Discharge the cap at about 2 volts over what is in the battery. You could do a few more volts over with no problem.

The if you reduce the capacitance (uf's), the volts will climb higher faster. But even so, you can make it trigger whenever you want with mechanical switch. With 555 circuit, you would have to adjust discharge frequency based on how often the cap gets to about 2 volts over the batt for whatever particular setup.