Will a degraded battery reduce CPU performance?

Cyflor-N-Ac wrote:

The value I am referencing is the full design capacity, in which my battery is at 82%. The only wait to view this info on intel macs is through coconut battery. However, it appears to be inaccurate due to the fluctuations macOS makes,

so 82% for my battery is just an average of all the percentages coconut battery displayed while both plugged in and on battery at different charge levels.

It is not macOS that causes those fluctuations, but the chemical reactions within the battery itself. The more you see fluctuations, the more likely the battery is failing. In fact that is one of the ways I can determine if a battery is failing since many times the battery condition will be "Normal" and the diagnostics do not report any battery failures. I use a command line utility to monitor the Full Charge Capacity while performing a system stress test to quickly drain the battery since that is when most issues will show up, then I stop the stress test and quickly charge the battery (usually the drain test is the most informative).

So, in short, unless a battery is very degraded or it is faulty or has an issue, it will not affect performance?

Correct.

he guy in the YT video I found said that when he replaced the old battery (which was at around 60% - 70%), the Macbook went from noticeably but useably slow to perfectly fast again. It was clear that mainly the battery was affecting the performance of that Macbook.

A battery that worn is much more likely to also have a hardware failure, so it is not terribly surprising that the battery may affect the power system where it could affect the system performance.

So, in general, the battery and the other components. The raw performance all comes down to the condition of the hardware, right? Unless there is a hardware issue or fault, the raw performance of the Mac should still be the same?

Yes, but as always there are exceptions. If the computer is running all cores at full the cooling system may not be able to keep the computer cool enough, so if the fans are at their max speeds, but the temps keep rising, then the CPU speed will be throttled to allow the cooling system to keep the temps within a safe zone to keep the system stable and to prevent heat damage. On the Macs, the CPU speed on the Retina models tend to go to default idle speeds (800Mhz) when this happens. Once the temps stabilize again, the CPU speed will increase again.

Of course third party software is a very common reason by computers tend to also have performance issues when the hardware is healthy. Anti-virus apps, cleaning apps, and third party security software are the usual culprits since these types of apps interfere with the normal operation of the OS.

File system errors or corruption can also affect performance in some cases.

What value are you referencing here? The current charge level which is sometimes displayed on the battery icon or the Full Charge Capacity relative to the Design Capacity? The latter value is not normally provided by macOS, although it may possibly be available with macOS 11+. I know this value is available for M1 Macs, but not sure about the Intel Macs. The third party app Coconut Battery does display this value.

The standard % level (displayed on the menubar) while running on the battery & charging should have no effect on performance assuming the battery is healthy and the laptop's hardware is fine.

A battery with a lowered Full Charge Capacity should not affect the performance of the laptop unless the battery has a hardware issue (again assuming no other hardware faults exist).

If there are actual power related issues with the laptop, then yes, they can affect the performance of the laptop. A faulty battery regardless of its Full Charge Capacity can possibly affect system performance, but a healthy battery should not cause any problems.

Keep in mind that when the Full Charge Capacity drops to 75% - 80% of the original Design Capacity the battery capacity is usually not sufficient to most people who require the laptop to run off the battery. Apple considers a battery Consumed once the Full Charge Capacity drops below 80% of the Design Capacity. If the Battery condition is "Service Recommended", then it means capacity has dropped below 80% and/or there is a hardware fault with the battery. Unfortunately some battery hardware issues are not detected even with the Apple Diagnostics.

Hardware faults and problems with third party software are the usual causes of performance issues. If you are having a problem with your laptop, then run the Apple Diagnostics to see if any hardware issues are detected. To look for possible software issues, run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.

Yes, I normally would not worry about those small fluctuations in most cases. That is just the uncertain nature of those chemical reactions.

Not very many people really look that closely at battery behavior. Until a few years ago I never looked that closely either, but after these Retina laptops began showing problems, I began to investigate more deeply since macOS and the diagnostics were reporting the batteries were fine. It can be hard to confirm an end user's issues since software can play a huge role in battery life.

When I began suspecting these "healthy" batteries may be the problem, I began to develop ways of performing better tests which showed me a bit more detail on how these batteries behave. I'm still not certain how a brand new healthy battery behaves since I haven't had the time to really thoroughly observe one to create a baseline for comparison. I just used my common sense to realize some of these Full Charge Capacity fluctuations were just too severe in scope and could be triggered by special tests sometimes causing the laptop to power off instead of hibernating. Since then I have noticed some commonalities with batteries which tend to misbehave the most. Sometimes it is really hard to draw the line on when to replace a seemingly "healthy" battery. Fortunately or unfortunately I have access to thousands of Apple laptops that I must support, so I was motivated to find something to help us determine whether an expensive battery was even needed (also want to keep our users happy, but not break the bank either).