Battery/electrical question

rowdie- stang said:

I moved my battery to the trunk a few years ago and now for some reason my battery want stay charged like I have a short or something. I tried to jump it off today and as long as the jumper cables work hooked up my car would run. As soon as I took them off it died and the cables were very hot. They were only on for maybe 30 secs. Does this sound like a short in the battery cable from battery to starter solenoid or could it be a short somewhere else? Thanks for any help

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Your alternator might be fried.
Take it in for testing

If the car runs while jumper cables are hooked up and stalls when you disconnect them, the it is either the battery or the alternator. But even with a bad alternator, a car with a good battery will run for awhile. I would load test the battery. I would lean more towards the battery being bad.
When the car is running with jumper cables hooked up, is the battery lite on in the cluster?

JRPony said:

If the car runs while jumper cables are hooked up and stalls when you disconnect them, the it is either the battery or the alternator. But even with a bad alternator, a car with a good battery will run for awhile. I would load test the battery. I would lean more towards the battery being bad.
When the car is running with jumper cables hooked up, is the battery lite on in the cluster?

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The battery is down because the bad alternator hasn't charged it. That's why it dies when the jumpers are removed. Even with a bad battery, the alternator "should" keep the car running after it's started. You should even be able to remove the battery after the car is started.

Do I win a prize?

I award you no prize and may god have mercy on your soul.

How do you know the altenator is bad? You tested it yourself, right? I'm not saying that the alternator is not bad, I'm was just saying that the battery is more likely to be the culprit.

JRPony said:

I award you no prize and may god have mercy on your soul.

How do you know the altenator is bad? You tested it yourself, right? I'm not saying that the alternator is not bad, I'm was just saying that the battery is more likely to be the culprit.

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What's more likely, that one or both be bad?
A bad alternator would cause all of his symptoms, while a bad battery would only do half of what he's experiencing.
The law of averages says it's the alternator.

That's Einstein talking there, not me.

Nice chat, TTYL

its really no big thing either way. could charge the battery and then just drive it to the parts store, pull it out and have it tested. if that checks good then pull the alternator and have them test it. shouldn't take long to do both if you got tools handy.

94GT347 said:

The battery is down because the bad alternator hasn't charged it. That's why it dies when the jumpers are removed. Even with a bad battery, the alternator "should" keep the car running after it's started. You should even be able to remove the battery after the car is started.

Do I win a prize?

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Disconnecting the battery form a running engine is very bad practice. The resulting electrical spike can damage the alternator, radio, computer and other electronic equipment.

Auto Electrics 101: Generators will run without a battery. The iron core for the field coils will retain enough magnetism to create the magnetic field required to make electricity.

Alternators like the ones used on Mustangs require a battery to power the rotating field windings. That's because the AC current they generate internally demagnetizes the iron core for the field coils when there is no battery current to power them. The diodes inside the alternator turn the AC current into DC current. There are 3 phases to the alternator’s internal circuitry & 6 diodes to rectify the AC current into DC current. The battery acts like a big sink or capacitor to smooth out the remaining pulses

There are some self exciting alternators, but I have never seen one on a Ford or GM car. The last self exciting alternator I worked on was a 200 amp 110 volt one in a Convair 580 twin turboprop airplane. That was over 30 years ago…

jrichker said:

Disconnecting the battery form a running engine is very bad practice. The resulting electrical spike can damage the alternator, radio, computer and other electronic equipment.

There are some self exciting alternators, but I have never seen one on a Ford or GM car. The last self exciting alternator I worked on was a 200 amp 110 volt one in a Convair 580 twin turboprop airplane. That was over 30 years ago…

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LOL, I didn't tell him to do it, but you could if you had to.

Once the alternator has been excited, it continues to run on it's own.
That is my understanding anyway.