Headlights dim when sub hits, How do I fix it?

[4 door 330]

Running Alpine MRP-F300 bridged to the speakers and MRP-M500 to the subwoofer.

Dual 4-Ohm voice coil in parallel so the amp sees 2-Ohm.

When the volume is up, not even to an extreme level, just loud, the headlights dim. It is not enuogh to make the idle drop down at a stop light but the dimming is very noticeable.

Battery is 3 months old OEM BMW from the dealer.

Is this a simple fix like changing out the battey for an Optima with more power, or do I need to replace the alternator with a new or higher power alternator.

The alternator is original, and the car is at about 105k.

Thanks in advance! -Dylan

EDIT: Search is my friend, looks like I'll be going for an Optima Yellow Top or Bosch Silver

[Mele4nyc]

I had the same problem as you when I had halogens. You can buy a capacitor to fix the problem, or do what I did and buy HIDs. I have the 50w HIDs and it doesn't dim as it did with the halogens.

[ca1242]

A high-performance battery would help.
Also, make sure:
- Battery terminals connections are clean and tight
- Amplifier connections (ground & power) are good
- Amplifier's gain is properly set up (no clipping)
- You are not boosting the bass at the head unit or amplifier (distortion)

[habbyguy]

Turn the music down? :4ngie:

The suggestion to get a (large!) capacitor installed across the power terminals at the amp is a good one. A capacitor acts like a parallel "battery" that can charge and discharge VERY fast (enough to supply the extra power for a drum beat, then recharge waiting for the next one).

The other thing you can do is to make sure the amp is wired as directly to the battery as it can be.

[ca1242]

Just one comment about the capacitor. If you consider the cost, vs. benefit, a capacitor is not a great investment. They do help a little, but are not really the best solution to this problem.

Something else to check is the alternator:
1. is it running properly
2. what size is it? Most e46's have a 120A alternator, but some have a 90A.

You can easily check the car's electrical system performance by using the cluster's voltmeter:
1. turn car off
2. hold down left cluster button while you start the car.
3. let go and push 19 times (test mode)
4. wait a sec and push 9 more times (voltage).
5. Number you see is voltage (i.e. 135 = 13.5v). Car running with alternator charging battery should be ~14 volts.

[4 door 330]

I'll definitely check the alternator Amp rating. The Volt Meter shows 14.2.

The amps are in the spare tire well and are run 8 gauge from amps to distribution block, 4 gauge distribution block to battery, both power and ground. The total cable length is less than 3 feet. I rechecked all the connections and they are solid, so looks like its either the battery or the possibility of the smaller alternator.

[ca1242]

8 gauge is kind of small. What are the amplifier requirements?

When you said connections are solid, how did you check them. The only way to ensure connections are good is to check with a voltmeter (especially grounds).

[habbyguy]

Keep in mind a bad connection or too-small wire will put LESS of a load on the car's electrical system, not more (since both will limit the peak current the amp can pull).

I'd check to see if you can see a negative voltage spike (using an old-school analog voltmeter) at the battery and the distribution block. If you're seeing it in both places, then your alternatives are 1) a big honkin' capacitor, 2) a bigger battery (which might not help at all), or c) live with the lights dimming.

[ca1242]

Not necessarily true, a bad connection causes a voltage drop. Amplifier will draw more current to make up for the voltage drop (depending on amplifier's power supply design). A small wire will also exacerbate this situation even further, although at only 3 ft, I doubt it will have much effect, even if it is a bit undersized.

As I said before, a big capacitor is not going to improve things a lot (although I do agree it will help just a bit).

A bigger battery (or more batteries as some people like to suggest in this forum) will only add more load to the car's alternator. A more efficient battery, however (optima, Stinger, etc) will help, since they waste less energy during charging.

Lights dimming is an indicator of the electrical system being strained. Not a good idea. Electronic components could get damaged.

I have 2200 total watts in my own car. NEVER had a problem with lights dimming, stock alternator, upgraded battery (and cap).

Keep in mind a bad connection or too-small wire will put LESS of a load on the car's electrical system, not more (since both will limit the peak current the amp can pull).

I'd check to see if you can see a negative voltage spike (using an old-school analog voltmeter) at the battery and the distribution block. If you're seeing it in both places, then your alternatives are 1) a big honkin' capacitor, 2) a bigger battery (which might not help at all), or c) live with the lights dimming.

[pastE36prsntE46]

Keep in mind a bad connection or too-small wire will put LESS of a load on the car's electrical system, not more (since both will limit the peak current the amp can pull).

I'd check to see if you can see a negative voltage spike (using an old-school analog voltmeter) at the battery and the distribution block. If you're seeing it in both places, then your alternatives are 1) a big honkin' capacitor, 2) a bigger battery (which might not help at all), or c) live with the lights dimming.


You should do some significant research before you give people incorrect advice.

[Eroc162]

Dude I did everything and my lights still dim a bit when I open the doors. I have the oem battery connected to a Kinetik 2000watt battery which is connected to my alpine pdx1000 amp my gain is set to the 3rd knotch (which is less than half). And they are all connected to 4gauge Monster cables. I have everything grounded to the same bolt that the oem battery is grounded to the frame. OH yeah my car came stock with a 120a altenator. Any body have any suggestions.

[habbyguy]

You should do some significant research before you give people incorrect advice.

You should probably study electronics before accusing someone of giving bad advice. :confused:

[4 door 330]

Alpine recommended 8 gauge to each amplifier, power and ground, and I ran 4 gauge fromt he battery to the distribution blocks, so this is all up to manufacturer recommendations.

I put the voltmeter on all of the connections from one terminal to the other, and they seem fine. All connectors are properly sized for their wire gauge, gold-plated, and crimped using the appropraite crimper.

My thinking is that the stock BMW battery, although new, simply doesn't provide enough powerat peak usage. I still need to check the alternator amperage because, if it is the 90 Amp, that could be part of the problem.

Stay calm guys, I'm accepting everyones response as opinions, so no need to flame each other.

Now to dig up an analog volt meter...

[ca1242]

Dude I did everything and my lights still dim a bit when I open the doors. I have the oem battery connected to a Kinetik 2000watt battery which is connected to my alpine pdx1000 amp my gain is set to the 3rd knotch (which is less than half). And they are all connected to 4gauge Monster cables. I have everything grounded to the same bolt that the oem battery is grounded to the frame. OH yeah my car came stock with a 120a altenator. Any body have any suggestions.

As i noted before, having 2 batteries causes problems. Get rid of one of the two batteries.

[ca1242]

When checking with the voltmeter, measure
- voltage at battery
- voltage at amplifier +/-
- voltage from battery to ground (car's chassis)

They should all be the same or really, really close.

Alpine recommended 8 gauge to each amplifier, power and ground, and I ran 4 gauge fromt he battery to the distribution blocks, so this is all up to manufacturer recommendations.

I put the voltmeter on all of the connections from one terminal to the other, and they seem fine. All connectors are properly sized for their wire gauge, gold-plated, and crimped using the appropraite crimper.

My thinking is that the stock BMW battery, although new, simply doesn't provide enough powerat peak usage. I still need to check the alternator amperage because, if it is the 90 Amp, that could be part of the problem.

Stay calm guys, I'm accepting everyones response as opinions, so no need to flame each other.

Now to dig up an analog volt meter...

[habbyguy]

When checking with the voltmeter, measure
- voltage at battery
- voltage at amplifier +/-
- voltage from battery to ground (car's chassis)

They should all be the same or really, really close.

Agreed - just make sure to do this with the amp at "ear-bleed levels" (or crankin' loud enough to cause the lights to dim). Otherwise whatever is causing the voltage drop won't be exhibiting its nasty behavior, and the voltage will be the same at all points, even if there is a wiring / connection problem.

The easiest way to check the cables and connections is to look for a voltage drop from one end to the other. Here are the checks I'd make with a voltmeter (with a drop of more than 0.1V being suspect):

Battery post or pad to battery cable terminal (this is a common point of high resistance, though it's not always possible to get to side-post battery pads to make the measurement). If in doubt, remove, clean, schmear with the protective glop you get in a ketchup pack at the auto parts store, and reinstall (I'd recommend using a small battery - a 9V will do - to keep your car "powered up" while you're doing this to prevent having to reprogram your radio and losing your MPG and other info.

Battery cable terminal (at the battery) to the distribution block terminal (or ground terminal, depending on which side of the battery the cable is connected to). Again, you're not connecting the meter across 12V, but from one point on the circuit to another point further down-stream.

You might also measure from the distribution block to the input to the amp if that's accessible.

I once had a 120W FM transmitter (amateur radio) in an old Honda Civic - when I hit the push-to-talk button, the lights dimmed a LOT. There was no way the little toy alternator and battery in that car were going to handle that kind of load without complaining at least a little). :4ngie:

And something else to consider - maybe the music is so loud it's causing your pupils to contract on the thumps, and the lights aren't really dimming at all... :craig: