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Back in February I bought an used 2015 MBP Retina and it had 251 battery cycles and 8440 mAh full charge capacity. Now after almost three months I felt like the battery life decreased a little bit and I was shocked to find in system Information that the full charge capacity dropped form 8440 mAh to 8119 mAh.

I would like to add that I added 8 battery cycles, but other than that I used it plugged in mostly.

Is there any best practice to avoid this situation?

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At this speed, the Mac will have a dead battery in around 6 years. Probably below useful levels in less than 5 years. As the Mac is already around 4 years old, that means around 7 to 9 years of battery life. Which seems quite decent considering that laptop batteries are expected to last 2 to 4 years, according to a quick Google search.

In other words, you are probably doing everything right.

As mentioned before, heat (and cold) is an issue. However, leaving it plugged in during work should not be a problem, as cycles add more to battery loss than leaving it at high charge. So the rule about 20% to 80% is more useful for storage than for an actively used Mac, as it will add cycles to always move between the two values. Reconditioning a battery also adds cycles, usually without any advantages. Do that only if the battery has serious issues. When you don't work, keep it unplugged, but make sure you reload it before it gets below 20%, and only up to 60% if you don't intend to use it for a longer time.

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Heat kills batteries. It’s not a good idea to keep a battery on the charger constantly, nor should you keep it almost dead. For the best battery life, keep it between 20 and 80%. Try a utility like “FruitJuice” which will also help recondition your battery.

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