Computer turns off at 30%-40% of charge

Batteries are considered Consumable Products. Over time it will degrade to a point where it needs to be replaced. The usage is normal as the capacity will go up and down all the time and never stay always at 100%. Apple Batteries are rated for 1000 Full Battery Cycles and / or 80% Capacity before needing Evaluation or replacement.

To check the Full Battery Cycles >> System Report and scrolls to Battery. On Right will show the condition of the battery and number of Full Battery Cycles. It may also mentioned Service Battery Warning too if it is determined it needs servicing.

For additional reading on Battery and Apple Computers this link may help. About battery health management in Mac notebooks and especially for the M1 and Big Sur If battery charging is paused or on hold on your Mac

Depending on the Age of the computer, some aspects from the above links, may or may not apply to your computer.

A feature introduced into Big Sur was to preserve the number of Full Battery Cycles - there was an Algorithm coded into the macOS. It is perfectly Normal for Big Sur to Pause / Suspend charging the battery beyond 80% and hold it at that level. One can nudge / over-ride this by clicking the Battery Icon on Desktop Top Bar and allow charging beyond 80%. From observations, it will Pause / Suspend again once reading 90%. Same procedure to over-ride and will charge to 100%

Computer-turnsoff-at-30charge wrote:

Hi,

I can't understand why when the battery is around 30% of charge, the computer turns off without any notifications. Then, if I put it on charge and turn it on again in few minutes after it turned off, it never says that it has a very low battery. Today on the battery notification there is "assistance recommended", I called Apple service and they told me to bring it to an Apple store but I would like to know if this event has ever happened to you before and what was the problem. My computer is a 2019 Macbook, its version is 11.6.

You can try resetting the SMC https://support.apple.com/en-us/HT201295 for power management