Power Source for Amp: Battery vs. Distribution Junction

[MyFriendScott]

Read some tech at http://www.madelectrical.com/electri...hreewire.shtml today regarding the differences between 1 and 3-wire alternators. It was definitely a good read, but also clued me in to how hooking up my amp's power source directly to the battery might have been a mistake.

A quick back-story: Last weekend I was driving the car at night, headlights and stereo on, first time actually for doing both at the same time since installing the new sub and amp. While driving at night, I noticed I could only play the music at half volume, where the sound quality of all speakers and sub became very poor. When this amp was installed and tuned, the head unit could be turned up to max without clipping and the gain on all 5 channels were set at appropriate levels...basically I could max the volume on the H/U without sound distortion or clipping after proper tuning. So now with the lights on, the amp's performance is very bad so I started investigating.



I tested resistance from the battery ground to the engine block, to the body (where my amp is grounded), to the subframe, and to the radiator support. Only the radiator support resistance test showed a measurement that could use improvement. I resolved that with a 4 ga wire from the radiator support to the subframe. There was a small improvement in the volume setting before sound quality deteriorated, but I still haven't fixed the problem. I ran out of time today but need to test voltage at the battery and amp next while a helper turns on the lights and starts turning up the H/U's volume. As an aside, all of the amp's specifications are listed at 13.8V so that's my target with lights on and volume up.

During restoration, the '67 Camaro received a new wiring harness and was upgraded to a 3-wire, 94 amp alternator, which gives it a voltage regulator sensing circuit. In very basic terms, this helps the alternator adjust output voltage to the main distribution block based on the voltage drop caused by the attached accessories. By design, and after finishing the reading of the link above, accessories should be attached to this distribution block where the voltage sensing wire is also attached, and not directly to the battery. Here's the basic idea borrowed from MadElectrical.



It's such a common practice to attach a huge power lead wire directly to the battery when installing an amp, but this now seems to be in conflict with how the electrical system is designed to support accessories when using a 3-wire alternator. This is where I am in my understanding now. If the amp's power lead is connected directly to the battery, then the battery is responsible for providing the necessary voltage, not the alternator. But it is the alternator's job to provide the necessary voltage for the accessories (while also providing a charge to the battery). It makes sense to me that the battery then, while being able to provide a lot of amperage, acts as a buffer and hides the alternator's ability to sense the voltage requirement of the amp (by the voltage drop that would occur if the amp were attached to the junction). If the battery begins to drop below 13.8V (and thus the amp's lead wire drops below 13.8V), the alternator only sees this as a very slightly discharged battery and won't adjust the output voltage appropriately so the amp again is getting at least 13.8V....right? Hooking up the amp to the junction instead seems like the logical solution to maintain a constant voltage to the amp (as long as the alternator is working correctly and can handle the demand).

[DoABarrelRoll]

I'd say you need a higher power amplifier. How big (watts) is your amplifier?

The reason you hook anything up directly to the battery is for a smooth power source. It applied to compasitive discharge ignition systems as well as stereos.


On a vehicle, think of the battery as a cup. Everything takes water from the cup. Your alternator is a pump whose job it is to keep the cup full. When you tap the cup directly, say from the bottom, you get a solid constant stream. When you tap the return line from the "pump" to the "cup" you get the water you need, but it isn't as solid a stream.


Unless I am way off base, I just started my cup of coffee.

[MyFriendScott]

I'm very sure it's not the amp's fault here. Based on the ball park current draw this amp is capable of, I could probably benefit from an alternator that puts out more than 94 amps to drive the H4 headlights and audio system at obnoxiously loud volume. But I get your point about using the battery for a smooth power source.

[chuckd71]

Things like amps and the starter go to the battery; other things can go to a distribution point fed primarily by the alternator like in the mad's 'new system' diagram. Or at least that is my understanding after talking to the guy on the phone. Makes sense to me.

[Kenny]

The battery is the best place for amp hook-up as previously posted, as well as using the shortest ground possible at the amp. It does sound like your alternator isn't keeping up with the demand of the amp and accessories. You can add some resistance to the "sense" wire and that will raise the charge voltage, which will alleviate some of the problem. Being so close to the limitations of the stock output, I'm not sure that is the best choice. Your local alternator rebuild shop can probably knock that dude up to 130+ amps and add a larger gold charging post for a very reasonable cost. Adding an adjustable internal regulator may be another option that is reasonable. The rebuild shops around here have always given me a better product than mass produced "box store" upgrade specials. Of course going big (over 140) would have me recommending Ohio Generator. As a side note, I have never seen any kind of reduction in battery life charged with a full 14.4v charge at the battery, but I have seen improvements in speaker/amp life at higher charge voltages.

[MonzaRacer]

A CSI 144 120 amp alternator will fit in the same general spot as a 10SI alternator, simply take crescent wrench and push up the upper mount slightly. I wired one into my 70 Monte Carlo and it lasted about 7 or 8 yrs between that car and my truck.
I have also used CSI 133 105 amp also on my Monza.
I had my big stereo in my Monte and loved the big alternator.
Power for accessories I always pulled from a second cable drawn from the extra terminals of my batteries as I always bought dual terminal batteries. Used a stereo brass block,,,then hooked a lot of add on stuff to the car.
I ground my amps directly to the chassis, and add in extra grounds to the battery also. Also many of the amps I have installed do not take large by huge cables,,,as most of them I have simply ran 8ga for and had zero issues.
Now for my starter I always try to source 2/0 welders cable for the power and then use many wide braided grounds in multiple places.

[PT Sportwagon]

Just to clarify what others have said. Go direct to the battery, the cup of water analogy is very correct. You want smooth flow to the amp. not fluctuation that an alt charging system. I would say you need a higher capacity alternator. Running a stock vehicle a 94 amps would be alright but once you add a stereo system and your high watt headlights. That 94 amps ain't going to cut it. Plus that 94 amps is probably at 2000rpm. idle it more than likely 60 amps maybe. You may also want to do the Big 3 upgrade. up grade your grounds, from Battery to block, from battery to frame, engine to frame. I have competed in Sound quality competitions for over 20 years. 99.9% of the competitors run direct to the battery. 100% of the high $$$, experienced, winning cars do. I really think the person at madelectical is an idiot.
Like Lee said ground your amps direct to the chassis, that is the biggest ground in a car.

Tim

[MyFriendScott]

Thanks for the info guys. As it turns out, the new amp has been asking for more power than the alternator can keep up with. In the past 2 months, I've noticed that the sound quality has been lasting for shorter and shorter periods while driving before the bass at loud volumes just becomes weak and muddled. Even though I've verified the existing 94amp alternator was keeping a steady 14+V charge at the battery, even with high beams and A/C running while playing at loud volumes, I had no way to see the amperage reaching the battery.

I've done the big 2 out of the big 3 so far and just installed my MechMan 150amp 12SI alternator (90 amps at idle). The MechMan alternator case has a built-in ground stud so I can complete the big 3. I've got new 4 AWG wires attached to the alternator charge and ground, now just waiting for the new battery clamps to arrive so I can hook everything up nicely.

[RSX302]

You are on the right track. My guess is lots of bass for the amprage you require. Look into a capacitor at the amp. They help alot with big amp hits. 1 farad per 1000watts. Heed the WARNING with these capacitors.