[Healeys] Electric Draw

[Bob Spidell]

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 at schoerner.se 
>> <mailto:per at schoerner.se>> wrote:
>>
>>     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 at gmail.com
>>     <mailto:050.rpl at 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, here’s what I’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 it’s either “good
>>     or bad” 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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