Let me try and expain power caps a little bit here. Basically, they are just a small battery, capable of holding a charge of electricity. But, this charge can be sent out, or discharged, very quickly. Quicker than the alternator, and quicker than the battery. Lights usually dim because of a voltage sag or drop when heavy bass hits. The amp is drawing its max power, or close to it, and draws alot of current, and the voltage drops because of this, causing the lights to dim. My first step of advise if you have the dimming problem, is to have a good, true deep cycle battery, like the optima yellow/red top. These batteries can discharge a lot of energy very quickly, and much faster than a traditional car battery. Use one of these deep cycle batts, and be sure to use the proper guage power wire. For a 300 watt amp, 10 guage will sufice, but 8 guage will be better. Larger wire has less resistance, and lets more amperage through with less voltage drop and less heat, and less resistance. Now a power capacitor will be a way of "stiffining" the voltage in the car. When the big bass hits, the cap imiadiatley discharges, but the voltage remains somewhat more constant, with less voltage drop, because the cap is a place of electrical storage, that discharges at lightening speeds, much faster than even a deep cycle battery can discharge. This extremely fast discharge is what keeps the voltage from sagging, or dropping. The rule of thumb for needing caps, is 1 farad for every 1000 watts of amp power. Caps aren't a waste of money, but they DO NOT add more bass, or more loudness by using one. Also, by adding a second battery, you will increase the demand on your alternator by 2 times now, so your voltage will sag even more unless you have a high output alternator capable of charging 2 batteries at high output. By adding a second battery, the only benefit of that is to extend the carr off play time. If you have the alternator power to charge additional batteries, then haveing a second battery or more can be very benifical because now you have more electrial storage ready to be used. With a 300 watt amp, i'd say you don't need a cap. If you experience dimming lights, try a optima yellow top battery, and use at least 8 guage power wire. I'd bet that will take care of your problems. By the way, I'm pushing over 2000 watts to my 15" eclipse sub, and my lights dim very little, with no caps. I'm using an optima yellow top, and 4 guage power and ground..I hope this helps..