Battery turned off even if it has life [duplicate]

Solution 1:

From my answer to Apple battery issue or something else? :

• As lithium-ion batteries age and their cycle count increases, their maximum storage capacity decreases and their internal resistance increases due to the deposition of electrolytic contaminants and microscopic mechanical breakdown.
• A Li-ion's voltage discharge curve depends on a number of factors, including instantaneous load and internal resistance, the latter of which rises with wear due to the above reason.
• Modern electronics use sophisticated power gating and performance scaling algorithms that cause wild fluctuations in instantaneous load.
• What happens when the load exceeds the maximum available power delivery capability is an undervoltage and/or undercurrent condition. This is detected by power management integrated circuits, which are found at all levels throughout the system and include brownout protection. The PMICs will force a power emergency shutdown rather than risk running the silicon out of spec.
• The SMC, being just another subsystem on the logic board, and being directly powered by the battery, also experiences this brownout -> emergency shutdown cascade. Because there is no backup power to the SMC, it has no way of recording the failure condition in the same manner that it is normally able to record most other failures to provide a nonzero shutdown cause code.
• The reason that this happens without warning at the "80-90%" battery charge level is because the battery level is determined by a heuristic algorithm that was built upon empirical, statistical modeling of voltage discharge curves. It is not a direct, real-time measurement of internal battery chemistry. These models are weighted towards newer batteries and they become less and less accurate as the battery ages. For a >5-year-old pack, it's perfectly reasonable to see this kind of sudden shutoff behavior.