Dual battery setups, battery sizes

[xan326]

Okay so I'm planning on doing a dual battery setup in my vehicle, and I was wondering if the batteries have to be the same. I don't know too much about the subject so I'm asking. Reason being, the front battery in my car is small, and I can't get a bigger one in unless I relocate it.

I'll be running somewhere around 2180 to 2660w rms on the full system, and somewhere around 2910 to 3630w rms or so after efficiency is factored in.

The stock battery I have no clue on it's specs, I just know it came from walmart or some place, came with the car. I just know I can't get a much larger battery due to it being at the front and the hood would interfere.

The alt right now is either 70 or 80 amp, it's the stock alt. I could change it if I need to, if anything fits.

1995 Corolla 1.8l is what the car is.

Any help is appreciated

Also, I understand the battery side of things, but not the alternator side of things. Alts are measured in amps, but how do you figure Ah for charging the battery? Also how can you figure the step down from 14v to 12v, and how the amount of power is affected?

And for the future, how can I figure out what size of battery I would need for a specific system?

[brent379]

im not sure how you came up with your rms numbers, normally factoring in efficeincy will drop the rms, not raise it. and there isnt a real way to guess your efficiency. and to be honest, even if you clamp everything, that is still not your true rms, there is what you call a power factor, it has to do with the phase of the amps and current and what not which drops your rms number anyway.

ive read that your supposed to run same group size batteries together, so if you have a group 35 under the hood, run a group 35 in the back. but there are also alot of people who run different group sized batteries and have no problems. but no matter what, if you have an agm battery under the hood keep your other batteries agm.

as for the alternator, get the highest amperage you can afford and fit. cuz most likely, like everybody else, your going to eventually want to go bigger. and it would suck to have to buy an alternator twice.

for stepping down you voltage, you do not have to worry about that unless you are wanting to run 14v batteries, they would need to be charged at 16v. factory vehicle systems are 12 volt systems, and alternators charge at 13-14 volts. for instance, my set up charges at 14.7-15.0 when idling. and resting voltage with car off is between 12.7 and 13. depends on if my battery is topped off.

i would HIGHLY advise on an alternator being one of you first investments, you can add all the batteries you want, but your stock alternator will kill itself trying to recharge them. get an alternator, do the big 3 in some quality 1/0, and put a good agm battery under the hood, either xs power if you have the money, or deka, and go from there. once everything is in check your voltage drop and add a battery if you have to.

im running an aq2200 at .5 ohm, i have a 270 HO alt, and a deka 9A35 under the hood, big 3 done in sky high 1/0 ofc. my car idles at about 700rpm, and at idle my voltage drops to mid to high 11s and if i hold the rpms at 2500, i very rarely drop below 13. i will be adding my 2nd battery here very soon.

you have to have strong electrical to keep you voltage up, the lower your voltage is the less power that is available for the amp and the less power the amp can put out.

[xan326]

Thanks. I've always known to figure how much power you need is to take the amp rms and the inverse of efficiency, usually 50% efficiency for A/B amps and 75-80% for D amps. I actually don't know my true rms yet since I don't have all my amps here. I'm running aq/sq 200.2s for front and rears, Q1-1200D for my sub though I may upgrade if I throw in a second sub later on, and I need a 100w amp for my tweeters. The 200.2s @ 2 ohms are rated for 340w per channel, and the 1200D @ 1ohm is rated for 1200w, then I need the 100w for the tweeters, so that would be around 2660w rated rms. Doing the rms + inverse of efficiency all amps being A/B except the mono block, rated rms+inverse of efficiency would be somewhere around 3630w. Just going off of those numbers, and the alt I have right now, and playing for maybe 4ish hours total a day; what size battery would you recommend?

Also I know factory systems are 12v, and alts are 13-14v, but how could I figure what size alt I need to charge a battery when I get to that point? Alts are rated in amps, but batteries are rated in size of amps and list a charge/discharge rate in Ah. Is it something as simple as finding the amount of watts and matching the numbers?

And on an unrelated note, how can I monitor my rpm if my car doesn't have a tach? I'd go out and find a gauge cluster that has a tach if it was easy to but I have yet to find one locally.