Dead alternator .. again?

In general use the alt is not designed to output max current, like with any electrical item they are spec'd for general use and can deal with periods of extended output, but extended periods of extended output will stress an aging component. The OP mentioned the alt was a recon unit - often recon companies will leave any functioning hardware and replace only the faulty/worn item - this is how running it at max current for a long period (i.e. dead battery, not just a one-off flat battery) could cause problems. I never once suggested that a single, one-off flat battery would cause a problem. Consider, if you will, running your home stereo system at full whack for 24 hours - sure it can do it for a short period but it wont be too long before you melt your voice coils and/or cook the amps due to the extended duty cycle, not due to the peak performance.

Boosting a remote flat battery in a system with a fully charged local battery is nothing /like/ the same load or duration as charging a dead (possibly shorted internally) battery with no other source of power. Its not like they remove the vans battery and replace it with the customers now is it? The extra equipment they have is negligible.

Indeed. In fact, the two coil ones had no specific current limiting at all!

I smoked a number of Lucas dynamos whilst trying to maximise the output for my Cooper S rally car. I only ever managed to get something like 17A out of the best of them.

Chris.

Many cars will get near the maximum output of the alternator when running with lights and heated rear window on, etc. Alternators *are* specced to work at their maximum.

Then don't buy crap re-cons. Is that your job? You seem to be making excuses for them.

Any semi decent amplifier will run all day at it's maximum rated continuous output. If it won't it's cheap rubbish. Speakers too at their maximum rated input.

The point is they are.

That depends on the item, I've never seen a TV melt

That tells you a lot more about HiFI than alternators.

Actually it's about 80% but that's still massively more than the battery will take.

Only on a small duty cycle usally backed up by the battery

Most amplifiers have both a max output and a max continous output , the continous output is usally falls between 60 and 80 % of the full output same with generators or any other piece of electrical equipment .

It is very rare that the max output and continous output are identical on any peace of equipment

On a fairly high duty cycle on most cars. Where fairly high in UK ambient temperatures & 100% are indistinguishable.

Apart from tvs, computers, ovens, kettls or almosy anything where it might be usefull. Which doesn't include amplifiers & speakers.

Everything will die eventually. One of the diodes failed.

We will have to agree to disagree then. They are spec'd with a peak and continuous current, as virtually all component level electronics are. The two are different and generally items are sold as under their continuous limit, not the peak. Peak limit is that which is only attainable for a short duration.

I don't, I recon them myself with original parts. However it isnt my job, just something I have experience of, and is in the same field as my career.

Thats what you'd expect, but you will need to go for high-end kit to get that. Most of your "decent" kit will not withstand continuous use without rapid life reduction. Electronic component lifespan is DIRECTLY proportional to heat dissipation, double your current and you get 4 times the heat output from any conductor (I^2R). Electronic devices are designed with an operating envelope in mind, but with peak values outside that envelope being tolerable for short periods. Things like your TV run at their max continuous output all the time because there is no way they can be taken outside their operating envelope. Items like a battery charger or lighting transformer operate at their continuous output but peak at a higher output for short times with no problems. If you run them near the peak for any length of time they dont last long (trust me, I've done both!). I can see we are never going to get anywhere with this so we might as well drop it!

will only be run at max in the first 10 minutes to heat the car then they are turned down lights are rarely all on at the same time

You wil notice that at low revs if you have heater , wipers screens and lights on the wiper blades run slower and the lights are dimmer

The maximuim capacity of the alternater is usally rreached when your engine is running at around onethird of the engines max revs give or take depending on vehicle

And if you look at the charge current into your battery it's similar

Yep, & it's cooling improves at higher revs than that.

Perhaps we're talking about different specs. A maker's workshop manual only states one figure for the output of an alternator and that is the one it will be happy at continuously. Of course an alternator is *never* run continuously at full output throughout its life. Could well be it could have a peak output slightly in excess of this.

Well yes. But then much cheap gear can't deliver its 'spec' anyway.

Really? I've got lots of gear here on continuously. Including some would consider obsolete.

So use it only in the freezer and it will last forever? ;-) Or do you mean the life is proportional *after* a certain temperature is exceeded? That's why good amps have decent heatsinks. Fans, even. Of course you could make an amp which could stand high currents until it overheated and died - but that would be cheap crap. Unless you state a conservative current limit as the spec.

Seems to me it's all about your idea of specs. But your original statement that a flat battery can burn out an alternator is still ridiculous.

Not so if you're in the North of Scotland etc in a dark cold winter. Indeed one car I owned couldn't balance the full load. As standard - no extras.

Depends on who makes the amplifier. I agree you can have a higher peak output if the heatsinks are inadequate. Make them larger and that becomes continuous. You pays your money... Many PA amps are designed to be run beyond their stated output without damage - as that's how they'll get treated.

Plenty of things I can think of. A light bulb. Electric heater without thermostat. Etc etc.

Most ford focus wont if you run full lights , heater , wipers front and rear screen heaters at tickover your voltage wont rise much above

Even at a steady 60 mph on the motorway your voltage wont get much above 11.5

Knock off the front screen heater and your straight up to 14.5 volts

Thats running a brand new battery and the alternators working within the makers parameters

A light bulb will run at twice its rated capacity it just fails earlier Same with an electric heater , the only factor with these is tier output is limited by the input voltage

I would respectfully suggest that something is wrong either with your meter, or with your Focus.

The ECU on these cars monitors battery voltage, and will progressively shut down high consumers as a "life-saving" strategy. The first one to go would be the front screen. This happens nearer 11.5v than 10.5.

An early Focus with a lead battery should be 14.4v, a later one with a silver technology battery 14.6v.

I agree absolutely with the premise that an alternator might not balance full demand current when the battery is completely flat; it won't kill an otherwise healthy alternator however.

As an aside, some cars now have alternators that produce 300A...

Chris

However none of the decent modern ones will run at peak output continously, they all shut down "gracefully". But if you're running a PA amp at peak you'll be clipping anyway.

Indeed - although of course you only need the front screen heater until the engine gets up to temperature when the car heater can take over. I was referring to a car without screen heater where long term loads could exceed the alternator output. It isn't a current model, though. It was also very common in the days of dynamos.

If you overload anything it can and will fail. But that's a different matter from using it at it's rated specification.

Indeed. And water cooled too.