Dramatic jump-starting result

[scipio84]

Good morning guys.
Two days ago the battery of my car was almost died. So yesterday I tried a jump starting, very simple exercise.
So I connected the + of the charged one to the + of the worn battery and then the - of the charged one to the body of the "discharged car".
I turn on the dashboard of this last one (not making the engine run, only dashboard), and everything was ok, the lights inside the car were on.
So I turn the dashboard off and start the motor of the car with good battery, left it running for a while and finally I try to start the car with discharged battery.
Result: everything off. Not a sound, no smell, no smoke. Also the lights inside the car went off and no way to start anything.
The engine of other car continued to run without slowing down.

Two other notes.
The car was a BMW 320i, gasoline, 2 liters, year 1991 (pretty old), electronic injection and the battery was a 60Ah and I believe 480A.
The car with good battery is a 1.1 liters with a battery of about 50 Ah.

Now, I understand that such a smaller car can fail to start a 6 cylinders... but I can't understand the total black out and where can be the failure in the bmw, considering also that the small fiat didn't show any sign of fatigue.

Thanks a lot!

 

[Lucien Nunes]

Probably a high resistance connection somewhere, e.g. a loose or corroded terminal, maybe even in the jump leads. The resistance was low enough to power the lights and dashboard but with the heavy current of the starter the voltage drop was too great and the heat developed made it go open-circuit completely.

Only the other day I was underneath the pickup truck fixing a high-resistance negative battery-to-engine connection. Big cable and lug, nice and tight, but with a film of corrosion that was preventing it making contact.

 

[scipio84]

Well I see... I was thinking of any fuse or similar that is interrupting the circuit, but the idea is the same.
Is there any check I can do to try a diagnosis?

 

[PEG]

Hi,is the battery of the BMW completely discharged? They can get to the point where they are a resistance,themselves,in the jump-start process.

With a battery that is merely flat,i would expect most of the internal electrics,lights,wipers etc,to function,once the leads were connected from a running jump vehicle.

There is not a "main fuse",as such,but there will be several large fuses for circuits other than the starting circuit.

 

[Octopus]

If the car you are trying to start is a diesel, you will need thicker jump leads.....

 

[scipio84]

Gents, Peg was right. Now it makes about 5V. When I connected the two batteries, it didn't act anyhow and lost again.
I tried a new battery and worked well.
The problem is now how to make a diagnosis to find where the dissipation is.
Measuring the current between the negative of the battery and hearting lug (cable disconnected and car turned off) there is a residual of about 30mA, and it's too high. As well, I found out that there is an absorption of 2-3V.
How can I find the point where it happens?

 

[PEG]

Hello,If you are saying the vehicle is drawing 30mA when stood,that could be normal. There will be current drawn,from items such as radio/ICE equipment,memory and security systems. Some modern,high-end vehicles can draw well in excess of this,and as such,are a joy,when shipped overseas,with the battery not isolated...

There are other components,depending on model and vintage,which have been known to discharge a vehicles battery. Alternator windings,being one.

I have LED interior lamps,in my Troll,adding up to about your 4 watts,and they have remained on for a weekend,with no detriment to the battery.

 

[shanky887614]

put the N straight onto a bolt on the engine mount or the bad battery