Don,
Reading your statement above ... assuming a power factor of 1.0, note that 15 amps times 120V does not equal 125 amps times 12V.
I guess a more accurate way to state it would be: "15 amps will do 125 amps @ 14.4 volts on a charger with a power factor of 1.0."
What you say is why I keep my little four-cycle, quiet, fuel-sipping, 650 watt Honda generator - it should be able to drive a 35 amp charger putting out 14.4 DC volts if the charger has a final power factor of at least 0.8 - should I ever want to buy another charger. ;)