36V DC charger with 12V DC charger and multiplier circuit?
With a voltage step up of 3x the available current from the output is proportionately decreased by at least a factor of 3, (e.g. I/3).  On top of this the overall efficiency of the step-up system also decreases the available power.  If the output voltage needs to be stable the step-up voltage may even need to be higher than 36V and then regulated back to 36V, creating additional inefficiencies.

With the above you would need to ensure that your available input current meets and exceeds the proportional requirements of the output. This is one issue that users find out the hard way (by continually replacing burned out fuses) when expecting a 12VDC to 120VAC automotive inverter to power medium power AC appliances.