SOT: Car battery conundrum

Dave Plowman (News) laid this down on his screen :

I agree, all but the very cheapest cars had an ammeter. When alternators appeared, there was much less need to know what was happening with regards to charging. The better models did then include a limited range voltmeter, instead of an ammeter.

Reply to
Harry Bloomfield
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In message , Harry Bloomfield writes

A friend and I did that a couple of times. A reversal of the battery (usually requiring longer connecting cables) and a quick splat of the dynamo field terminal, and the job was done. Fortunately, the fuel and temperature gauges were hot-wire, and the indicator flasher was a bi-metal strip - so all were polarity insensitive.

Reply to
Ian Jackson

FSVO 'fairly sophisticated' :-)

Reply to
The Natural Philosopher

I think the point was that alternators were not adjustable: mechanical regulators were adjustable, so could go out of calibration.

If you cant adjust it, what's the point of knowing whether its working (badly) the light told you it was working or not.

Reply to
The Natural Philosopher

In article , Ian Jackson scribeth thus

Yep, used to do that on Old Minis which were Pos earth!.

Anyway what's the fuss about fitting ammeters these days no modern driver would have a clue what it was or did;!....

Reply to
tony sayer

Reply to
Andrew Gabriel

And that's how an ammeter works. It is a voltmeter measuring the voltage drop across a shunt. The big difference is it gives you a direct reading - no guesswork needed.

Reply to
Dave Plowman (News)

You have a multimeter which measures fractions of an ohm accurately? Pray tell which one?

Reply to
Dave Plowman (News)

but nearly every car has a rev counter.

Reply to
charles

The beauty of a suppressed zero voltmeter is it costs no more to make than an ammeter - but doesn't need heavy duty expensive cable to it. And also gives a (vague) indication of the state of the battery.

Reply to
Dave Plowman (News)

But could be a problem if any of the various electric motors - wipers, heater etc, were permanent magnet types. And of course a transistor radio...

Reply to
Dave Plowman (News)

Quite. But then some early alternator regulators weren't that brilliant either. ;-)

Reply to
Dave Plowman (News)

In message , "Dave Plowman (News)" writes

Some of the real cheapies appear to be remarkably accurate (well, my Fluke agrees with them!). Of course, you've got to carefully zero them beforehand.

Reply to
Ian Jackson

It's an interesting point.

You could obviously make a dynamo of any size - same as an alternator. The speed it runs at is determined by the gearing from the engine.

You would also obviously use electronic control these days with a dynamo.

Of course the lack of high current mechanical switching - commutator and brushes - on an alternator is a plus point - and I'd guess they are cheaper to make overall. And probably allow a far wider actual rev range than a dynamo, again because of the commutator. But you could use a viscous drive to limit the top speed of one too. But one thing is true - a dynamo produces more output at low revs than an alternator. So an alternator has to be geared to run faster relative to engine speed to match it.

Reply to
Dave Plowman (News)

plenty will - its all down to having a battery capable of delivering decent current.

Reply to
The Natural Philosopher

yes. Odd that. Since most cars are electronically regulated/gas flow strangled to never get near over revving

Reply to
The Natural Philosopher

swap the wires over. But of course which way a wiper motor goes is almost irrelevant

And of course a transistor

Valve radio. In those days, if you had a radio at all.

Reply to
The Natural Philosopher

no, that's the one thing that is false.

In the end it's speed an flux density and that's the same for both. There were practical reasons why alternators DOD produce more, but that was down to the fact that materials and form factor changed AND the real gotcha, they didn't have an 'on off' type regulator, so overcharging could be controlled better,

In short they fitted BIGGER alternators. Not better ones as such.

Reply to
The Natural Philosopher

In message , "Dave Plowman (News)" writes

Yes, you're right. While I think that it doesn't matter which way the wiper motor turned, obviously the heater blower motor would need to be reversed. However, on the couple of cars I was involved with, there didn't seem to be a problem. [Maybe we did swap the blower connections, but I don't recall doing that.]

There's a story there...

A friend and I did an elderly positive earth car for a bloke at work.

In those days, in many (if not most) cars, a radio was an optional extra, so he had bought one. He had managed to fit it himself, but it didn't work - so could we have a look?

We soon established that the reason that the radio didn't work was that it was negative earth (can't recall whether it had a polarity switch), and had blown up.

At the time, most modern cars and radios had gone over to negative earth, so to avoid it happening again, he asked us whether it was possible to modify his car. It turned out that it was, so one lunchtime we did the business.

After the change, the car functioned as normal - but a couple of days later, he came in, and reported that he had acquired another radio - but that too didn't work. We had a look, and found that it had a polarity switch - set to positive earth!

I'm glad to say that he gave up asking us for our assistance.

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Reply to
Ian Jackson

Not so sure about that. If measuring fractions of an ohm accurately, things like the resistance of the test lead connectors etc come into play.

Reply to
Dave Plowman (News)

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