[OldTee]

I removed the ground wire from the battery of a running car AND IT DID NOT DIE. I did not think an alternator could excite itself and it needed a few volts from the battery to keep (as they say at drivein movies) putting out.

Electrical gurus explain please. Car is 86 Carrera.

ARF

[Jase007]

Dunno the answer except that pre-computer cars ... alternator will keep it going (prolly not good for alternator). Computer cars ... voltage fluctuations / spikes can't be good for DME / ECU when connecting disconnect a running car.

Everything I've read says don't do it ... so I don't.

[Vicegrip]

Bat only gives initial bootstrap. Once spinning the alt excites itself. You can bootstrap most alts with a 9 volt battery if you don't have any charge in the car battery or no battery at all during a push start. Does not take much to get a little electromagnetism in the field winding.

[matt de maria]

Kurt's right. Commutator in alternator powers itself off of the output of the stator through 3 diodes. Once alternator is running the power to charge is from the alternator itself. However the regulator which controls the amount of charging sees (when battery is disconnected while running) a very dead battery and goes into a very full charging mode. This can drive the alternator into charging at 16.5 volts or above taking out various computers in a fraction of a second.

[VaSteve]

What the purpose of battery cut off switches on race car then?

[Trak Ratt]

Originally Posted by VaSteve

What the purpose of battery cut off switches on race car then?

Purpose is to kill motor and electric system all at once. The function is tested at tech by running motor to 2K-3K, switch off see if it actually shuts off.

[smdubovsky]

Originally Posted by VaSteve

What the purpose of battery cut off switches on race car then?

They usually leave the alternator on the battery side of the switch (so both disconnect from everything else.) Thats why they test it at high rpm @ inspection to make sure you did it right.

FWIW, its very dangerous to electronics to disconnect the batt while running. The charging current in the stator inductance can't instantly decrease and can generate hundreds of volts for an instant. Its called a "load dump" transient. Most modern good stuff is well protected against it. Old or cheap stuff is a crapshoot.

Also note that leaving the alternator on the battery side of a cutoff switch prevents load dump. Stator energy has somewhere to go.

[Vicegrip]

Do not leave the alt connected on the battery side of the disconnect switch. This defeats a large portion of the safety systems intent.

The most common switch setup has the alt dump to ground through a ballast resistor at the same time it is disconnected from the battery. Don't get a battery disconnect switch confused with a proper $50 racing disconect/kill switch. The proper switch does three things at the same time. It removes the battery from the system de-energizing as much wire as posable, it cuts power to the ignition system and provides a dump to the alternator. It is 3 switches in one that all change state together. One large heavy duty switch for the battery power and two smaller ones. Of the two smaller switches one changes from open to closed to provide the alt a resistor dampened power to ground dump and the other that changes from closed to open to cut power to the ignition system. Wired per the directions it disconnects everything from the battery, alt hot lead wiring included. The wire from the alt to the battery is often the wire that runs to the starter (a 911 is an example) and it is large, unfused and easily capable of starting fires if shorted. The reason you want as little wiring live after you pull the disconnect is live wire could be shorted by distorted metal during a wreck and cause a fire. The kill switch should be located as close as you can to the battery and still have the disconnect method survive a wreck.