleep mode prematurely. This is independent of the batteries and their calibration status, but will affect the system's power usage. Therefore it may help to reset the SMC for the machine that you have swapped the battery to.
Tip: Live swapping without external power
While you can always shut down your system to swap out the battery, or keep it plugged in to an external power source when swapping, you can also make use of the Mac's deep-sleep mode if you only have a battery available.
Be sure to save your work if you can, and then put your system to sleep by using either the Apple menu or by press the power button once and choose the sleep option from the presented options. Wait for the light on your Mac to begin pulsing, which indicates the RAM's contents have been written to the hard drive, and then swap the batteries. When the new battery is in place, power on the system and you should see the system wake from deep-sleep mode with a gray progress indicator shown over the desktop.
Proper care for batteries is something that is often overlooked by laptop owners, and Apple has some dedicated resources available for helping people ensure their batteries last as long as possible. These include tips on storage, calibration, and overall use of the batteries on both laptops and Apple's other mobile devices.
In addition to Apple's resources, we have a few articles on tips and suggestions for managing and maintaining laptop batteries: