AirDrop randomly failing between iPhone XS Max (iOS 18.0.1) and MacBook Air (Sequoia 15.0.1); please help troubleshoot and fix

1. I don't know, because I bought it refurbished in 2019.

OK.

2. 85%

That's OK. (<80% is the critical point.)

3. How could the battery cause AirDrop to fail?

AirDrop relies on the iPhone's Bluetooth & Wi-Fi antennas being sufficiently powered by the battery to maintain a connection between devices.

4. Is it not poor software design if the device's battery can interfere with code execution?

No, all code execution in the integrated chips requires sufficient power from the battery.

...As the battery appears to be okay, wait for suggestions from others here and also an iOS update is likely coming very soon.

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ethereality wrote: "...it's not apparent any update will be released for the XS Max."

I wouldn't expect a third party article to make that apparent. You'll just have to apply the next macOS & iOS updates and see if they fix the AirDrop problem. If not:

Contact Apple for support and service - Apple Support

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ethereality ~ The long process of applying an iOS update to an old iPhone with a weak battery can be the proverbial "straw that broke the camel's back." So...

Does your iPhone XS Max still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health & Charging

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ethereality wrote: "I bought it refurbished in 2019."

On second thoughts, five years is a long time for a battery to still be healthy at 85% maximum capacity. So I wonder if that's a false read-out. An Apple store can test the actual health of the battery.

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ethereality wrote: "Thank you for this information. I had not thought of the circuitry. Why do you think an update is coming soon?"

You're welcome. See HERE.

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