m18use

Would it be best to run a earth from the battery to the boot?
Have it earthed in the back where was told would be best under the shelf,
But it is just not up two the job as running 3 phoenix golds and have battery in boot aswell One fuse just keeps blowin and there £3 each bought about 10 now so getting fed up lol

chiark

Earth cable to the battery? No.

The chassis is a lot better than a separate earth - even with its nasty welding etc. If you're running two batteries, definitely use the chassis as earth. I'd personally look to the floor pan for an earth as that'll have solid welds all the way through to the front. It doesn't hurt to upgrade the cable between battery and chassis either - in fact, if you haven't done this, do it *right now*. Go on, get outside. Now

What makes you think it's not up to the job? If you're blowing fuses, that's because of something else going wrong. I'd check for strands of wire across terminals... There's no way an "insufficient" earth can cause a fuse to blow - current is constant throughout a circuit... It has to flow back to the battery somehow.

What amps are they? Are the amp fuses blowing, or your master fuse?

If earths are insufficient, you'll get brown outs where things start working strangely... You won't get fuses blown.

babber

I disagree with this. Have you ever measured the Impedence between the earth on the battery and the boot of an Impreza? It's a couple of OHMs, when I measured it on Mouses car. My impreza is exactly the same. The earth point we are using is from the rear screen heater earth point, as seeing that is pulling a fair few amps, thought this would be a good place.

I think that running a cable from the earth point at the front to the earth point of the spare battery could only help the Impedence issue??

Surely if the amp is trying to pull more current due to the bad earth, then this will blow the fuses?? A bad connection can also cause voltage drops, that would increase current draw, and blow the fuse again?

The blade fuses on the amps are not blowing, it's the gold plated 30 amp fuses that appear to be blowing

We have check all the connections for strands of stray wires, and haven't found any.

Thanks Phill

chiark

Hi Phill, good to see you around

Yes, I did measure the resistance... From the spare wheel well to my battery, it was .1 ohm. That was after I'd changed/beefed up the OEM earthing in the engine compartment.

I would not use an OEM earth point, but rather make my own. I don't believe at all in putting any holes in any part of a car, but this is the one concession I'll make. The best place will be somewhere on the floor pan that hasn't got any spot welds connecting it to the rest of the car... Again, the OEM earthing needs sorting, but the impedence shouldn't be that great.

I see what you're saying about the voltage drop across the earth causing the amp to try to draw enough current, but I think that most amps are current limited - could be wrong on that tho. Give 'em more voltage and they make more power, but they don't alter the current that they draw. Give 'em less and they normally brown out. I don't see the current being affected.

What amps are you fusing to 30A? Bear in mind that that'll "only" be about 360W nominal current delivery, and amps are not normally more than 75% efficient - so if the amp is more than 250W rated, 30A probably isn't enough.

babber

Well thankyou mate, nice to get a message like that

Maybe it is worth changing the OEM Live and Earth cable in the front, do the earthing mod and earth the ECU properly. At the same time run an earth cable to the point on the chassis (a new one ) where we earth out the rear battery. It can only help, and won't have an adverse effect.

I thought that the amps would be current limited, but aren't sure. The amp BTW are the daddyos Pheonixs gold, with line drivers. Maybe this is causing the amps to draw a larger current, but I really don't know.

The fuses are 30 Amp, I think, but will check when Mouse gets home. I'm sure they are the same rating as the blade fuses on the amps, but these bad boys never blow. When you say a 250 Watt rated amp, do you mean output? Surely a 250 Watt rated (output) amp should be using a lot more than 250 Watts.

Phill

chiark

It can't hurt, can it? But I really don't see it stopping the fuses being blown. TBH, I'm only going off physics a-level from 13 years ago, so am probably wrong. I'd definitely have a wander over to talkaudio.co.uk/vbb and ask some of the gods on there. There's some fantastically technical people.

In my first install I did run 4AWG to the back of the car, and split earths out from there. It didn't do any harm, but when I started messing with a multimeter, I found it wasn't worth it - plus I could conceivably be causing more problems with earth loops.

Another worthwhile thing would be a call to BBG's tech support to ask 'em what fuses they suggest for the amps.

When I say 250W rated, it will be using a lot more than 250 watts - they're normally about 75% efficient tops, so it'll be "using" 330W minimum. At 12V, that's about 28A. Hence 30A is decidedly iffy if you've got everything set up correctly.

Just a thought, and it's probably a bad theory, but if there's a large bank of capacitance on the 12v side of things in the amp, this could cause a large instant draw when a bass hit tries to empty the cap... It could draw more than 30A.

babber

So did we get a view from the BBG's gurus mate? Be good to sort Mouses system out once and for all

Thanks Phill

m18use

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