I got my stereo put in a couple of weeks ago and I didn't realize how much it would pull the battery down.How do I figure what size of a capacitor to get?

Pioneer DEH-P7700MP
Polk Audio 6.5 DB650
Polk Audio 6x9 DB690
2 Polk Audio 12's in ported box
Bazooka Amp BA1500 750 watts-subs
Bazooka Amp BA460 600 watts-front speakers

 

Polk Dual db124 12” Subwoofers with Enclosure (DB212)

Here are the subs

 

Basically the way I figured mine out was by going for overkill... LOL I started looking for a cap on e-bay and found a Voodoo 1.2 farad cap for like $80. I jumped on that and I love the cap it's done nothing but good for my car and stereo. The higher capacitance the better off you'll be. Basically picture it like this. You have a CD player. It takes 3V to run (2 AA batteries) now lets say it's an OLD CD player that eats batteries for breakfast. So in 2 hours you've burned through a set of AA's wouldn't you rather have C batteries that last 8 hours then? The cap basically acts as a second battery for you with it building a charge and holding it dispensing it as necessary. When there is a draw on teh system (When bass hits) the cap takes the slack instead of putting the extra strain on the battery/alternator. That way the alternator works at a steady pace instead of harder when teh bass hits. The bigger cap the more charge is stored. I'd say you could do with a 1.0 farad or 1.2 should be plenty for you. I suggest running the power line directly to the cap then running the - of the cap to your splitter then to each amp.

 

The rule I follow is for every 1000 watts you need 1 farad worth of cap. I got a .5 farad cap bc my system is only 300 watts.

 

The primary source of power when the vehicle is not running is the battery. The primary source of power when the vehicle is running is the ALTERNATOR. This is a very important point so remember it. The primary functions of the battery are to start the vehicle and provide power when the vehicle is not running.
Depending on your car's factory electrical system, you may not have enough power to run your aftermarket stereo system. You probably do unless you have a high output system or an extremely low output alternator.
Adding a Second Battery
Many people are under the impression that adding a second battery will cure their electrical problems. If one battery is good then two must be better. Right? Wrong. Remember that the function of a battery is to start the vehicle and to provide power when the vehicle is not running. The only thing a second battery will help you with is "parking lot" listening time. Basically it will provide you the additional capacity to run your stereo longer without starting the vehicle. After the vehicle is started the second battery becomes another load on an already overworked alternator. If you do replace your battery I would recommend getting an Optima Gel Cell
Upgrading Your Alternator
Frequent dead batteries are a good indication that your electrical system isn't keeping up and you may need to upgrade your alternator. This will give you more juice to run all of your electrical components, including your stereo system, without looking to the battery for help. The problem is your alternator is undersized for your new electrical requirements and has to look to the battery to provide the extra power, resulting in shortened battery life.
Power Capacitors
Another solution is to add a capacitor. This device acts as an electrical storage "buffer". It stores power until needed for high electrical demands such as heavy bass notes. If your headlights dim when you have you stereo cranked then you may benefit from one of these. go to howstuffworks.com and type in capacitor to find out more info about capacitors