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I have a MacBook Pro M1 16”. Can it safely charge when the charger (with multiple USB-C ports) will fluctuate between 45 watts and 100 watts, based on number of cables plugged into it?

Can the MacBook handle the fluctuating power, and if so, will it be safe for the battery in the long run? Or, is it recommended to avoid it?

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Can it [MacBook] safely charge when the charger (with multiple USB-C ports) will fluctuate between 45 watts and 100 watts

Can it safely charge? Short answer: Yes.

There’s a fundamental flaw with the premise of your question: the wattage of the power adapter doesn’t fluctuate maximum wattage. Whatever the rating is (i.e. 100W), that is the maximum amount that can be drawn by the connected devices. These increases/decreases in overall power draw has absolutely no influence on the battery of your MacBook.

The wattage rating is the maximum that the adapter can supply to the device(s) attached, not by the “cables” attached.

Current is drawn, not pushed.

First, power is drawn not pushed. The device will draw the power it needs and if the adapter has the capacity to supply it, it will draw the current. For example, if you plug a cell phone that has a maximum power draw of 12W into a 100W charger, it will only draw 12W. If you plug in a second phone to that charger, they will only have a combined 24W draw. The maximum wattage of the charger will stay the same at 100W.

Power delivery is negotiated

Secondly, USB 3/4 devices don’t just pull power at some random rate causing power fluctuations in the power adapter. These devices negotiate their current delivery per the USB-PD (Power Delivery) Specification. The device will “ask”, the charger will “reply” and they will agree on how much power they get. This is why you can charge a new MacBook that requires a 90+W adapter with a 10W charger; just very, very, slowly.

Your battery is protected

Built into your battery is a PCB which does many things like keep data and stats about your battery like SN#, charge cycles, etc. However, the most important function it’s responsible for is that it provides overcharge/undercharge protection.

First, there is the charging circuity that’s controlled by the SMC to regulate how power is distributed through the logic board including to the battery. Secondly, the battery itself has it’s own charging circuitry to to protect it from receiving too much or too little charge as well ensuring it doesn’t over or under charge (detrimental to the battery).

The bottom line is raw power isn’t being dumped into your Mac or the battery. It’s being regulated to ensure it get’s the correct charge (within it’s parameters) notwithstanding any changes occurring at the charger.

TL;DR

If the adapter is rated for 90W, for example, it will always be able to supply up to 90W. Just because it’s only delivering 45W at a particular time means the device attached is only drawing current to consume 45W; nothing more.

As for being “safe” for your MacBook, it’s inherently safe. The charging of separate devices on the same charger don’t influence how much your MacBook has negotiated. Nor, can a change in current adversely affect your battery since the builtin (to the MacBook) charging circuitry manages the delivery of current to the battery, not the USB port.

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The MacBook would be fine with such a charger. It will pull as much power as the charger says it is allowed to and no more and the charger (short of an outright blowing up style failure) can't push more into the MacBook than it needs.

It is entirely allowed for a USB power delivery source to change the available wattages and voltages. Many cheap chargers will do so by just resetting all of the ports and renegotiating from scratch but the specification does allow on the fly renegotiation of an existing agreement.

The other current answer seems to be focused on the common misconception that chargers push power into devices and explaining that that is not the case. My interpretation of the question here is the case of a multiport charger that limits the power available to certain ports depending on the number of devices connected. Some of them may allow 100 watts if only one USB PD device is plugged in but only 60 and 30 for each of two ports if two devices are plugged in.

https://www.eevblog.com/forum/projects/renegotiation-of-an-established-usb-pd-contract-as-source/

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    I did which is why I included "...common misconception that chargers push power into devices and explaining that that is not the case." in my answer. That part of your answer is not wrong but I think you misinterpreted the question. Here is a thread discussing how many chargers just reset the ports to reallocate power reddit.com/r/UsbCHardware/comments/pitd6u/… I think the original question was about a charger like the ones the user in the Reddit thread was trying to avoid.
    – Jonathan
    Commented May 8, 2023 at 17:39
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    Do you not read that as equivalent to your statement "current is drawn, not pushed"? I'm not trying to contradict that. I was trying to point out that I felt you were answering a different question than was originally asked. Not saying your answer was wrong, given what I think you were actually answering.
    – Jonathan
    Commented May 8, 2023 at 19:38
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    @Jonathan: I agree with your interpretation of the question. Your answer would be improved by explicitly answering the question "are such chargers safe for MacBooks?"
    – dhag
    Commented May 10, 2023 at 14:01

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