Check your battery terminals

Stempy · Post by **Stempy** » 25 Apr 2005, 23:56

Thought I'd post this up following the recent self distruction of my alternator. It would appear that the reason it blew up was that the chassis terminal on the battery was a bit loose and apparently it only takes a momentary loss of contact for the alternator to destroy the regulator as they don't like to be run off load. There was never any indication on the dash of a charging fault, even when the alternator was in full smoke mode.
Looking at the battery terminal itself, although it looked to be fully tightened, it could actually be pulled off fairly easily, so a big bump might have dislodged it enough to allow the alternator voltage to rise and to blow the regulator.[xx(]

vanny · Post by **vanny** » 26 Apr 2005, 00:29

hmmmm, that could well explain my busted alternator!

AndersDK · Post by **AndersDK** » 26 Apr 2005, 00:51

Dont think so. The regulator is exactly there to keep the voltage within some 14Volts - whether it charges the battery or not.

fastandfurryous · Post by **fastandfurryous** » 26 Apr 2005, 01:41

But... if it's on load, generating 20 amps say, and then the battery cable comes off, the spike through the regulator is enough to fry it. That's why there are big warnings about not disconnecting the battery cables with the engine running. It's even not reccomended to connect or disconnect booster cables with the engine running, although many people do it. I've killed an alternator by doing this.

derekjl57 · Post by **derekjl57** » 26 Apr 2005, 03:11

Sorry, Anders, I'm with the others on his one, the regulator only works if it has a load to work against. Remove the load and the regulator just can't work anymore... hence fried alternator.

oscarloco · Post by **oscarloco** » 26 Apr 2005, 03:43

Good thing to keep in mind!

Post by **Peter.N.** » 26 Apr 2005, 03:52

I agree with Anders, the internal resitance of the regulator and exiter windings are sufficient to load the regulator, the damage is done when an arc causes a high back EMF due to the inductance of the alternator windings, producing voltage high enough to destroy the rectifier diodes or regulator IC or both.

alan s · Post by **alan s** » 26 Apr 2005, 03:59

Just out of curiosity, had any problems with corrosion in the cooling system as per this?
http://www.andyspares.com/discussionfor ... IC_ID=8275
Alan S

AndersDK · Post by **AndersDK** » 26 Apr 2005, 04:20

Alan - I believe that is highly relevant for the few XM-diesel owners who have to replace their radiator every 4-5 years or so.
As for the first discussion :
1) What alternator load is suddenly interrupted by removing one (or both) battery terminals - when engine running ?
2) How long and how heavy does the alternator charge the battery - when engine running ?

Post by **Peter.N.** » 26 Apr 2005, 04:30

Depends on the state of the battery. If fully charged the current flowing is negligable and if you removed a battery terminal under these conditions probably nothing would happen (only don't try it and sue me)as the battery voltage would be more or less the same as the regulated alternator voltage, the alternator would take over any load that the electrical system was imposing and you probably wouldn't notice the change. If however the battery is in a very low state of charge there could be up to 80 amps flowing, disconnect a lead at this current and you have a potentially very destructive arc

AndersDK · Post by **AndersDK** » 26 Apr 2005, 04:44

Peter -
Why would "the alternator take over any load" ?
Do you believe the battery is delivering any needed energy ?
In that case where would the battery get it's energy from ?
My point is :
The battery is there to provide an energy source when the engine must be started, and to provide emergency energy when the engine is stopped.

fastandfurryous · Post by **fastandfurryous** » 26 Apr 2005, 04:46

So either way, you shouldn't disconnect battery cables with the engine running! be it due to the sudden load change (which I'm sure can't be good for the alternator) or a back EMF spike due to an arc.
As PowerLee has regularly recommended, get rid of the standard (and rather rubbish) battery terminals, and fit some decent quality ones.

Post by **Peter.N.** » 26 Apr 2005, 04:47

True, but if the alternator is not rotating sufficiently fast to provide all the power needed, it will be shared between the altenator and the battery.

AndersDK · Post by **AndersDK** » 26 Apr 2005, 04:52

True Peter -
Can you run a car engine no battery installed in the circuit - i.e. jump starting it ?

fastandfurryous · Post by **fastandfurryous** » 26 Apr 2005, 04:55

Surely this never happens? even at idle a modern alternator can chuck out about 80% of its maximum output, which is more than enough to achieve 14.4V after a couple of seconds.