voltage stabilizer has the same principle like those big caps in car audio.
it backs up your battery during high current demand surges since the capacitor releases more energy rapidly than the battery (ex. during engine cranking) but it doesn't help on continuous power demand (load)
it smoothens/minimizes ripples on the output of your alternator/source coil therefore raising the voltage a little bit.
for those who like it diy:
this version has a soft starter (industrial grade)
also, you can add more caps as you please
use at least #10 gauge wires for battery terminals
