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Habitation Lights Dimming To bass from Car Stereo

6878 Views 29 Replies 9 Participants Last post by  mooro

Just added a second 110ah leisure battery in parrallel.

Decided to run a lead from these to the dashboard stereo, so I could have music when on site, without flattening the car battery.

Unfortunately now if I have the music at any great volume, the bass notes are causing the habitation lights to dim. I guess the extra current being drawn by the stereo is causing this to happen.

Anybody know how to remedy this?


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That sounds a bit odd, as the power used by LED's is so low, maybe it's a harmonic response of some sort, you may have in vented some new disco lighting tackle.

turn the music down?

Sorry couldn't resist that.

Two 110A batteries should be able to stand any normal stereo system not perhaps one of those boom box arrangements (from Maplins) that need 100A gold plated fuses.

I would look at the new battery wiring
Sounds like a high resistance somewhere. Check your connections.
What is the power output of the stereo ??
Thanks guys.

The stereo is a bog standard Sony 40w x4 (so only 10w per speaker as I understand).

There is a 10a fuse in the back of the stereo. I hooked it up to the battery by way of an 8a power lead, with a 7.5a fuse (which isn't blown so presumably the total draw is less than the 8a capacity of the power lead).

I connected the two batteries up using "high capacity" cable from Maplins. The two batteries are right next to each other so the run is only inches.

Re Disco: I had thought of that. May just go with multicoloured L.E.D.'s and enjoy the show ;-)
you say it does it to the bass note, I assume you've tried it with the bass turned down, but have you?


I used to like doing it to the bass note, now I can't remember what it was I liked doing.
A battery is a very stiff supply and such a low impedance that it acts as a capacitor would do smoothing out fluctuations.

Is any part of your wiring to the stereo shared by the lighting circuit or does any of it run side by side?
sallytrafic said:
A battery is a very stiff supply and such a low impedance that it acts as a capacitor would do smoothing out fluctuations.

Is any part of your wiring to the stereo shared by the lighting circuit or does any of it run side by side?
Well there is one strange thing going on.

When I initially connected the stereo via its own fused lead to the leisure battery I opted to leave it connected to its original earth i.e. I did not run a cable from setreo earth to battery earth.

I thought I would try putting that right. So I ran a 15a lead from stereo earth to battery earth and no change. I then (accidentally) knocked the earth lead from the battery and the stereo still worked! By my understanding this must mean it is earthing somewhere else. Not via the aerial since it worked with this disconnected also.

The only conncections to the stereo now are an aerial, a 12v lead from leisure battery, and 4x 2 speaker cables.

It does get better with bass low definately. I did run a 15a power lead from the battery to the stereo, but same problem. Only non-standard thing I am doing is putting one input into both the strero continuous/ignition circuits via a push-splice.

And I know this is no help but for one minute while I was messing around things seemed o.k.! I don't know why!!

The lighting/charging/stereo lead all leave the positive pole of one of the two batteries, earth from the negative of the other, as suggested by cleverer folk than me on here ;-)

p.s. One other thing I have changed come to think of it: I changed a halogen bulb to a l.e.d. The halogen was running off 10w, the l.e.d. off 1.3w.
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If I read the posts correctly the problem started after fitting a second leisure battery?. Have you done any work on the earth cable from the 2 leisure batteries to the vehicle chassis?

These cables do get less efficient as they get older (but don't we all) so inspecting this at both ends and perhaps a good rub down with something rough and emery may solve the problem. Don't forget you've just doubled the amount of amp hours and this will affect the performance of the cables.

Another thing to try is to run an earth cable directly from the stereo to the vehicle earth (not to the battery negative terminal) and see what happens, I would suggest if the stereo has known good clean earths then it should stop affecting your lighting circuit and you can start investigating where the bad earth joints are.

Frequency does affect the performance of earth cables too so use a nice thick cable, not a skinny thing.

Just thought of another test, what happens if you revert to just the 1 battery?.

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Just reread the thread looking for answers what part number or rating is: "high capacity" cable from Maplins

Perhaps your supply isn't as stiff as it should be.

Also some stereos have several earth connections the case for example is connected to earth internally. On some vehicles that is connected to earth by the mounting frame.
OK....dont shout........ but could the vibration caused by the bass be shaking the bulbs loose in their holders?

I'll get my coat :oops: :oops: :oops:
have a read here bass led you migt have this effect by accident, try putting the old bulb back in.
Well folks been playing around some more. Almost definately an earth issue, as now only earths when the aerial is connected.

I think also something wierd is going on with the head unit.

I've had the manual out and the pin postion for the iso output just doesn't match. For example pin 8 should be black earth. It's actually got a yellow wire and the unit won't work unless a live feed goes to it.

Simlarly there is no wire to pin 5, where one should find the yellow cont. power input. Then red switch power is on the wrong pin.

I'm pretty certain the previous owner has modded the head unit. I think I'll buy another and see where I am - I was going to for direct ipod connection anyway.

I did try back to halogen same prob.

And don't worry I won't be playing music that loudly. The only real time this is likely is on a motorway where a 20 y.o. diesel engine can make a clatter. Won't be necessary on site.

However you know how it is - when you know something is wrong, that little d.i.y. gremlin gets hold of you until you fix it!

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So today I bought a new head unit:

(see my review if you're looking for a good, cheap ipod compatible stereo).

The problem persists.

I have switched to one battery, no change.

I have also re-earthed the stereo to the chassis - no change.

If I change lighting from leisure to car battery it behaves normally - so it is not the bass shaking a loose connection in the lighting circuit.

I'm stuck.

Any more ideas?


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Ok when you have eliminated all the likely things what about the speaker cable runs. Its possible that this is pick up between the radiated signal from the speaker wires or speakers and the electronics in the LED array.

[Had a case once where the 400Hz of a electrically driven pump was being picked up in a pilots headset. No direct connection but using a scope found a wire that was doing nothing most of the time but was acting as an aerial for the 400Hz at one part of the aircraft and reradiating it into the intercom at another.]
When the radio is wired independently to the battery as you indicate, i.e. at no point does it share part of the cabling with the lighting, that surely the only explanation is that somehow the radio must cause a voltage drop at the battery?

Check the clamp on the battery posts (is it tightened properly, is there dirt between pole and clamp?) as it seems unlikely that the battery itself would have a significant voltage drop caused by a relatively small load.

Otherwise the only explanation is indeed that your battery is poor. Can you measure the voltage accross the battery poles? How much does it drop when you put say 30W of lights on? And with 60W?

Could you have pinched a speaker cable or got any part of the speaker cable run to earth?

When you tried the new unit did you hook up to the speaker cables that have been connected all along? If so try a direct connection to the speakers to eliminate this.

Good luck
O.K. guys, thanks for above.

There is definately a voltage drop at the battery.

I was reading 12.63v, then turned the stereo on. At full volume the voltage drops to 12.5v., and flass and rises with the music.

I then turned off the stereo and turned everything on in the van: extractor van, lights, pump etc. The volatge went down to abour 12.45 volts.

I tried cleaning and tightening the posts. I will have a look at the speaker runs tomorrow, thanks.

Do I have a problem?
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Yes you do, (I thought we had eliminated battery problems by your earlier fault finding). I am surprised that your new battery behaves like that. I went and did a similar check on my battery and it does not fluctuate at all. I suspect one or more of your batteries has high internal resistance ie is sulphated or you still have a poor contact

Note to anyone else reading this you either need a very fast Digital Volt Meter, Oscilloscope or an analogue meter. Cheap DVMs are too slow to follow a music beat.

Although I think you say the fault exists on just one battery How thick are the cables joining the two batteries Maplin high current I think you said. I would use 25mm square as a minimum.

Iam making a guess that a 40W stereo won't draw more than 4A on full volume. If that is so then a drop of .13V equates to a resistance of 0.03 Ohms. Does anyone know what it should be for a leisure battery? Its way too high for a starter battery.

BTW for every 100 Amphour of capacity I have seen it quoted as being equivalent to having a 1.5 Farad capacitor that is one hell of a smoothing capacitor, given how big a Farad is. We normally talk in microFarads.
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