Battery Revisited

The message from Simon Dean contains these words:

Well, yes, but of course the melting was caused by power dissipated from a voltage drop across a bad connection. Once you make the connection properly the on-load voltage shouldn't droop so much so there's the opportunity to draw more current.

Class A in a car? ;-)

I'm more worried as to why the volts on the battery droop to 12 volts when he jacks the volume up. The first major impression is that he's putting the alternator on it's knees (as well as his eardrums).

PDH

Can you explain this bit for me?

Voltage drop across a bad connection??

Unless? Ahhh. Is it a case that a lower voltage supplied by the battery would equal more amps and more heat or something?

The fuse holder itself as I said ended up being melted. But with the fuse out, there was no contact between to the two, well, erm, contacts. So I ruled out a short circuit there. The fuse plastic was melted, but the fuse remained in tact.

thanks Simon

Ok, most designs that deal with any real power will be a compromise of class AB, I stand corrected. Efficiency is now 60-65%, however, practically, with real audio sources bother As and ABs actually operate at much lower than 50%

Bad connection = one that has unusually high resistance. The high resistance could be from a mechanically poor connection (e.g. little contact area), or from dirt or other non-conductive contact, reducing the contact area, or introducing a series resistance between two conductors.

Once you have a tangible resistance then Ohm's law kicks in, and the power dissipated in the joint (as heat) is a function of the resistance and the current drawn ( P = i^2 x R ); So, lets assume 30A is being drawn (the rating of the fuse) and the resistance of the bad connection = 0.1 Ohm, then the power dissipated in the joint = 30 x 30 x 0.1 = 90 Watts, a hell of a lot of power in what's likely to be a small area. Some of this will be conducted away (you've got a nice copper heatsink in the form of the supply cable), but a lot is gonna start heating the local fuse holder, with the resulting melt-down that you've seen.

Good site:

Reckons 8 gauge is good for 100A.

You raise an interesting point, though - if you're truely sapping the power from the alternator then that cabling (twix battery and alternator) might well need upgrading (along with the alternator!)

No, they'll be a class D with 90% or more at full output. Or a crappy dual rail class H (or is it G) design.

Bah, most (well, the 2 that I looked at) respectable manufacturers claim AB on their websites

Class D?, not really HiFi (but then, neither is the car environment).

Class H?, ok, I need to research that one

Most amps built into subs are class D, many others now are also class D. As for being HiFi just remember that most new pro-audio amplifiers are heading to class D, and the power supplies are switchers in a lot of the A-B amps.

Either G or H, can't remember which, allows for a steady power rail and a high voltage short drain rail for transients - as used in the built in amps in many car radios.

The message from Simon Dean contains these words:

Look, it's not a short circuit, it's a failure of the fuse holder to properly hold the fuse. If instead of a nice low resistance contact between the socket and the fuse there's a dodgy high resistance one then not only will there be excess heat there but also you'll drop more voltage across the connection.

It needn't be a fuse to do this - it just happens to have been in your case.

Well I didn't know. :-(

I just raised it in case there was a thought that maybe I was drawing more than 30 amps but the fuse didn't blow because the link was shorting...

I understand now. Thanks to you and Mike for that.

So, long story short, fuse wise, I probably don't need anything more than 30 Amp at the time being. There was just a problem with the fuse and/or holder causing a high resistance, of which, I'll never get sussed now or even get my money back under guarantee...

Right?

Cheers Simon

Ahhh. Gotcha. I understand now. Thanks.

60 amps on 8 gauge is pushing it a bit, my sources rate 8 AWG as being good only up to 70 amps in enclosed runs, however even at 50 amps, the voltage drop on 12 foot of 8ga is almost a volt (0.9V), which doesn't sound much but its 7.5% lost in the wire to resistance, and resistance here equals heat.

For the sake of safety I would invest in a run of 4Ga for the long run from main battery to amp (or terminal block) and just use the 8ga for short connections (less than 2 feet) from the block to your devices.

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Think I might just do that. I fancy sticking some 4 guage in too... maybe two lots, because at some stage, I want to stick some kind of bass tube in there to annoy the locals.

:-)

Where do you put your shopping?

Ta Simon

In the rear footwells :)

Stops it rolling about or needing repacking when I get home.

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