Looks like the light is finally shining!
As to the problem with Battery Chargers and AC/DC (usually built in, but also applies in general to an external supply).
First, no power supply can be 100% efficient. A well designed Switching Power Supply might be about 80% on average.
So if you have a 100W input AC/to DC power supply, 80W is about the maximum usable output.
There are additional losses in the charger that reduce the actual charge capability further.
As an off the wall number to play with, assume that the charger's circuitry is at best, also 80% efficient.
So, our 100W from the AC power source is down to about 64W charge capability under almost ideal conditions.
In practice, the actual charge capability might be something like 50W, Gee, that's familiar!.
With a DC charging source, usually an automotive battery- - -
A fully charged battery is usually about 13.8V, and the DC voltage from the automotive electrical system (engine running)
using the same battery is usually no more than about 14.6V. This translates to a slightly higher voltage than many of the built in AC/DC
supplies includes in the 50W rated chargers, with a slight increase in charge capability.
One of my chargers lists about 40-45W using the internal AC/DC power supply, and up to ~50W when using an external DC source, such as a battery or
vehicle electrical system with the engine running. Allowable DC voltage from an external source is listed as 11-18 VDC.
Many of the 50W chargers can charge up to a 6 cell Lipo using the internal AC/DC power supply, or an external supply.
Obviously, the internal charger circuitry has a switching power supply section that can roughly double the DC input voltage.
Given the small size and fairly light weight, it's rather obvious that the switching circuitry in both the built in AC/DC power supply
and the switching circuitry in the charger itself must run at fairly high frequencies. If the charger manufacturer "cheaped out",
the filtering may be inadequate, possibly causing problems with a vehicle's electronic "stuff" if the vehicle's engine is running, etc.