Re: Low-voltage DC is just USB now
There's a reason for that - 24W is barely enough to turn those kinds of things on, let alone operate-and-charge at the same time. Even my old laptop would fail to charge on certain 19v chargers - it would accept them, but if you did anything vaguely interesting, the battery charge would fall WHILE it was running on mains... you just need more oomph.
Now, granted, it should warn about that scenario, but there's no way on earth you're ever going to power a laptop in any fashion from an ordinary 500ma/1A/2A USB cable. The voltage is only 5v, for a start, which is not enough to charge a 17/18v battery at all, no matter the current - the physics just isn't there. There's a reason we have 19v chargers and why USB-C's high-power mode is 20v. You'd actually be better off with a PoE charger... at least that can hit 47v!
Phones generally only have a 3.7v or thereabouts battery. That's why they can trickle-charge from just about anything. Hell, you could arrange three AA's and it would charge.
Without voltage transformers (which cause even more loss of power), there's no way "ordinary" USB can charge something like a laptop, especially not if it's running at the same time.
You need the "full" charger, as you say, which is one that provides the 20v negotiation. Not all chargers, cables or devices are capable of utilising it - it was a much later standard and required complete hardware redesign and extra chippery to manage it.