[Xringer]

Those volts wouldn't be wasted. Volts are like pressure. Like PSI in a compressor tank.
Amps are 'flow', as in letting the air flow from the tank.. cubic feet per minute
is kinda like Amps, only that's electrons per second..

For nerds, 1 amp is 1 coulomb flowing per second (a coulomb is approximately 6.241×10 to the +18th electrons)


Anyways, if you were to design a battery charger for 12V,
you would start off with a DC power source of at least 15 volts.
And it should the ability to provide some good current flow at that voltage.

The tricky part is to sense when the battery is up around 14v and ease back on the current, so as not to over-charge.

But, if I had to design a 12v charger using a 100 volts source,
it's not going to be easy, and the parts will be a lot more expensive.
Yes, there will be a bit of waste, as the output transistors turn off and on, they will run hotter..
My 48v charger never gets hot. Warm maybe.