What is the source impedance of the thing that supplies the impulse? I will assume it is low because otherwise you have very little power available and if so then it would take an impractically long time to charge up a battery. There are two obvious choices to charge the battery. The first is to let any of the impulses that exceed the battery voltage feed directly into the battery through a rectifier and perhaps a current limiting resistor.
The second is a bit more complicated. You would feed the impulse through a schottky rectifier to a large value capacitor. The charge would build on the capacitor over many impulses until it reached, say, 0.8 volts or more. At that point you can have a switching regulator (boost type) switch on and charge your battery until the capacitor voltage drops below a threshold and then turn the switcher off. I've seen this kind of thing used for charging a battery from a solar cell (I think I saw an app note on Maxim's site, to do with solar cell battery charger circuits.