Mounting an amplifier- right next to the battery?

[HustleRussell]

I've been fitting an upgraded stereo to my E34 BMW 525i, and since I am trying to avoid using any boot space whatsoever, I have decided that a cavity under the rear seats would be a perfect place to fit two small 2-channel amplifiers. The only problem is that these would be within half an inch of the battery which lives under the rear seat in these old 5-series.
Is there any reason why, with careful installation, I shouldn't fit the amplifiers so close to the battery? I am aware the amplifiers will generate a little heat (which I can deal with), but also is there a risk of electrical interference between the battery and the amplifiers?

[Ozzie Osmond]

No problem.

[Herbs]

It's been a long time since I have done car audio but I cannot imagine it would great for sound quality - being so close would it not pick up interference?

I'm happy to be told otherwise as there are far more knowledgable people on here than me.

Could you not use one 4ch amp and put it under the set that doesn't house the battery?

[Krikkit]

How will an amp pick up interference from a DC battery? The worst source around there would surely be the fuel pump.

[HustleRussell]

Krikkit said:

How will an amp pick up interference from a DC battery? The worst source around there would surely be the fuel pump.

I didn't give that any consideration at all but a quick google reveals that the fuel tank lift pump lives in the boot next to the spare wheel well, so a good 18" away from the proposed amplifier location and seperated from it by the rear bulkhead.

Thanks for your re-assurance Ozzy Osmond, and Herbs- I was thinking along simular lines but hopefully it won't be a problem.

[Herbs]

Krikkit said:

How will an amp pick up interference from a DC battery? The worst source around there would surely be the fuel pump.

I was thinking more along the lines of cable runs going to the battery rather than the battery itself as the speaker cables may well end up running alongside the power cable.

[kambites]

You wont get any distortion if the battery is drawing steadily. You might hear a click when you turn the lights or rear window demister on; and you might get some distortion while the starter motor is spinning while starting the engine.

[HustleRussell]

Herbs said:

I was thinking more along the lines of cable runs going to the battery rather than the battery itself as the speaker cables may well end up running alongside the power cable.

I am re-routing the main battery earth so that both of the main battery cables come from the offside of the car, and the amplifiers are to the near side, for this very reason. Hopefully that'll do.

kambites said:

You wont get any distortion if the battery is drawing steadily. You might hear a click when you turn the lights or rear window demister on; and you might get some distortion while the starter motor is spinning while starting the engine.

Is there any way of mitigating these little noises, Kambites? (without moving the amplifiers to the boot like a sensible person would!)

[renrut]

The battery will never draw 'steadily' FWIW, a DC system is never truly DC.

Next to the battery won't be a problem as others have said. Running the various speaker etc cables next to the power might be so try to keep them separate. Good earthing should sort out the worst of it anyway.

[kambites]

HustleRussell said:

Is there any way of mitigating these little noises, Kambites? (without moving the amplifiers to the boot like a sensible person would!)

They might not be noticeable anyway - electromagnetic interference is a complicated enough subject that you'll never know until you true it. You could probably shield the amplifier and cables to an extent if you have to.

[Dogwatch]

HustleRussell said:

Is there any way of mitigating these little noises, Kambites? (without moving the amplifiers to the boot like a sensible person would!)

Yes, don't operate any switches!

It is only a possibility that you would hear interference at all. Bit different if you have AC circuits around - but you haven't.

[HustleRussell]

I'm fabricating a bracket from a sheet of honeycomb aluminium which will also act as a heat sink and a physical barrier between the battery and the amplifiers. Perhaps this will also perform a shielding function.
Anyway I have been re-assured that it's not a safety issue or anything, so I'll just have to try it and see how it goes.

[SuperchargedVR6]

What kind of power does this amp kick out? As you've already mentioned, heat is going to be the biggest issue if it's a grunty one. I used to be big on ICE and my genesis amps mounted in the boot used to get too hot to touch after an hour's heavy use.

Modern class D amps will be OK as they're super efficient and don't get very hot, but older class A/Bs won't like the confined space and on long journeys, you may find the internal protection circuits kick in.

In my old E34 tourer I mounted it behind the left storage panel and put an OE looking vent at the top of it to let the hot air escape.

[HustleRussell]

The amplifiers are ~20 year old Alpine 3541 (class A/B). They only make 45w per channel RMS, but as you say they'll run hotter than modern class 'D' amps.
I am aware that it is a potential problem, my intention is to get them fitted and try them out without the rear seat squab fitted to get an idea of what I'm up against temperature-wise. I'm hoping that the fact they are low powered will help but I am willing to play around with cooling fans if it looks like it's going to be a problem.
I would use the same compartment you did except that is where my sub woofer is going to go.

[renrut]

Motors are about the noisiest for EM you can have in a car, but they're unlikely to be operating continuously other than the fuel pump so as long as you keep clear of that and its wiring you should be fine.