Check the battery's health

Cycles, maximum capacity and condition.

1. System Settings → Battery → Battery Health (the "i" icon).
2. Details on the command line:

   system_profiler SPPowerDataType | grep -E "Cycle Count|Condition|Maximum Capacity"

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3. A "maximum capacity" below 80 % or the "Service Recommended" condition means a replacement should be considered.
4. Turn on "Optimised Charging" to slow ageing.