A cap (power capacitor) stores up power to release when your system is pulling a heavy amount of current from the battery. They are measured in farads. 1 farad is equivelant to roughly 1000 watts. So a 6 farad capacitor is capable of 6000 watts of storage. Your car runs off of a 12 volt system and everything else electric inside of your car is pulling current off the battery. When you add amplifiers to your car it pulls more from the battery then the battery can supply. That is when the cap gives off some of its powere, instead of the battery overworking and eventually dieing as a result. Everytime the cap is used it continuously recharges itself.Sometimes you can turn your car off after a long drive and the cap will stay on until it is recharged fully. It does not drain your battery in any way, however a common fallacy is that they will help stop your lights from dimming when you system is turned up really loud. They dont. Only changing the big 3 will help that. If you are trying to buy a cap, add up how many total watts of power your system contains and round up to the next farad.