secondary battery, diode isolator, am I going to burn it down?

[Eamon Conheady]

Hello! I am brand new here to this forum, but so far, I have found it INCREDIBLY useful; seems like a great community!

So, here's what I'm thinking:

In my '95 Subaru Impreza wagon, I have:

-Higher output alternator (140A, while factory is 80A I think)

-Standard battery up front

-Upgraded "big 3"

-Rockford Fosgate T1000-dbCP 1000watt amp

-2x 12" Alpine type-r subs - in a sealed box

-crummy, dying streamline 90W x 4 inline door speaker amp (worthless)

-German Maestro 2-way speakers up front, Alpine Type-S in rear strut column (hatchback)

-Sony GS810BT head unit

I am going to be adding:

-Rockford Fosgate T400 or T600 4-channel amp (60watt rms or 100watt rms, respectively)

-New speaker wire to run from trunk to door speakers (I'm guessing 14 or 12 gauge, respectively)

-Would either do a stiffening cap or the secondary battery in the rear for the two amps

-A diode battery isolator

I already have 4 gauge power from the battery to the 1000W amp. I'd like to not have to replace that with 0/1, and instead use an isolator to charge the rear battery, and allow the amps to draw on that. In theory, I think this would eliminate the need for large gauge power lines coming to the back since it shouldn't ever need those huge draws, only the amount/rate that the battery recharges at.

What do you think? Will that work?

QUESTIONS:

What kind of draw would a battery pull from the alternator, through the isolator, to charge the battery while running the audio?

-This would allow me to determine what size of isolator I would need. I've been told that even an 80A would be over kill because batteries don't recharge beyond 40A?

What size battery would I require for this set up? Since it cannot recharge faster than it can draw, I feel like any size would be fine for shorter drives around town, but getting out into 4+ hour trips, I think the battery would be discharging quite low no matter the size. After looking at a bunch of comparisons in these forums, it looks like XS Power is the only battery to consider.

Am I incorrect on thinking that?

Is there a monitor to tell how quickly the secondary is discharging?

Does anyone have a diode isolator that they recommend? I was really liking the quality on charging capabilities of this one: http://www.invertersupply.com/index.php?main_page=product_info&cPath=263_266&products_id=1166

I like the feature that allows it to bypass/top-off the battery without the voltage drop incurred by the system - at least, that's what I think it does...

If I am mistaken on any of this, or you see somewhere that you think my plan can be improved, please do tell me about it!

Thank you so much for your help!

[im_quagmire]

i read most of your post but not all, but as I've read enough to ask.. why are you going to run an isolator? Just Upgrade your front battery with the biggest battery XS has to offer for your vehicle, and a smaller battery to supplement your amps. IMO no isolator is needed. Those are used for more complex vehicles with multiple accessories. For bass and mids/highs, you really do not need one. Run an AGM for car audio. cheers

[Eamon Conheady]

I'd like to add the isolator to protect the rest of my car's systems. I still get voltage drops and some headlight dimming when it's pounding, and that'll get worse with the addition of another amp. I was considering a cap, but a battery can accomplish nearly the same thing, but provide other benefits. I'd prefer to not put a huge battery up front, which would then require 1/0 line being ran to the rear.

I'd like to be able to limit the amount of power that can be transferred from front to rear, to something that the alternator can keep up with and not cause the voltage drops across the whole car. It would force the audio to run on the rear battery while leaving the front battery out of the equation. If the limit could be like 80A, or less even, it would never cause undue strains on the charging system. It'd be nice to not be destroying alternators, which happens when the system can draw more than the alt can put out. I'd be OK with a small battery for driving around town, but it could really drain the battery on longer road trips if I wanted to bump it loud and draw more power from the rear than the battery being charged with, for hour after hour of driving.

Just let the system draw the rear, and limit the drain on the car's charging system to like 80A.

[im_quagmire]

you'll be fine with what i recommended.

As of now i have a d3400 up front running 2800 watts total. I have not done my dual battery set up yet, and I only dim while at a light. I say that to say this...

You will be running 1400/1600 watts max. with a upgraded battery under hood and one to supplement in the rear, you'll have plenty to spare. Go with XS and you'll be fine. no need for an isolator. your draw will balance out with the dual battery setup. No need to get crazy with electrical as your pull isn't outrageous.

[Eamon Conheady]

Ah, that is a pretty sweet set up! I'll have to take another look at this option. The big deterrent is needing to purchase two expensive batteries and then running all that thick power cable, that when I get a new car soon, I'd have to likely buy all new 1/0 line...

I think what I'm looking for is a "charge controller" like what is used for solar panels. It caps the amount of power being allowed to flow to the batteries. Instead of coming from a solar panel, I'd want it to come from the alternator. I think that the isolator would then prevent the load equalizing potential. In this regard, I could get a small XS Power battery for the rear, and limit the flow to it. Perhaps it could be adjustable, so if I was driving during the day, I could let more power go to it since the car wouldn't have all those accessories running.

Also, it's not just for the practicality of the current system. It's also fun to stretch things and make it interesting! I like a good puzzle. Also, if I get the infrastructure in place for more powerful systems, I'll likely upgrade piece by piece and utilize it all!

Does anyone know much about charge controllers and/or a set up like this?

[ShadeTreeMechanic]

If you add a battery in the back then that will stiffen it up pretty good. Hook it straight parallel and put a fuse at each battery in case the power run gets pinched. Anything you do to prevent the rear battery from charging will cause the voltage to sag and potentially hurt your amp. The only real way to limit the current to the rear battery is with the volume knob. If you put 1/0 in your car you can always pull it out when you sell it. Don't let the small things keep you from doing it right. Just hook it up like normal and let do what it does.

[Broke_Audio_Addict]

Worst idea ever,

You bought some expensive amps but want to cheap out on the charging system, what sense does that make?

Nothing good will come from what you're wanting to do, either spend the money required to power it properly or don't run it because there is a high chance you'll burn this stuff up.

[Eamon Conheady]

I appreciate the input. I know it seems like a strange idea for powering it. I just don't usually keep my cars around long enough to make it worth spending the money on a HO alternator.

I'm not looking to be a cheap a$$ by not wanting to spend the money on an alternator, I'm wanting to have a fully powered system that I can move from car to car instead of leaving behind yet another HO alternator. This recommendation came pretty strong, and makes sense once I looked at the science behind it:

Using this: http://www.hellroaring.com/bic75150.php

It is a two pole isolator; power in from alt, power out to battery. Doesn't allow it to flow the other way.

Two ways to set it up, starting battery isolation OR auxiliary battery isolation.

The Auxiliary Battery Isolation method seems like it's what I'm looking for.

*Auxiliary Battery Isolation*

-leave the starting battery connected to the alternator as normal

-run that 20' 8 gauge line from the alt, through the isolator, and on to aux battery, fused with 60A

This allows the car's battery and power system full demand.

It should limit the amount of power going to the aux battery for the system to draw to 60A. It wouldn't burn off the excess power as heat either, which would defeat the entire point of doing this system by just adding even more demand.

*starting battery isolation*

-disconnect starting battery from alternator

-connect alternator to the auxiliary battery using large gauge wire

-use a second 20' section of 8 gauge wire from alternator to that isolator, then from the isolator to the starting battery

-60A fuse close to starting battery
Supposedly the resistance in that length of wire will act as a limiter on how much demand that system can place on the alt.

The starting battery doesn't need much power once car is running, allowing the rest of the alt's output to go to the aux system.

Almost seems pointless IMHO, although it would probably reduce any chance of headlight dimming or more apparently the speed of the blower fans.

** for the aux isolation method**

I was told that by having 20' of 8 gauge wire ran like this, only about 60A will get through to the battery, allowing the battery to be the reservoir and provide all additional power needed by the audio system, albeit for limited times of extreme demand (dependent upon the amp-hour rating of the battery and the size of the draw deficit). Then when the music ISN'T hitting hard, the 60A will continue to flow into the battery and recharge it, preventing excessive demand on the alternator.

I could even potentially throw in something like this: http://stores.tgelectronics.org/n8xjk-120amp-booster/

That would make the decreasing battery voltage not affect the audio system.

I could put a couple of the XS batteries in parallel in the trunk to create a larger reservoir so the deficit could go on for a while, like on a road trip.

I'm not sure about the real life draws of the system. Rockford's website says that average draw for the T1000 is 70A (max 140A) while the T600x4 is 40A (max 80A). I doubt that most of the time I am drawing anywhere near that kind of power, and if I am, I've already put my earplugs in.


Any thoughts on how well that could work? If the 20' line would actually act as a pseudo-limiter? I was told that it wouldn't do it through heat loss, or pose a fire hazard. I question that though... lol
Thanks for sticking with me on this one! I know the easiest way is to get a bigger alt, but I'm sure there is a way to design something really interesting that could be done safely!

[ShadeTreeMechanic]

If you don't want to get a bigger alt then don't. Your amp is going to need every bit of power it can get from the alt. Don't try to limit that. The best you can do is hook up an extra battery in parallel on 1/0 with a fuse at each end and call it a day. I know its fun to figure out different ways to do things but everything is a compromise. All this stuff about different ways to charge the batteries so the lights don't dim are pointless because it comes at the expense of potentially damaging your amp or your car or both. Use the search function to learn all you can about this would help instead of making the longest posts I have ever seen.

A wise man once said "Do it right or don't do it at all; If you can't do it right then don't do it".

[Jhunt94]

It makes no sense to try and starve your system. Trying to do what you talking about will not only limit the amount of current, but the rate at which the secondary battery will able to be recharged. When you have voltage drop you want the charging system to react and bring the voltage up as quickly as possible, I wouldn't be surprised if you lengthened that time with the set up that you are trying to do.