First off you wouldn't connect a battery directly to the output without being sure it was designed for that. You would need some kind of a regulator circuit to adapt the changing generator output voltage to the charging requirements of the type battery you are using. And the battery in itself is essentially a low-pass filter to which the RC network would add nothing except loss and what 'high-frequency spikes' you talking about from a slow RPM DC generator? Without carefully matching the generator output to the specific battery you could under charge the batt rendering the whole project disappointing, or overcharging the batt reducing battery life and/or damaging the generator. Other for that it sounds like a good idea.