Time for a beefier alt, or do I need another battery?


bananapehl777

CarAudio.com Newbie
Mar 27, 2020
4
1
Aurora, CO
Hello all,

I'm brand new to this forum and I am really excited to connect with everyone else who's as obnoxious as I am driving their cars down the street! But I do have a question for those who are a bit more experienced than I am, as I don't want to waste money throwing upgrades at my system that it doesn't need.

I currently am running 1700W RMS in my 2005 Subaru Outback XT (it has all the bells and whistles, from heated seats to heated mirrors and wiper de-icer, basically there is a lot of factory electrical parts) and ever since I installed the system, I've had headlight dimming and sometimes at an idle, my engine RPMs will drop when the bass hits. Currently, I have a DC Power 270 AMP alternator, Big 3, and an Interstate MTP 35 as my only battery. My question is, is the headlight dimming and engine RPM loss due to the fact that my alternator is too weak for my system, or is my battery too weak for my system and I need to add a second one? I have a feeling I need to upgrade to a second battery, but others have told me that the symptoms I'm having is due to the alternator, and that adding a second battery will make it worse because it puts more of a strain on the alternator. I believe DC Power says at idle my alternator puts out 180 AMPs, but I'm not sure what that means, because I'm not sure if engine revolutions is the same as alternator revolutions (i.e. if my engine is idling at 700 RPM, is the alternator revolving at that too?) Any advice is greatly appreciated!
 

Bobbytwonames

Professional Fisherman 🎣
Aug 28, 2018
1,808
-193
You really shouldn't be getting headlights dimming with your setup. Try a good sized AGM first. Just my $.02
 
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bananapehl777

bananapehl777

CarAudio.com Newbie
Mar 27, 2020
4
1
Aurora, CO
You really shouldn't be getting headlights dimming with your setup. Try a good sized AGM first. Just my $.02
Thank you, I may go with the AGM first and then go from there. When I was planning this system, I was trying to make sure that the electrical system wouldn't be a problem, yet here we are lol.
 

Popwarhomie

Team Lethal Pressure
10+ year member
Jan 22, 2010
7,574
928
FL
Halogen headlights are very sensitive to voltage drop. 1700 rms before rise shouldn't cause any crazy voltage drop. You shouldn't be seeing any drop with a DC power 270 as well.


I would guess your battery is dying.

You are going to have RPM drop. As the alternator will strain the engine when its working hard. In my 2010 I have to turn the stereo down to get on the highway.... lol
 
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bananapehl777

bananapehl777

CarAudio.com Newbie
Mar 27, 2020
4
1
Aurora, CO
Halogen headlights are very sensitive to voltage drop. 1700 rms before rise shouldn't cause any crazy voltage drop. You shouldn't be seeing any drop with a DC power 270 as well.


I would guess your battery is dying.

You are going to have RPM drop. As the alternator will strain the engine when its working hard. In my 2010 I have to turn the stereo down to get on the highway.... lol
I did think my battery was dying, so I just replaced it with a brand new one, but the same model, the Interstate MTP 35. The headlight dimming is still there, but not as bad I think. I think the addition of an AGM is going to help considerably. I may also install a voltmeter just to keep tabs on the system when I install the second battery. I have heard that halogens are very sensitive, but even the dash lights (which are not halogen) dim a little with bass notes. What my system really does not like is the punch bass where the subs are playing quick, short bursts. Makes my headlights into strobe lights almost lol.
 

ThxOne

AudioFreak
Jun 8, 2018
1,794
316
Saint Marys, GA
Remember, the alternator is NOT what is powering your stereo. When the stereo demands power it pulls it from the battery as the battery is designed to discharge large amounts of power quickly. The alternator will then kick in and slowly fill the battery back up and will only do this at a set rate, set by the voltage regulator. Pretty sure you did well upgrading the Alt as the stock one is about 110a.

So, you have plenty of alternator power to keep up with things and then some. You have purchased a new battery so that should not be an issue either. This "Big 3" you have done... what cables are going where and what size are they? They should be no smaller than 0awg and should be pure copper wires. How are these cables terminated? What battery terminals are you using? It is ok for headlights to flicker... not dim. A quick flicker when heavy bass notes hit is pretty normal with higher power if everything is good and tight. Dimming is a sign that there is a bad battery, too small of cables and or bad/poor grounds. Lastly an amp pushing a load it is not designed to push could cause the amp to draw too much current.
 

Bobbytwonames

Professional Fisherman 🎣
Aug 28, 2018
1,808
-193
Remember, the alternator is NOT what is powering your stereo. When the stereo demands power it pulls it from the battery as the battery is designed to discharge large amounts of power quickly. The alternator will then kick in and slowly fill the battery back up and will only do this at a set rate, set by the voltage regulator. Pretty sure you did well upgrading the Alt as the stock one is about 110a.

So, you have plenty of alternator power to keep up with things and then some. You have purchased a new battery so that should not be an issue either. This "Big 3" you have done... what cables are going where and what size are they? They should be no smaller than 0awg and should be pure copper wires. How are these cables terminated? What battery terminals are you using? It is ok for headlights to flicker... not dim. A quick flicker when heavy bass notes hit is pretty normal with higher power if everything is good and tight. Dimming is a sign that there is a bad battery, too small of cables and or bad/poor grounds. Lastly an amp pushing a load it is not designed to push could cause the amp to draw too much current.
It always goes back to math for the most part. Variations in cars is a factor, but math is math. Do your math and adjust accordingly. Every car is different. My car was a unicorn. Modern cars have a lot of shit going on with computers and stuff. "It's better to be a 1/2" too big than a 1/2" too small," That's what she said.
 

yamomluvsme

itssobig
Dec 4, 2018
349
16
Remember, the alternator is NOT what is powering your stereo. When the stereo demands power it pulls it from the battery as the battery is designed to discharge large amounts of power quickly. The alternator will then kick in and slowly fill the battery back up and will only do this at a set rate, set by the voltage regulator. Pretty sure you did well upgrading the Alt as the stock one is about 110a.

So, you have plenty of alternator power to keep up with things and then some. You have purchased a new battery so that should not be an issue either. This "Big 3" you have done... what cables are going where and what size are they? They should be no smaller than 0awg and should be pure copper wires. How are these cables terminated? What battery terminals are you using? It is ok for headlights to flicker... not dim. A quick flicker when heavy bass notes hit is pretty normal with higher power if everything is good and tight. Dimming is a sign that there is a bad battery, too small of cables and or bad/poor grounds. Lastly an amp pushing a load it is not designed to push could cause the amp to draw too much current.
the alternator is a generator except its output is DC. it can and will power everything and its constantly charging. voltage sag is the problem capacitors are ideal for this situation. put a small cap inline with the lights. or get a cap bank or VERY low esr battery. deep cycles have a much higher ESR. don't use them for car audio
 

n2audio

OPTIDRIVEN
10+ year member
Dec 29, 2001
5,687
56
Lawrence, KS
Remember, the alternator is NOT what is powering your stereo. When the stereo demands power it pulls it from the battery as the battery is designed to discharge large amounts of power quickly.
Is that a typo? Because anyone familiar with the basics of automotive charging systems knows the alt is always the primary source when the engine is running.
 

ThxOne

AudioFreak
Jun 8, 2018
1,794
316
Saint Marys, GA
It is not a typographical error. To prove my point, remove your battery and start your car. The alternator is like a battery tender/charger in a way when the vehicle is running. Which is why it has a voltage regulator. When it senses the drop in voltage (from the battery) it will supply more power to the battery to bring it back up to the correct voltage. The battery is what is connected to the fuse panel, not the alternator because the battery is supplying the power, the alternator is maintaining the battery. Can an alternator run a car, sure, but that is NOT its job.
 

ScottyBoyBasshead

CarAudio.com Newbie
Feb 26, 2020
11
-1
Kent, Ohio
Hello all,

I'm brand new to this forum and I am really excited to connect with everyone else who's as obnoxious as I am driving their cars down the street! But I do have a question for those who are a bit more experienced than I am, as I don't want to waste money throwing upgrades at my system that it doesn't need.

I currently am running 1700W RMS in my 2005 Subaru Outback XT (it has all the bells and whistles, from heated seats to heated mirrors and wiper de-icer, basically there is a lot of factory electrical parts) and ever since I installed the system, I've had headlight dimming and sometimes at an idle, my engine RPMs will drop when the bass hits. Currently, I have a DC Power 270 AMP alternator, Big 3, and an Interstate MTP 35 as my only battery. My question is, is the headlight dimming and engine RPM loss due to the fact that my alternator is too weak for my system, or is my battery too weak for my system and I need to add a second one? I have a feeling I need to upgrade to a second battery, but others have told me that the symptoms I'm having is due to the alternator, and that adding a second battery will make it worse because it puts more of a strain on the alternator. I believe DC Power says at idle my alternator puts out 180 AMPs, but I'm not sure what that means, because I'm not sure if engine revolutions is the same as alternator revolutions (i.e. if my engine is idling at 700 RPM, is the alternator revolving at that too?) Any advice is greatly appreciated!
Yeah, you should add an XS Power battery. I got the D3400 and my headlight dimming is much more faint, but I've heard that if you got halogen headlights, it's gonna be inevitable. Your amp demands so much power that you're gonna see it in the headlights. If u got a upgraded alt, then a agm will work wonders for ya. I'm running 2000 watts on a factory 150 amp alt and xs power d3400 agm and seen a major difference. But, I still technically should get an alt. Just don't wanna spend the money.
 

BCotrell

Senior VIP Member
May 27, 2015
975
178
irvine, ca
Is that amperage rating 180 for 700 rpms? The newer Subarus tends to be on the tail of gutless unless you have their teenager cars (for the sake of efficiency they promote) which correlated to generally lower idle rpm than a ho alternator may be designed for.

anyways, if you’re listening to the stereo with engine on disregard posts nerding out on alt/battery stance...car is on alternator is providing more than enough amperage to feed a 1700 watt amp musically

-Are Big 3 grounds all good? What wire you use?
-at the amp what is the voltage (Drop) from baseline(14 ish with car on) that you’re seeing on when you have music up and car is on?
 

n2audio

OPTIDRIVEN
10+ year member
Dec 29, 2001
5,687
56
Lawrence, KS
It is not a typographical error. To prove my point, remove your battery and start your car. The alternator is like a battery tender/charger in a way when the vehicle is running. Which is why it has a voltage regulator. When it senses the drop in voltage (from the battery) it will supply more power to the battery to bring it back up to the correct voltage. The battery is what is connected to the fuse panel, not the alternator because the battery is supplying the power, the alternator is maintaining the battery. Can an alternator run a car, sure, but that is NOT its job.
This is wrong. Totally and completely wrong.
You have NO understanding of how sources of electrical power work.
 
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ThxOne

AudioFreak
Jun 8, 2018
1,794
316
Saint Marys, GA
You should read and understand this: http://www.bcae1.com/charging.htm
Doesn't get much more basic than that.
I am sorry that this is what you are basing your argument on. However, it is not 100% factual. I will agree with you that the alternator is a necessary part of the automotive charging system.

How many amps of current does your stock cars electronics demand to work properly from the factory?
Engine Computer, Injectors, Body Control Module, A/C System, Factory Stereo, Head Lights, Wipers, All Sensors on the Engine, All Sensors on the Body, TPMS, Fuel Pump, Oxygen Sensors...

Lets say your stock alternator is 135amps
Lets say your stock max electrical load pulls 200amps
The car still runs even though the max amp output of the alternator has been exceeded
The battery is capable of supplying the current needed to fill that deficit because that is what it is designed for... not just starting the car.
Hell, let's match the alternator, let's say the car will pull a max of 135amps. Better hope there isn't a hidden load somewhere right.
But we know it is fine even then because there are several hundred more amps of available current sitting there waiting to be used.
Now you add your aftermarket stereo with its (just for an example) 100amp load. Now you are WAY over the max output of that alternator... but the stereo still plays. Thank the lord for the stored energy in that battery.
The energy in that battery will still diminish even with the alternator charging it so to compensate, the output of the alternator is boosted higher via voltage a few volts above the resting voltage of the battery. So now you are charging your battery with a 2v 135a power supply. - In this example.

Lastly, automotive batteries weren't designed for car audio use. They are designed to run a 12v electrical system. The majority of car audio amplifiers require 14+v for their "rated" power. This is one reason that 14v and 16v batteries were invented for the car audio industry.

It is always a good idea to upgrade to a higher output alternator to help recover/charge the system quicker.
 
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bananapehl777

bananapehl777

CarAudio.com Newbie
Mar 27, 2020
4
1
Aurora, CO
UPDATE: Added an XS Power 3400D AGM to my brand new Interstate MTP 35. Seems to have mostly resolved the headlight dimming issue. At an idle tho, the RPMS will drop when the bass hits full tilt, but I am assuming this is normal, similar to when cooling fans turn on ect. But after they drop, it goes back right to the original idle speed very quickly even if the subs are still playing. They don't drop enough to shake the car or stall it either. Here's a picture of the install. Never heard my system sound as loud as it does now. Thanks everyone for the advice!
 

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n2audio

OPTIDRIVEN
10+ year member
Dec 29, 2001
5,687
56
Lawrence, KS
I am sorry that this is what you are basing your argument on. However, it is not 100% factual.
It's not what I'm basing my argument on, but it is certainly 100% accurate. It provides the fundamentals, or basics as I originally stated.

I didn't really disagree with your dissertation other than your original statement that the alt isn't powering the system. It is absolutely the primary source, this is known because it operates at the highest voltage. The battery is ONLY the source for momentary high current demand and at times when the alts peak output is exceeded. If those two things create a situation that makes the battery the source with much regularity, that alt won't last long.
 

Lasherž

CarAudio.com VIP
Apr 27, 2020
312
31
United States
I am sorry that this is what you are basing your argument on. However, it is not 100% factual. I will agree with you that the alternator is a necessary part of the automotive charging system.

How many amps of current does your stock cars electronics demand to work properly from the factory?
Engine Computer, Injectors, Body Control Module, A/C System, Factory Stereo, Head Lights, Wipers, All Sensors on the Engine, All Sensors on the Body, TPMS, Fuel Pump, Oxygen Sensors...

Lets say your stock alternator is 135amps
Lets say your stock max electrical load pulls 200amps
The car still runs even though the max amp output of the alternator has been exceeded
The battery is capable of supplying the current needed to fill that deficit because that is what it is designed for... not just starting the car.
Hell, let's match the alternator, let's say the car will pull a max of 135amps. Better hope there isn't a hidden load somewhere right.
But we know it is fine even then because there are several hundred more amps of available current sitting there waiting to be used.
Now you add your aftermarket stereo with its (just for an example) 100amp load. Now you are WAY over the max output of that alternator... but the stereo still plays. Thank the lord for the stored energy in that battery.
The energy in that battery will still diminish even with the alternator charging it so to compensate, the output of the alternator is boosted higher via voltage a few volts above the resting voltage of the battery. So now you are charging your battery with a 2v 135a power supply. - In this example.

Lastly, automotive batteries weren't designed for car audio use. They are designed to run a 12v electrical system. The majority of car audio amplifiers require 14+v for their "rated" power. This is one reason that 14v and 16v batteries were invented for the car audio industry.

It is always a good idea to upgrade to a higher output alternator to help recover/charge the system quicker.
I don't think anything on http://www.bcae1.com/charging.htm disagrees with what you said about exceeding the rating. It says, "As long as the engine is running, all of the power for the accessories is delivered by the alternator. The battery is actually a load on the charging system. The only time that the battery would supply power with the engine running is when the current capacity of the alternator is exceeded or when engine is at a very low idle."

I think you have a decent understanding of that, but this example of yours in particular is problematic.

The energy in that battery will still diminish even with the alternator charging it so to compensate, the output of the alternator is boosted higher via voltage a few volts above the resting voltage of the battery. So now you are charging your battery with a 2v 135a power supply. - In this example."
The alternator is always at the same voltage, usually 13.8 to 14.2v some as high as 14.5v as determined by its voltage regulators. The difference in an alternator under load vs not is the amount of current it's putting out and it's regulated through the resistance of the system (particularly the battery's internal resistance which is why you shouldn't disconnect your battery while the car's running). When the voltage drops, not only is your alternator put at full torque due to low resistance loads from accessories, it's also going to continue a heavier load until the battery is recharged and the internal resistance is once again elevated.

It seems like some of that was probably based on misunderstandings so I don't want to come down hard on you, but the example in particular wasn't in line with reality as far as how alternators work.
 

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