I've tried this sort of thing with USB power banks, which generally have no idea what they're being connected to nor what exactly is charging them. Obviously that just drains it eventually via losses. But I hadn't tried it with my MagSafe 3 cable before on a Mac - connecting it to one of its own USB-C ports. MagSafe 3 is really USB-C on one end and the latest version of Apple's proprietary MagSafe auto disconnect power input on the other end. But these are really just a means to connect USB-C power and I've tried using all sorts of power sources, including 5V USB power banks and relatively low powered USB-C supplies or even USB-A supplies with an adapter.
It was really weird too. It took a while to stabilize. Originally it stated that was connected (little lightning in the center of the battery icon) to power but also stated that it wasn't charging. After maybe a minute it was saying that it was charging. In System Report it's reporting a 7W power adapter, and it has no ID # like most AC power adapters. And the really weird thing is that it's giving me an estimated time until it's fully charged. So either I have a perpetual energy machine, or it's calculating based off of numbers that simply won't add up. I'm guessing it's not accounting for the current power drain from USB-C but calculating the estimated time based on the current power input level alone.
I also tried connecting a USB-C cable to two USB-C ports on the same machine. Charging through a USB-C port is secondary, but it can theoretically be charged with any USB-C source through one of its USB-C ports. It asked If I wanted to connect one Mac to another Mac, and apparently it has no idea that it's connected to itself. In System Report it's indicating that it's connected to a 15W AC power adapter. It always lists "AC Power Adapter" even if it's connected to a DC source like another Mac or a USB power bank. But it is slowly draining the battery by trying to provide power that is then trying to charge. It also takes a few minutes before it starts giving an estimate for when it will be fully charged.
I suspect that it may not actually be charging the battery, but that's just the way it's reported.
It was really weird too. It took a while to stabilize. Originally it stated that was connected (little lightning in the center of the battery icon) to power but also stated that it wasn't charging. After maybe a minute it was saying that it was charging. In System Report it's reporting a 7W power adapter, and it has no ID # like most AC power adapters. And the really weird thing is that it's giving me an estimated time until it's fully charged. So either I have a perpetual energy machine, or it's calculating based off of numbers that simply won't add up. I'm guessing it's not accounting for the current power drain from USB-C but calculating the estimated time based on the current power input level alone.
I also tried connecting a USB-C cable to two USB-C ports on the same machine. Charging through a USB-C port is secondary, but it can theoretically be charged with any USB-C source through one of its USB-C ports. It asked If I wanted to connect one Mac to another Mac, and apparently it has no idea that it's connected to itself. In System Report it's indicating that it's connected to a 15W AC power adapter. It always lists "AC Power Adapter" even if it's connected to a DC source like another Mac or a USB power bank. But it is slowly draining the battery by trying to provide power that is then trying to charge. It also takes a few minutes before it starts giving an estimate for when it will be fully charged.
I suspect that it may not actually be charging the battery, but that's just the way it's reported.
