Re: 12v conversion messy

Posted by Gerald J. on November 11, 2006 at 20:07:28 from (67.0.102.73):

In Reply to: 12v conversion messy posted by oddjobbob on November 11, 2006 at 17:58:29:

Why do you want to throw away a good information source, the ammeter? A voltmeter tells you lies and half truths about charging.

Having 14.2 volts (which is the proper 12 volt charging voltage) does not mean the battery is charging. Only that if the battery was connected it should charge. ONLY the ammeter shows if the battery actually gets charge.

Then unless that voltmeter has more precision than any 2" was ever made it won't show the state of the battery charge unless its connected AT the battery, but then it loads the battery when the tractor isn't running and you can't tell the difference on the meter between full charge and half charge because its less than a pointer width.

I tried the pair of meters in my '64 VW. I used lab quality instruments. The voltmeter gave me very little information, while the ammeter was far more informative.

What the ammeter shows is a peak of charging current right after starting, that tapers to zero as the battery charges. That shows that the battery is coming up to full charge (providing the charging voltage is set to 2.366 volts per cell). The ammeter shows the loads when the ignition is on and the engine not running. If the battery current never tapers to zero, the voltage is too high or the battery is sulfated. If the battery current tapers to zero too quickly, the charging voltage is too low. If the battery is sulfated charging current won't taper and in that case a voltmeter will show the battery voltage has come up to the proper charging voltage. That's the only time a voltmeter is useful, but you really still need the ammeter to detect that the current hasn't tapered. But then the sulfated battery probably won't start the engine the next day either.

The reason Detroit uses a voltmeter is its a whole lot easier to wire than a proper ammeter, but with a mark at 8 volts and another at 18 volts I begin to think its like the oil pressure "gauge" that comes up to mid scale and never moves until the engine stops and the oil pressure sensor is a switch.

Oh by the way, that charging voltage should rise for battery temperature above 68 F and for temperature below 68F. Trouble is the vintage relay type voltage regulator is so imprecise that temperature compensation isn't worth the bother.

Gerald J., Electrical engineer since 1963

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