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If I use the CPU heavily while charging a Mac, how much slower will the charging be? It would be interesting to hear if someone has data how many minutes it takes to:

  1. Charge 0-100% while using ~0% CPU
  2. Charge 0-100% while using 100% CPU

PS! Another interesting question (probably a separate question) would be: If a heavy 100% CPU job takes 1 hour on a 100% charged machine, how long does it take on a charging machine?

  • The second question is trivial and addressed in the answer. CPU gets all the power it wants. Charging or not doesn’t affect performance. – bmike Jan 23 at 12:21
  • Not sure what kind of answers you expect here. A system under load needs more power than an idle one, so of course charging is faster if the system is idle. But CPU is only one place where power is consumed, there is also memory, the GPU, disk access on HDD or SDD, network access etc. Don‘t see a meaningful answer here. – nohillside Jan 23 at 16:25
  • You can check the battery amperage (charging or discharging) in System Information - Power - Batter Information. – amdyes Jan 25 at 9:24
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The CPU is boss of the power scheme and will get all the power it requests. The charger is boss of the maximum power it will output.

  • Only thermal conditions throttle the CPU.
  • If the charger is offering more power to be delivered than the charging controller is being asked to provide to the CPU, you have excess power available.
  • When the battery needs replenishment and excess power is available, the energy will be applied there in the usual non-linear LiPo charge curve off fast charge to roughly 80 percent capacity then slow charge.

The CPU can pull from two sources and the charger can deliver to two sources.

It’s a pretty simple systematic first glance, but the nonlinear charge and variable CPU/GPU loads make si ole linear time estimates not match reality.

So, if your charger is not providing the power needed to keep the CPU fully powered, the battery will drain and eventually the Mac will shut off because there’s no more battery to make up the deficit in power. And that is why the CPU can run at full speed.

I don’t think there is any general common timing even if you restrict yourself to only USB-C charging models. But, the great news is you can just open the terminal and check the current power budget and time estimates and not rely on me or anyone else rule of thumb on the internet.

 pmset -g sysloadlog
 pmset -g thermlog
 pmset -g pslog

Those three commands show engineering summaries and details for the overview of the system state (sysloadlog) and the thermal CPU throttling (thermlog). The last power log will answer the specifics of any charge scenario for any model Mac and the charger you have connected.

  • The question is in a situation where the Mac is charging (ie. connected to a charger), thus the following sentence seems out of place: "The battery will drain and the Mac will shut off". – forthrin Jan 23 at 11:09
  • @forthrin I am saying you can provide a charger that doesn’t have the wattage to keep the CPU running. The CPU can drain the battery entirely while connected to power depending on which model of MacBook Pro and which specific charger you have chosen. – bmike Jan 23 at 11:17
  • This is also an example of why site norms are to only ask one question per question. When there’s only one question, then the answer can adequately address it. When you ask multiple questions you might not get all the answers you want – bmike Jan 23 at 11:18
  • After some consideration I tend to agree and thus rewrote the question. – forthrin Jan 23 at 11:37
  • Awesome refinement @forthrin - what model year and specification of Mac and what charger is in play? A MacBook with a 29 watt USB C charger will behave differently than a MacBook Pro 13 touchbar. The GPU also matters when there are two of them, so you’ll get a different power budget connected to an external display and running some but not all programs. – bmike Jan 23 at 12:06

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