Why is my voltage dropping when bass hits?

I have a 2006 Dodge Charger SRT8.

I’m running a 320 AMP alternator with a 44mm pulley, all 1/0 100% copper cables with hydraulic crimped copper lugs. Big 3 upgrade. I’ve got two Yellow Top Optimas wired in parallel and an external regulator setup.

The sound system includes two 12” T1s on a T1500 amp. The amp is grounded to the body and powered by the second battery. Alternator power goes to the primary battery. Both batteries and the alternator are grounded to the body.

At idle, I have a steady 14.5+ volts, but as soon as the bass hits, the voltage drops, and my lights dim. When cruising, it’s fine. What am I missing here?

Is the voltage on your first battery under the hood below what the alternator is putting out? That can stress the system. You’ve upgraded everything, so it’s worth checking. By the way, what’s the first battery? Is it stock? You mentioned a second battery—when did the issue start? After adding the second battery or upgrading the alternator?

@Morgan
Both batteries are Yellow Top Optimas, and they’re both located in the trunk. They show 12.5+ volts when fully charged.

Mai said:
@Morgan
Both batteries are Yellow Top Optimas, and they’re both located in the trunk. They show 12.5+ volts when fully charged.

How about the alternator?

Morgan said:

Mai said:
@Morgan
Both batteries are Yellow Top Optimas, and they’re both located in the trunk. They show 12.5+ volts when fully charged.

How about the alternator?

With the car idling, it shows 14.8 volts.

@Mai
Your alternator should keep a minimum of 12.6VDC, or the batteries will start discharging. Are your connections clean? No frayed wires? Poor wiring could be an issue. Try grounding the alternator bracket to the same frame location you’re using for your main ground.

@Morgan
I’ve grounded the alternator casing with 1/0 100% copper welding cable, the same cable I use throughout the car. All connections are new with crimped copper lugs.

Mai said:
@Morgan
I’ve grounded the alternator casing with 1/0 100% copper welding cable, the same cable I use throughout the car. All connections are new with crimped copper lugs.

Man, have you tried better batteries that supply more consistent voltage?

@Morgan
Yep, upgraded the main battery to the biggest Yellow Top Optima and added a second one just like it.

Mai said:
@Morgan
Yep, upgraded the main battery to the biggest Yellow Top Optima and added a second one just like it.

Dang, sounds like you’ve done it all. Maybe it’s time to let a shop check it out.

@Morgan
I’ll do a load test on the batteries first, and if they’re good, I might take it in.

Mai said:
@Morgan
I’ll do a load test on the batteries first, and if they’re good, I might take it in.

Sounds like a plan. Sometimes those load tests reveal what a voltage test can’t. Let us know how it goes!

@Mai
When you’re testing, make sure to account for all electrical loads like lights, AC, and anything else running while driving. That way, your voltage numbers reflect real-world conditions.

Honestly, this sounds pretty normal. Most alternators don’t put out much amperage at idle, so your amp is relying on the batteries.

Do you have a dyno sheet for the alternator? What’s the idle output?

@Finley
I had the alternator dyno-tested after the build. It hit my target of 300+ amps, but I didn’t check the idle output. I even asked for a smaller pulley before leaving to improve low-rpm performance. I figured it would hold 14.5+ at idle with the system running moderately, but the lights still dim at idle. That doesn’t seem right.

How much does the voltage drop? Alternators don’t usually output full power at idle.

Erie said:
How much does the voltage drop? Alternators don’t usually output full power at idle.

Before I added the external regulator, it dropped as low as 11.8 volts. Now it stays around 12.5.

@Mai
Somebody mentioned checking the alternator’s output curve. Knowing the amps it produces at idle versus higher RPMs would help. Honestly, 12.5 or above at idle isn’t bad.

Did you add any extra ground points on the chassis to distribute the load?

Marlon said:
Did you add any extra ground points on the chassis to distribute the load?

Yes, I welded an extra stud for the second battery’s ground and another for the amp’s ground.