What should I know about replacing my computer's battery?
Here are some tips about the batteries that go inside Apple computers and devices.


lithium cells will catch on fire and burn with such intensity that they can ignite almost anything that is combustible if you damage during the replacement. To save space, Apple engineers designed the case of your MacBook to protect the battery - once you open the case, the battery is extremely vulnerable to physical damage or a short circuit. If you lose one tiny metal screw behind the battery and then shut your case it could easily squeeze, crush or puncture a battery cell - setting you up for disaster down the road.
that being said, assuming you power off your Mac before you start the replacement, you won't lose any data and you probably won't damage anything else as long as you are slow and careful. Pay attention to not damage the delicate cables or lose any parts. Be willing to take it to a trained technician if you can't be absolutely certain you did the replacement right.
dispose of the spent battery properly and insure it is safely protected and packaged so it doesn't get punctured and start a fire. Lithium batteries are considered hazardous waste in many places and should it catch on fire, release of very toxic gases will happen.
as long as the battery contains the proper parts, there is no break-in procedure whatsoever. You might be on your own if you select an aftermarket part have to do some research with the vendor to be sure you have the correct part.