I've seen a lot of systems and have noticed that some have a second battery for their car audio. I've seen cars running a total of 100 watts with an extra battery and other cars with 1000 watts using only the single battery under the hood. So I was wondering, what size should a car audio system be when it becomes a good idea to use a second battery?

Also, in my case, should I replace the battery that came with the p5 with one of those red or yellow ones that you see in all those pics of installs. I'm running 800 watts RMS and will soon be adding another 400 RMS. Should I replace my one battery or just get a second? Would I need a larger alternator too? Does it even matter?

 

A second battery won't do you any good unless you like to park and chill with the stereo on for long periods of time.

With 800w and adding another 400w, i would go for a Optima or SVR battery (i have an SVR) You may be ok on the alternator for a bit, but if you play it loud alot, then i'd look into a higher amperage alternator or have yours rewound.

 

also a cap would be good also because i had 4 10s in my old s-10 and had 1200 watts out of my amp to push them and had stock battery with a cap and never had problems.........and yes all of that will be going in my car soon

 

Really! No problems? Great! I already have a 1 farad cap so maybe I'll be ok if I don't worry about batteries.

 

well true a cap should, i also have a 1f cap, and it helped sonically, but 600watts just for my subs is still diming the lights.
I may add another alt in some frankenstein fashion....

 

After reading your post I went out to my car(in the evening), started it up and turned up the bass. I only had the volume on 17 out of 30, the BBE turned on and the subs set at -12(real low) and the headlights were dimming quite a bit. Looks like I'll need another capacitor.
A battery won't do anything to help this, correct? I'm sure a better alternator would help but I don't want to replace my current one because it is very new and still good. So, by elimination I figure another capacitor would help. Since my sub peaks around 1200 watts, maybe 1 farad isn't enough. What do you think?

 

another battery will not help, another cap may help, but the cap stores energy for short bursts. While is does charge and discharge quickly, faster than a battery can, it's not the fix all device. It's more for sound quality than for power issues. The alternator will need to put out more amps.

 

A second battery will not fix the problem, you will not be running the two batteries in parallel, the norm is to have them on a isolator and just switch back and forth.
Caps may help a little, but make sure to get ones with very low ESR's- some of them can actually be more of a drain than a help. In any case it is deminishing returns on caps, the effect is not linear in the increase. Remember it is for instantanious peaks, not long shots of really loud music.
Get a volt meter, and see what the output it at the amp on the power cables. If it is dropping there then you have an issue.
A unperfect answer is to stick a high output alternator on the car, but that cost buck, and count on Mazda waiving bye bye on any future electrical problems you might have. Even high out has to run some rpm to get more power though. And don't forget there is no such thing as free power, a higher output alternator will require more of the torque of the car to produce those additional amps.
Most all alternators do not produce much current from an idle. If you go to any serious audio competitions, they do not set at idle, depending on the santioning body, you run the car at a specificed rpm, mostly between 2000 and 3000 rpm.
Have someone idle the car at 2500 rpm, and measure the output voltage at the power cable into the amp. Thanks to resistance, and most folks having amps in the rear, long power cable runs can reduce voltage. Consider four guage wire if you are not already using it.
Overall, most every car at ture idle will dim it's lights when the stereo is pushed. Unless the car is not producing the the basic required voltage for all sources (lights, amps, defogger, heater) most stuff you will be able to change will make minor differences, cost a lot and take the car over the edge from a daily user to a more audio competition level beast. I know, I have built and owned several-
By the way an optima is a nice battery but it is a bit of snob thing is audio. The only consideration is getting one is if you plan to play the stereo until thhe battery is "dead" of highly discharged, then a yellow optima will recharge better than many convential batterys. The spiral cell technology is cool but it cost more, and despite some marketing claims, does not really last any longer in normal use.
Your description of 1200 watts is actually peak power draw, not what it can pull all the time, unless you are into burping subs. The peaks are at best transient brief moments. To try to pump the full wattage would be either killing the speakers, which are not really designed for that sound level on a constant rate, or clipping the amps which will not be good for them or the speakers.
If you plan to audio competition, then you will need to bet a bigger alternator. Plan on anywhere from 400.00 dollars up for a good one with the mounting brackets and the adaptor plugs. Ohio generator is the mainstay of the competition alternator companies, they are pricy, and they will tell you if you ask them (but you have got to ask) that there alternator will love every battery on the market except an Optima! The charging rates of batterys is a whole other topic, but charging can make the difference in a battery lasting 12 years or two weeks. On the vechile I am currently using a high output alternator on, a 225 amp unit, I am using a interstate battery that is functioning just fine.
My key observation is that there is a line out there between a nice car stereo, and audio competition cars, and the day in day out usefulness of a car will get compromised as that line is crossed. For each audio enhancement in that realm, for the gain you take away something else. There is a reason that most independents who build a sound car to compete have a second car, and the audio car may look nice, but is a one to ten year old car with 65,000 plus miles on it!