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OK, folks, I gotta eat crow. I'd never heard of it before, but I got to
thinking about it and realized that alternator diodes are a direct path
to the battery, and I did some research:
/"//A leaky diode also can allow current to drain out of the battery
through the alternator when the vehicle is not being driven."/
/-
/https://www.knowyourparts.com/technical-resources/starting-and-charging/starters-and-alternators-common-misdiagnosis/
In my experience, diodes fail open, sort of like fuses, but if one
failed short--or 'leaked'--it could draw current from the battery.  I
think the way to test for this would be to disconnect the
alternator->battery lead and see if the current draw ceases (or charge
the battery to a known voltage, disconnect the alternator, and see if
the battery remains charged overnight). I don't think a generator could
behave like this, but I'm gonna think about it before I press 'send'
before I've had my second cup of coffee.
My apologies to all (including the OP's mechanic).
Bob
//
On 6/7/2019 9:09 AM, Bob Spidell wrote:
>
> Uh, I'm not an electrical engineer--but I play one on email lists--but
> it sounds like your mechanic isn't one either (not uncommon). The
> diodes in an alternator perform the same function as the brushes and
> commutator in a generator; i.e. they rectify the alternating
> current--produced by rotating an electromagnet--into DC current, which
> your battery requires for charging and all your accessories require
> to, well, accessorize. Hence, they aren't doing squat when the engine
> isn't running--when there should be no current coming to the
> alternator to energize the rotor--and if you had a short somewhere and
> your alternator was getting current with the engine stopped it would
> likely heat up (see if your alternator feels warm after sitting
> overnight). Diodes can fail by either shorting out or opening up;
> most alternators have 6 of them and when one fails by opening up you
> lose one-sixth of your charging ability (I'd have to research it, but
> as solid state devices--sort of one-third of a transistor--they
> probably mostly fail by opening up as the silicon junctions are
> relatively fragile). I'm not sure if it's an open or shorted diode
> that causes it--maybe both--but with a bad diode you will get all
> kinds of radio noise, which varies with engine speed (when I used to
> fly light aircraft you could tell when someone had a bad diode with
> serious noise in radio transmissions).
>
> Others have offered good suggestions so I'll just add that it's
> conceivable your ignition switch is worn allowing some current flow
> when in the 'off' position (after all, it's Lucas ;)). See if it
> feels warm after sitting for a while (in fact, if you can get to them
> see if any of the electrical devices in your car feel warmer than
> ambient after sitting in the shade). Your battery ground doesn't pull
> a load--something has to draw current that the ground cable returns to
> the battery--so it isn't the problem. A nearly half-amp current draw
> in a 12V system will produce 6 watts of heat (nearly half of what a
> 60W equivalent CFL light rated at 13W will generate).
>
> Bob
>
>
> On 6/7/2019 6:13 AM, Michael Salter wrote:
>> Real Healeys don't have alternators.����
>>
>> On Fri, Jun 7, 2019, 8:55 AM Per Schoerner, <per@schoerner.se
>>
>> It sounds more like your cutout switch is the culprit here.
>>
>> Per
>>
>> Skickat från min iPhone
>>
>> > 7 juni 2019 kl. 14:47 skrev R. Lindsay <050.rpl@gmail.com
>> >
>> > For the past few weeks I have been fighting a parasitic draw on
>> my BJ8. It was running about .48 amps and would drain my battery
>> in a day or so. I was told by a mechanic that a diode in my
>> alternator must have blown therefore causing the draw. He also
>> mentioned that my ground wires in the boot were loose which he
>> tightened. I brought the car home, turned off the power in the
>> boot and 2 days later the battery was dead.
>> >
>> > So, hereâ??s what Iâ??ve done. I jumped the battery and started the
>> car and ran it for 10 minutes. I disconnected the charger and
>> tested the battery - 12.48 volts. I then removed the cutoff
>> switch and bolted the grounding wires together. I tested for a
>> draw and got a zero reading. That was last night. I went out this
>> morning (12 hours later) and the battery reads 12.33 volts.
>> >
>> > My questions are 1. Is .15 volts a reasonable drop in voltage
>> for a sitting car in 90 degree heat and 2. Can a diode in an
>> alternator work sometimes and not at others. If itâ??s either â??good
>> or badâ?? all the time, I guess I need to presume the alternator is
>> not the source of the draw. If a .15 volt loss over 12 hours is
>> reasonable then removing the cut out switch in conjunction with
>> presuming the alternator is good (as it works fine now) solved my
>> problem.
>> >
>> > What does the wisdom of this great resource think?
>> >
>> > Price Lindsay
>>
>
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<p>OK, folks, I gotta eat crow. I'd never heard of it before, but I
got to thinking about it and realized that alternator diodes are a
direct path to the battery, and I did some research:</p>
<p><i>"</i><i> A leaky diode also can allow current to drain out of
the battery through the alternator when the vehicle is not being
driven."</i></p>
<p><i>- </i><a class="moz-txt-link-freetext"
href="https://www.knowyourparts.com/technical-resources/starting-and-charging/starters-and-alternators-common-misdiagnosis/";>https://www.knowyourparts.com/technical-resources/starting-and-charging/starters-and-alternators-common-misdiagnosis/</a></p>
<p>In my experience, diodes fail open, sort of like fuses, but if
one failed short--or 'leaked'--it could draw current from the
battery.  I think the way to test for this would be to disconnect
the alternator->battery lead and see if the current draw ceases
(or charge the battery to a known voltage, disconnect the
alternator, and see if the battery remains charged overnight). I
don't think a generator could behave like this, but I'm gonna
think about it before I press 'send' before I've had my second cup
of coffee.<br>
</p>
<p>My apologies to all (including the OP's mechanic).</p>
<p>Bob</p>
<p><br>
<i></i></p>
<div class="moz-cite-prefix">On 6/7/2019 9:09 AM, Bob Spidell wrote:<br>
</div>
<blockquote type="cite"
cite="mid:350680f4-6bd3-42ae-2506-4fcb3a9af4ce@comcast.net">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<p>Uh, I'm not an electrical engineer--but I play one on email
lists--but it sounds like your mechanic isn't one either (not
uncommon). The diodes in an alternator perform the same
function as the brushes and commutator in a generator; i.e. they
rectify the alternating current--produced by rotating an
electromagnet--into DC current, which your battery requires for
charging and all your accessories require to, well,
accessorize. Hence, they aren't doing squat when the engine
isn't running--when there should be no current coming to the
alternator to energize the rotor--and if you had a short
somewhere and your alternator was getting current with the
engine stopped it would likely heat up (see if your alternator
feels warm after sitting overnight). Diodes can fail by either
shorting out or opening up; most alternators have 6 of them and
when one fails by opening up you lose one-sixth of your charging
ability (I'd have to research it, but as solid state
devices--sort of one-third of a transistor--they probably mostly
fail by opening up as the silicon junctions are relatively
fragile). I'm not sure if it's an open or shorted diode that
causes it--maybe both--but with a bad diode you will get all
kinds of radio noise, which varies with engine speed (when I
used to fly light aircraft you could tell when someone had a bad
diode with serious noise in radio transmissions).</p>
<p>Others have offered good suggestions so I'll just add that it's
conceivable your ignition switch is worn allowing some current
flow when in the 'off' position (after all, it's Lucas ;)). See
if it feels warm after sitting for a while (in fact, if you can
get to them see if any of the electrical devices in your car
feel warmer than ambient after sitting in the shade). Your
battery ground doesn't pull a load--something has to draw
current that the ground cable returns to the battery--so it
isn't the problem. A nearly half-amp current draw in a 12V
system will produce 6 watts of heat (nearly half of what a 60W
equivalent CFL light rated at 13W will generate).<br>
</p>
<p>Bob</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 6/7/2019 6:13 AM, Michael Salter
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAB3i7LLeik9r7cZyMa3Ph8tObwjUZA1VXpT5mQ0jPFZQGQVoKw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html;
charset=UTF-8">
<div dir="auto">Real Healeys don't have
alternators.����</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, Jun 7, 2019, 8:55 AM
Per Schoerner, <<a href="mailto:per@schoerner.se";
moz-do-not-send="true">per@schoerner.se</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">It sounds
more like your cutout switch is the culprit here.<br>
<br>
Per<br>
<br>
Skickat från min iPhone<br>
<br>
> 7 juni 2019 kl. 14:47 skrev R. Lindsay <<a
href="mailto:050.rpl@gmail.com"; target="_blank"
rel="noreferrer"
moz-do-not-send="true">050.rpl@gmail.com</a>>:<br>
> <br>
> For the past few weeks I have been fighting a parasitic
draw on my BJ8. It was running about .48 amps and would
drain my battery in a day or so. I was told by a mechanic
that a diode in my alternator must have blown therefore
causing the draw. He also mentioned that my ground wires in
the boot were loose which he tightened. I brought the car
home, turned off the power in the boot and 2 days later the
battery was dead. <br>
> <br>
> So, hereâ??s what Iâ??ve done. I jumped the battery and
started the car and ran it for 10 minutes. I disconnected
the charger and tested the battery - 12.48 volts. I then
removed the cutoff switch and bolted the grounding wires
together. I tested for a draw and got a zero reading. That
was last night. I went out this morning (12 hours later) and
the battery reads 12.33 volts. <br>
> <br>
> My questions are 1. Is .15 volts a reasonable drop in
voltage for a sitting car in 90 degree heat and 2. Can a
diode in an alternator work sometimes and not at others. If
itâ??s either â??good or badâ?? all the time, I guess I need to
presume the alternator is not the source of the draw. If a
.15 volt loss over 12 hours is reasonable then removing the
cut out switch in conjunction with presuming the alternator
is good (as it works fine now) solved my problem. <br>
> <br>
> What does the wisdom of this great resource think?<br>
> <br>
> Price Lindsay<br>
<br>
</blockquote>
</div>
</blockquote>
<br>
</blockquote>
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