In a tritium-powered betavoltaic cell, the power output obviously decreases as the tritium decays. Is it the voltage, amperage, or both that would decay?
The voltage, in theory, is simply the energy of the betas. These don't change, they are defined by the substance.
The amperage is the number of betas, which is simply a count of the remaining tritium. This, obviously, reduces over time.
So the voltage is largely constant and the amperage falling off with the half-life.