is there a minimum advisable charging current for a sleeping MacBook?

Grant Bennet-Alder wrote:

That USB-C charging protocol requires a smart device on each end. No substantial power will flow into your Mac unless/until it has negotiated the terms with the sourcing device.

This introductory article touches on some of the issues.

http://www.electronicdesign.com/interconnects/introduction-usb-power-delivery

Executive summary: This is unlikely to work without USB- type C.

The USB-C MacBook Pros do charge off of 5V USB power supplies, but it's slow. There's someone who tested it and reported results in this thread. The poster used an inline USB voltage/current monitor.

Can I charge my Macbook 2015 from a 2A 5V usb wall charger?

I'm not sure of some of the conclusions though, such as a claim of how long it might take.

That USB-C charging protocol requires a smart device on each end. No substantial power will flow into your Mac unless/until it has negotiated the terms with the sourcing device.

This introductory article touches on some of the issues.

http://www.electronicdesign.com/interconnects/introduction-usb-power-delivery

Executive summary: This is unlikely to work without USB- type C.

Crispin Miller wrote:

I have a MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports) and gave myself a scare by misplacing the charger brick, because all the other ones in the house are MagSafe and won't fit. Thought maybe I should invest in a backup route that can charge it (via its USB-C/Tbolt3 ports). An obvious route, if it'd work, is to get a cable that's USB-C on one end and male USB-2 on the other, and plug the other end into a USB power outlet (iPad charger, iPhone cube, etc.). Which might well offer just a trickle-charge and take overnight with the MacBook off or asleep, but hey, if it got me recharged I'd be happy.

Bounced this idea off a young man in an Apple shop and he claimed that I could cause battery damage by supplying too LOW a charging current. (And/or, perhaps, too low a voltage, by drawing more current than the supply could maintain full voltage for.)

I have trouble believing this. Sure, if I tried to RUN the machine while it was plugged into a supply that provided less than the machine's power need, I'd be draining that battery, but that was not my question. Is it REALLY damaging to the battery, or charging circuit, to connect it to a smaller power source than usual?

Not really. If it were then the battery management system would refuse to charge it at all. Most worries are about charging too quickly. The battery management is designed to safely charge using anything from a USB-PD compliant power adapter (such as the one that came with your MBP) to a low-powered USB-A power adapter.

Also - I'm not sure you have quite the correct terminology. It sounds like you're thinking about USB-A (not USB-2), which has been the standard from USB 1.0 up to USB 3.0. USB-C is for USB 3.1. Most USB cables are "male" on both sides except for some specialty cables such as extensions.

The biggest issue with using a USB-A power supply that is it's going to be almost painfully slow. It's also going to be charging at 5V instead of the 12V from a USB-PD compliant power adapter. There are relatively inexpensive USB-PD power adapters. Here's an example (currently $21.99 from Amazon's US site):

https://www.amazon.com/AUKEY-Charger-Delivery-MacBook-Nintendo/dp/B071H2JGJ6