The Hydra ZPM makes a step further, letting behind the Hydra X concept, which was battery powered. Instead, ZPM is using a ultracapacitor bank consisting of 2 x 310 Farads (1 Farad equals 1.000.000 uF) which will provide lower internal resistance and longer life and reliability, compared to a battery solution.
This charging of this huge capacitance is no trivial task, therefore is being done by specialized circuitry. Once capacitor bank is fully charged (it may take up to 4h first time!) , the output voltage will remain constant assuming that Hydra ZPM is left powered on.