SOT: Car battery conundrum

Not so. A dynamo can have a higher current flowing through the field winding than an alternator.

An alternator is inherently self limiting (safe) as regards maximum output. A dynamo not.

Some also contrived to fit alternators which were still too small.

Rolls managed to fit dynamos which were always up to the job.

All down to cost.

In message , "Dave Plowman (News)" writes

That's what the meter's 'zero button' is for (assuming there is one).

So you measure the resistance with the leads shorted and that is zero, then measure the load.

Another bit of decorative bling;!...

Have it your own way. No point in paying lots of money for a specialist device to measure tiny resistance. Any DVM will do it just as well.

You pay money for ease of use and accuracy, if you only need accuracy to a couple of percent its cheap.

That's the point. A couple of percent accuracy of full scale is fine for most things. But not for measuring a high current shunt.

Have you ever tried to make a shunt? I have. You tend to end up AUT it.

I would have said a modest screen TV at that time, there were no large screen TVs apart from specialist projection types, would have cost nominally £50.

Ian Jackson expressed precisely :

Do you remember the hot wire regulators, they used to provide a steady voltage to the instruments? You would hear it ticking when ever the ignition was turned on.

It happens that tony sayer formulated :

Why would you need one?

I've never seen it, but my car watches the charge and announces in words on the screen when there is a charging issue.

tony sayer submitted this idea :

Well you would feel deprived, if it didn't have a rev counter. Apart from a battery of warning lights, all I have is rev counter temperature gauge and fuel gauge. Then there is the panel, which tells you in words, what it has detected in faults, such as lamp failures. It even tells me which lamp has failed on the caravan when I'm towing, not that I've ever had one fail on the current set up - but I had to take a lamp out, just to prove it worked as described lol

Dave Plowman (News) explained on 14/01/2015 :

It didn't involve any wiring changes, just a matter of reversing the initial magnetism of the field in the dynamo. The usual reason to do it, was so that you could fit a more modern negative earth radio system.

I never came across any problems with other motors, due to the polarity swap.

The Natural Philosopher brought next idea :

I had a Motorola valve car radio for a while. Tiny low voltage acorn valves, it had a beautiful tone.

Dave Plowman (News) submitted this idea :

I came across my first alternator equipped car around 1970. A guy had broken down outside my door late at night, a Hillman something or other. He had no charge from the alternator and never having seen one before, I was pretty clueless. I accidently found the problem, when I unplugged the alternator connection - it was just salt corrosion on the plug.

In message , Harry Bloomfield writes

Do you mean bi-metal regulators? To be honest, 'maybe'!

Hot wire meters work because a resistance wire under tension gets longer, and sags when the current heats it up. The relaxation of tension is passed on to the needle movement. They have a slow response, so presumably will show only slight rhythmical twitches as the bi-metal regulator makes and breaks. This sort-of rings bell with me.

The Natural Philosopher used his keyboard to write :

The early ones had external regulator packs and I think maybe external diode packs.

Ian Jackson brought next idea :

Yes, they were a small metal case unit, three terminal unit, tacked on the back of the instrument panel. The idea was to even out the variable voltage across the battery, before feeding it to the instruments. I seem to remember the output averaged 10v. Before those, the fuel gauge etc. would wonder about as you reved the engine.

The field had some residual magnetism, which started the whole process off. Once started, the field coils would energise to enable current to be produced. Which explains how you could so easily change the polarity of the dynamo - change the polarity of that residual and it would then change the polarity of the output.

Dave Plowman (News) explained on 14/01/2015 :

The only way to measure such low resistances is to inject a large current and measure the voltage developed across it. An Ohm meter will just not hack it.