It might be easier if you convert from joules to Watt/seconds... 500 joules = 500 Watts per second. So, regardless of the Voltage, you have to deliver 500 Watts per second. And Watts are Volts multiplied by Amps.
So if Volts go down, Amps have to go up, and visa versa. Your only trick to do now is to make sure it all happens within the proper time domain. Do capacitor banks ring a bell?
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