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I'm concerned I may have fried the circuitry of a 14" MBP Pro on a week-long cross country car trip during which it was plugged in at all times to a 12 volt cigarette outlet using an inverter. I know that it can be bad for laptops to be connected to a power supply with less charge than they require.

Toward the end of the trip the computer would sometimes not charge unless the inverter was jammed in harder.

I had specifically requested a car with an AC outlet 6 months in advance, but of course on the day of picking up from the rental place that car was not available, necessitating getting the inverter.

Since the car trip a couple of things have happened:

For a day the backlight of the display was flickering a bit (that was once back on normal AC power), but that has since stopped.

My Samsung A54 cannot be charged from dead from the MBP. It previously could. In fact during this trip I was sleeping in the back of the van and charging all of my devices from the computer (the Samsung a54, an Apple Watch, an iPhone, and a wireless headphones case).

When I plug the Samsung A54 into the MBP from dead, it does charge and gets up to 1%. But if I turn it on, it then turns off after a minute or so and says it's starting to charge again. It doesn't do that when plugged into a wall outlet and once it has started up from the wall outlet, it can continue to charge from the MBP. Before I could charge it from dead from the MBP, though.

The computer can still charge my iPhone, Apple Watch, and wireless headphones—but those all use less power. I believe the Samsung A54 has higher power requirements due to having fast charging.

One strange thing I've noticed under System Report is that it lists 0 for the amount of power the device requires:

https://imgur.com/a/kewdyeX

Versus showing 500 for any other devices I plug in:

https://imgur.com/a/sAs1nrt

(The extra operating power and sleep power is for Apple devices only I believe—but I have Sony wireless headphones for which I plugged in the case and they charge normally showing 500 required).

I have tried restarting the computer which on an Apple Silicon Mac also is how you reset the SMC. I've tried switching ports. Still the same.

I ran Apple Diagnostics and it showed no issues, but I've not found Apple Diagnostics to reliably find issues in the past. I have tried charging it with no other devices plugged in, and it doesn't make a difference. Somehow the MBP was able to provide enough power to start it up before and now it can't.

I have also tried with another USB-C cable with no difference.

The only other thing is that the display may seem a bit dimmer on full brightness, but it may be because I am now in a much brighter area of the country. The flickering I noticed only happened for a few hours and then went away. Not sure if that was related or not.

But the issue with not being able to charge from dead persists. It's not bothersome really, but more wanted to find out if it's indicative of anything having been damaged as I am under AppleCare Plus and could have it repaired if I needed to (but given my situation would mean needing to send it away so would rather not if it's OK).

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    For hardware problems, you really need a hands-on inspection. I'd suggest taking the MBP to an Apple Store or other Apple-certified repair shop. At the very least, they'll provide you with an accurate diagnosis of what's wrong, and what, if anything, needs doing.
    – benwiggy
    Oct 17, 2023 at 11:09

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You never mentioned the make/model of the inverter, which is key here. Speculation. I believe you got one of the cheapest inverters since this was to be something used on a trip, not for long term, heavy duty cycle power generation.

When I plug the Samsung A54 into the MBP from dead, it does charge and gets up to 1%. But if I turn it on, it then turns off after a minute or so and says it's starting to charge again. It doesn't do that when plugged into a wall outlet and once it has started up from the wall outlet, it can continue to charge from the MBP. Before I could charge it from dead from the MBP, though.

It's likely your phone is charging off the 5V rail. The 9V and 15V are not used or "activated" when negotiated from the USB PD specification. See this post iPhone 6S only charges from one side of lightning charger if connected to laptop through USB, but shows green charging icon on both sides for more details as to the build quality of Type C power adapters.

But the issue with not being able to charge from dead persists. It's not bothersome really, but more wanted to find out if it's indicative of anything having been damaged as I am under AppleCare Plus and could have it repaired if I needed to (but given my situation would mean needing to send it away so would rather not if it's OK).

The link above goes into detail about charging from dead. That said, when sourcing a power adapter, spend the extra money and buy quality. Yes, it's much more experience but it ends up being much more cost effective in the long run.

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