General Phantom electricity after disconnecting battery?

[alexGS]

I've just seen something that I think is very strange, though I might be missing something obvious, and I'd like to ask the forum for an explanation...

Today, a certain yellow Uno that's not mine (fitted with 1580cc engine, points ignition) suffered a sudden flat battery. The battery is a fairly-new maintenance free type that was previously in perfect condition. Apparently the engine started normally this morning, then after a short drive, couldn't restart. Voltage below 12V - starter wouldn't crank.

Therefore, there is some problem with the alternator (voltage with engine running measured the same as voltage with engine stopped).

BUT this is the strange part - when I disconnected the battery terminals. I tried to take the negative terminal off first, but I couldn't. So I took off the positive terminal. The negative terminal was still connected to the battery. The positive terminal was disconnected - a few seconds later, it touched some of the metalwork (near the battery tray) as would usually happen, and - FAT SPARKS!

Why? If the battery positive terminal was off (and the battery positive post not touching anything else), where was the voltage coming from - no circuit was formed?

It's almost as though something (such as the alternator) has developed the ability to store a charge. I checked the ignition coil but it was cold. I should also mention that the ignition was off and had been switched off for an hour or two.

If this seems obvious to someone then please say so
My hope is that it might lead me to the fault that caused the battery to run flat.

Thanks,
-Alex

 

[punto5dr]

Unless the alternator fault has changed the polarity of the battery

 

[Chas Uno]

My suspicion is the coil. It acts as a big capacitor and stores a high voltage charge that is discharged when the distributor signals it to do so. If the engine was switched off while there was still a charge in the coil then it would in effect act as a temporary charge store. As the negative battery terminal was still attached then any part of the metalwork touched by the positive terminal would complete the circuit if there was any 'live' current still in the car (hence the advice to always disconnect the negative terminal first). Did the positive terminal touch the coil or any metalwork near it?

 

[alexGS]

1986Uno45S (Chas), I agree with the logic (and the coil was my first suspect too), except that the ignition is off; so the coil is out-of-circuit.

punto5dr - I reckon you're on the right track.

I received a tip-off yesterday that the heater hoses had been recently disconnected, dropping a load of coolant into the alternator...

Here's my final hypothesis. I reckon the alternator diodes have been shorted out, meaning that the field (stationary, stator) coils are now running off the battery. That is, normally they would feed into the battery but with the diodes bypassed, current flows the other way - so the stator becomes an electromagnet! (The alternator couldn't turn because it's effectively an AC, not DC, motor - there is no commutator - so all the EMF goes into heating, I would guess!)

Then, when the battery is disconnected, the magnetic field in the alternator breaks down when the battery positive lead (which is connected directly to the alternator coils) contacts a grounded metal surface.

This would also explain how the battery suddenly went flat - pouring its energy into the alternator!

Hopefully if I remove the alternator and clean/dry it out, perhaps normal function will resume. Otherwise I'll try to test the diodes and swap them with a spare (not easy, as the diodes are soldered into a heatsink).

-Alex

 

[punto5dr]

I had the same happen on a ford cortina many years ago, it gave the same symptoms as your car.

 

[Steely]

are all fiats negative earth?? i know some cars car be positive earth..

 

[punto5dr]

My uno is negative earth.

 

[Uno driver]

As a student of electrical engineering I would say the only possible cause for sparks are capacitors in radio or amplifier. Coil can act like a capacitor for very short time (let's say a second) and there is no way to get electricity from alternator without spinning.

 

[alexGS]

As a student of electrical engineering I would say the only possible cause for sparks are capacitors in radio or amplifier. Coil can act like a capacitor for very short time (let's say a second) and there is no way to get electricity from alternator without spinning.

Yes, I was a student of electronics as well. As pointed out to me, the discharge time of an inductor cannot be anywhere near as long as that of a capacitor.

Nevertheless the problem is now solved - I replaced the diodes in the alternator, which were clearly shorted-out by brush-wear debris and damp residues, and also swapped the brushes which were so worn that they had only been making contact with the slip rings over 1/3 of the circumferance.

-Alex