[DML] Re: Bad Battery / Good Alternator
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[DML] Re: Bad Battery / Good Alternator

In my tenure as a locomotive mechanic I saw batteries behave in all
sorts of strange ways (yes -- with a load on them. You diagnose the
batteries while trying to crank the engine).

Of course there's the voltage drop we're all familiar with.

One I'd never encountered before is a battery "going negative" --
voltage remains the same but the polarity reverses (we called it
"going negative" because that's what a digital meter shows on its

Automobile starter motors are fairly gentle on a single battery. A
locomotive generator (there are starter windings inside the generator)
puts an incredible load on a bank of batteries. 

Re: Dead plates -- there's a point at which a battery isn't going to
hold a charge, no matter how much acid you pour into it. Being very
expensive, we'd bring dead batteries back to the shop, pour out what
was mostly water by that point (in the field you just top them off
with a hose pipe), add new acid, hook up a charger, and hope for the
best. Sometimes they'd come back, sometimes they wouldn't. 

In such cases it was explained to me that the lead plates inside the
battery had lost their reactive properties and would not shoot
electrons across anymore, no matter what they were immersed in.

Bill Robertson

>--- In dmcnews@xxxxxxxxxxxxxxx, Martin Gutkowski <martin@...> wrote:
> Quoting Elvis <elvisnocita@...>:
> >
> >> Regarding good alternators on bad batteries: If the plates inside a
> >> battery are dead, but the alternator itself is good, it is not going
> >> to produce 13.5 volts (typical charging nomenclature). I defy you to
> >> produce charging voltage on a dead battery (not a battery that
> > simply
> >> needs specific gravity, but one that has bad plates).
> >
> > Is there any real explanation for that ????
> > What do you mean with dead plates ? I had dead batteries in my car
> > before and I am still alive - and my voltage gauge still showed
> > 13.8V when the engine was running at a bit higher speeds than idle !
> > (of course NOT with all the loads on that I could find !)
> > What are we discussing here ? Do we need to discuss basic high school
> > electricity laws ? Starting at Ohm's law ?
> I don't think Bill would know what you meant if you started referring  
> to current by the letter "I" :-)
> Bill, a dead battery (as opposed to a flat battery) is almost always  
> an open circuit, not a dead short. The alternator relies heavily on a  
> battery's capacitance to regulate correctly, and in this respect, even  
> a dead battery is often still good. Generating a potential (measured  
> in Volts) does not require a load or current flow. However for an  
> alternator to provide a stable potential, it does need a load. So if  
> your battery's dead, turn the lights on... there's a nice resistive  
> load in the order of a couple of ohms.
> Martin

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