LAB is correct, the 2MC only holds a fraction of a seconds electricity. Its sole purpose is to store enough to ensure a reasonably constant supply from the alternator - which would otherwise be a rectified sinusoidal wave - your lights could flicker in the valleys between the peaks.
Owen has deleted whatever he posted, but giving the electrics an 'easier life' (or whatever the words were) somehow doesn't sound too scientific. Capacitors are myriad in electronic equipment though, they do absorb peaks and troughs of voltage - more relevant to digital waveforms and equipment. Than to the analog stuff - like britbike charging systems.