[D20MCK]

40 amps @ 4.2 volts is 168 watts, 168 watts @ 15.8 volts is 10.63 amps , its a 200 watt supply mark.

Having never charged @ 40 amps myself or have a charger capable of doing so I'm only going by the above numbers.

Assuming your supply output voltage is 12 volts with a charge rate of 40amps @ 4.2 volts the process must be really inefficient, what is happening to the other 132 watts? That's a lot of power to loose in heat.