The inventive subject matter provides a circuit and a method for efficiently charging a battery. In one aspect of the invention, the circuit includes an oscillator that generates a series of current pulses at a frequency that corresponds to a resonant frequency of the battery. In some embodiments, the series of current pulses includes constructive resonant ringing that is constructive with respect to the charging of the battery. The constructive resonant ringing includes decaying oscillation of currents generated in response to a current pulse.