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I have a MacBook Air 2015 (13-inch). Bought it in oct 2016 and has been used decently since (5 years). Obviously, I have started having issues with shorter battery life (coconut battery says 30% - 40% capacity). I am thinking of getting a new battery, but before that I wanted to clarify something.

Whats weird is that the laptop shuts off (or sleeps) at 80%-90% battery (as said by the indicator at the top). I found this weird as generally, if the battery was bad the battery percentage would drop to 0-10% and then shut off and the battery would drop quickly. Why would a bad battery shut-off at 80%-90% indicated.

I searched this up and many websites said to reset the SMC, which I did but did not get anywhere with battery turning off at 80%-90%. I also tested the SMC if it was working. The keyboard lights works, the auto-brightness function works, the apple start-up chime works.

So my question is, is the battery bad?, is the SMC bad even though it seems alright? or both? or something else? I do not want to waste getting a new battery unneccesarily.

$ pmset -g log | grep -i "shutdown cause"
2021-11-04 17:47:51 +1100 ShutdownCause         SMC shutdown cause: 5: Software initiated shutdown                                    
2021-11-05 09:00:23 +1100 ShutdownCause         SMC shutdown cause: 5: Software initiated shutdown                                    
2021-11-06 12:25:48 +1100 ShutdownCause         SMC shutdown cause: 5: Software initiated shutdown                                    
2021-11-07 15:39:17 +1100 ShutdownCause         SMC shutdown cause: 5: Software initiated shutdown                                    
2021-11-08 10:42:34 +1100 ShutdownCause         SMC shutdown cause: 5: Software initiated shutdown                                    
2021-11-09 08:25:13 +1100 ShutdownCause         SMC shutdown cause: 5: Software initiated shutdown                                    
2021-11-10 07:38:39 +1100 ShutdownCause         SMC shutdown cause: 0: Battery disconnected                                           
2021-11-10 10:28:37 +1100 ShutdownCause         SMC shutdown cause: 0: Battery disconnected
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    I would expect your Mac to be super unreliable if Coconut Battery says 30-40% capacity. Anything less than 80% capacity and I would start worrying.
    – Gilby
    Commented Nov 10, 2021 at 0:40
  • What do you see with pmset -g log | grep -i "shutdown cause"?
    – pion
    Commented Nov 10, 2021 at 5:53
  • @pion it looks to be the battery at the most recent happened today in the morning which is shown in the log at the bottom. Although what does "software initiated" mean. Is that just me pressing the shutdown button at the end of the day?
    – Dstarred
    Commented Nov 10, 2021 at 6:31

1 Answer 1

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  • As lithium-ion batteries age and their cycle count increases, their maximum storage capacity decreases 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.
  • 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.
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  • Alright thanks, mostly likely gonna replace the battery at a genius bar.
    – Dstarred
    Commented Nov 10, 2021 at 7:51

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